Valve and Transistor Audio Amplifiers

Valve and Transistor Audio Amplifiers
VALVE AND TRANSISTOR
AUDIO AMPLIFIERS
JOHN LINSLEY HOOD
AMSTERDAM BOSTON HEIDELBERG LONDON NEW YORK OXFORD
PARIS SAN DIEGO SAN FRANCISCO SINGAPORE SYDNEY TOKYO
Newnes
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First published 1997
Reprinted 2000, 2001, 2003, 2006
Copyright 9 1997, John Linsley Hood. All rights reserved
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CONTENTS
ix
PREFACE
CHAPTER 1
CHAPTER 2
ACTIVE COMPONENTS
Valves or Vacuum Tubes
Solid State Devices
I
11
PASSIVE COMPONENTS
25
Inductors and Transformers
Capacitors
Resistors
Switches and Electrical Contacts
CHAPTER 3
CHAPTER 4
VOLTAGE AMPLIFIER STAGES USING VALVES
25
30
36
39
41
Circuit Configurations
Grid Bias Systems
Cathode Bias Resistor Calculations
Anode Load Systems
Grounded Grid Stages
The Cascode Circuit
The Long-tailed Pair
The Cathode Follower
The/.1 Follower
Tetrodes, Beam-Tetrodes and Pentodes
Load Lines
41
41
45
45
47
48
48
49
50
50
53
VALVE AUDIO AMPLIFIER LAYOUTS
57
Single-ended vs. Push-pull Operation
Phase Splitters
Output Stages
Output (Load-matching) Transformer
Effect of Output Load Impedance
Available Output Power
59
61
64
65
68
69
vi
Valve and Transistor Audio Amplifiers
CHAPTER 5
CHAPTER 6
CHAPTER 7
NEGATIVE FEEDBACK
71
Benefits of Negative Feedback
Stability Problems
Series and Shunt NFB
The Effect of NFB on Input and Output Impedances
Influence of NFB on Output Impedance
Effect of NFB on the Harmonic Components of the Signal
Practical Valve NFB Systems
The Baxandall Negative Feedback Tone Tontrol System
Frequency Response Equalisation
Filter Circuits Based on NFB
73
74
75
77
80
81
82
83
85
86
VALVE OPERATEDAUDIO POWER AMPLIFIERS
93
The Mclntosh Amplifier
The Williamson Amplifier
Distributed Load Systems
The Quad System
Baxandall's Amplifier
The Baxandall 5 Watt Design
The Radford Valve Amplifiers
The Leak TL/12
The Mullard 5/10 and 5/20 Designs
The GEC Audio Amplifiers
The GEC 912 + Amplifier
Other GEC Audio Designs
The Brimar 25P1 Design
The Sonab Design
Increasing Available Output Power
Conclusions
94
95
98
99
101
104
105
106
106
107
109
110
110
112
113
115
SOLID STATE VOLTAGE AMPLiFiERS
117
117
Junction Transistors
119
Control of Operating Bias
121
Stage Gain
122
Basic Junction Transistor Circuit Configurations
124
Emitter-follower Systems
126
Thermal Dissipation Limits
127
Junction Field Effect Transistors (JFETs)
131
Insulated Gate FETs (MOSFETs)
Power BJTs vs. Power MOSFETs as Amplifier Output Devices 134
135
U and D MOSFETs
137
Useful Circuit Components
138
Circuit Oddments
140
Slew Rate Limiting
Contents
CHAPTER 8
CHAPTER 9
EARLY SOLID STATE AUDIO AMPLIFIER DESIGNS
vii
141
The Lin Circuit
Quasi-complementary Output Stage Asymmetry
Listener Fatigue
Alternative Circuit Choices
Class A Operation
Fully Complementary Designs
Gain Stage Designs
HF Compensation Techniques
Symmetry in Circuit Layout and Slewing Rates
Stability of Output Current
143
145
148
150
153
156
158
160
162
163
CONTEMPORARY POWER AMPLIFIER DESIGNS
167
Remaining Design Problems
167
Power MOSFETsvs. Bipolar Junction Power Ttransistors (BJTs)168
169
Frequency Response of MOSFET Devices
171
MOSFET Linearity
174
gm Values for BJTs and MOSFETs
175
Quiescent Current Stability in Push-pull Output Systems
176
Other Iq Control Techniques
178
The Quad Current Dumping Amplifier
180
The Sandman Class S System
180
The Technics Class AA system
181
Voltage Amplifying Stage Gain and Overall THD
182
Basic Gain Stage Constructions
186
Output/Input Signal Isolation
187
Effect of NFB on Signal Distortion
187
Symmetry in Gain Block Structure
188
Negative Feedback and Sound Quality
190
Power Amplifier Output Stages
191
Output DC Offset Control
193
Hybrid Systems
193
Listening Trials
CHAPTER 10
PREAMPLIFIERS
Gramophone Record Replay Equalisation Characteristics
Replay Equalisation Circuitry
Practical Circuit Layouts
Very Low Noise Input Systems
Low Noise Devices
Headroom
Tone Controls and Filters
Signal Channel Switching and Remote Controls
197
198
199
203
206
209
212
215
218
9
9
VIII
9
Valve and Transistor Audio Amplifiers
Voltage Controlled Gain Systems
Digital Inputs
CHAPTER 11
POWER SUPPLIES
High Power Systems
Solid State Rectifiers
Music Power
Influence of Signal Type on Power Supply Design
High Current Power Supply Systems
Half-wave and Full-wave Rectification
DC Supply Line Ripple Rejection
Voltage Regulator Systems
Series Regulator Layouts
Over-current Protection
Integrated Circuit (Three Terminal) Voltage Regulator ICs
Typical Contemporary Commercial Practice
Battery Supplies
Switch-mode Power Supplies
INDEX
219
221
225
225
226
229
229
230
231
232
232
233
234
236
239
239
241
243
PREFACE
An old friend of mine was in the habit of remarking that an engineer is someone who
can do for five pounds what any damn fool can do for fifty. Needless to say, he was
an engineer. I suppose, in a sense, that this is a fairly good description of the major
aims of engineering practice; that the designer should seek elegant, efficient and costeffective ways of achieving a clearly specified objective. Only when satisfactory ways
of doing this cannot be found is there a need to seek more elaborate or costly ways to
get this result.
In the field of audio amplifiers there has been a great interest in techniques for making
small electrical voltages larger ever since mankind first attempted to transmit the
human voice along lengthy telephone cables. This quest received an enormous boost
with the introduction of radio broadcasts, and the resulting mass-production of
domestic radio receivers intended to operate a loudspeaker output. However, the final
result, in the ear of the listener, though continually improved over the passage of the
years, is still a relatively imperfect imitation of the real-life sounds which the engineer
has attempted to copy. Although most of the shortcomings in this attempt at sonic
imitation are not due to the electronic circuitry and the amplifiers which have been
used, there are still some differences between them, and there is still some room for
improvement.
In this book I have looked at the audio amplifier designs which have been developed
over the past fifty years, in the hope that the information may be of interest to the user
or would-be designer, and I have tried to explore both the residual problems in this
field, and the ways by which these may be lessened.
I believe, very strongly, that the only way by which improvements in these things can
be obtained is by making, and analysing, and recording for future use, the results of
instrumental tests of as many relevant aspects of the amplifier electrical performance
as can be devised. Obviously, one must not forget that the final result will be judged
in the ear of the listener, so that, when all the purely instrumental tests have been
completed, and the results judged to be satisfactory, the equipment should also be
assessed for sound quality, and the opinions in this context of as many interested
parties as possible should be canvassed.
Listening trials are difficult to set up, and hard to purge of any inadvertent bias in the
way equipment is chosen or the tests are carried out. Human beings are also
notoriously prone to believe that their preconceived views will prove to be correct.
The tests must therefore be carried out on a double blind basis, when neither the
listening panel, nor the persons selecting one or other of the items under test, know
what piece of hardware is being tested.
X
Valve and Transistor Audio Amplifiers
If there is judged to be any significant difference in the perceived sound quality, as
between different pieces of hardware which are apparently identical in their measured
performance, the type and scope of the electrical tests which have been made must be
considered carefully to see if any likely performance factor has been left unmeasured,
or not given adequate weight in the balance of residual imperfections which exist in
all real-life designs.
A further complicating factor arises because some people have been shown to be
surprisingly sensitive to apparently insignificant differences in performance, or to the
presence of apparently trifling electrical defects- not always the same ones - so, since
there are bound to be some residual defects in the performance of any piece of
hardware, each listener is likely to have his or her own opinion of which of these
sounds best, or which gives the most accurate reproduction of the original sound - if
this comparison is possible.
The most that the engineer can do, in this respect, is to try to discover where these
performance differences arise, or to help decide the best ways of getting the most
generally acceptable performance.
It is simple to specify the electrical performance which should be sought. This is that,
for a signal waveform which does not contain any frequency components which fall
outside the audio frequency spectrum - which may be defined as 10Hz-20kHz - there
should be no measurable differences, except in amplitude, between the waveform
present at the input to the amplifier or other circuit layout (which must be identical to
the waveform from the signal source before the amplifier or other circuit is connected
to it) and that present across the circuit output to the load when the load is connected
to it.
In order to achieve this objective, the following requirements must be met.
9 The constant amplitude (+0.5dB) bandwidth of the circuit, under load, and at all
required gain and output amplitude levels, should be at least 20Hz-20kHz.
9 The gain and signal to noise ratio of the circuit must be adequate to provide an
output signal of adequate amplitude, and that the noise or other non-signal related
components must be inaudible under all conditions of use.
9 Both the harmonic and intermodulation distortion components present in the
output waveform, when the input signal consists of one or more pure sinusoidal
waveforms within the audio frequency spectrum, should not exceed some agreed.
level. (In practice, this is very difficult to define because the tolerable magnitudes
of such waveform distortion components depend on their frequency, and also, in
the case of harmonic distortion, on the order (i.e. whether they are 2nd, 3rd, 4th or
5th as the case may be). Contemporary thinking is that all such distortion
components should not exceed 0.02%, though, in the particular case of the 2nd
harmonic, it is probably undetectable below 0.05%.)
Preface
X J
9 The phase linearity and electrical stability of the circuit, with any likely reactive
load, should be adequate to ensure that there is no significant alteration of the form
of a transient or discontinuous waveform such as a fast square or rectangular wave,
provided that this would not constitute an output or input overload. There should
be no tinging (superimposed spurious oscillation) and, ideally, there should also be
no waveform overshoot, under square-wave testing, in which the signal should
recover to the undistorted voltage level, +0.5%, within a settling time of 20~ts.
9The output power delivered by the circuit into a typical l o a d - beating in mind that
this may be either higher or lower than the nominal impedance at certain parts of
the audio spectrum- must be adequate for the purpose required.
9 If the circuit is driven into overload conditions, it must remain stable, the clipped
waveform should be clean and free from instability, and should recover to the
normal signal waveform level with the least possible d e l a y - certainly less than
20~ts.
In addition to these purely electrical specifications, which would probably be difficult
to meet, even in a very high quality solid state d e s i g n - and most unlikely to be
satisfied in any transformer coupled s y s t e m - there are a number of purely practical
considerations, such as that the equipment should be efficient in its use of electrical
power, that its heat dissipation should not present problems in housing the equipment,
and that the design should be cost-effective, compact and reliable.
Since it is improbable that all these performance requirements will be met, in any
practical design, it is implicit that the designer will have made certain performance
compromises, in which better performance in certain respects has been traded off
against a lesser degree of excellence in others. For myself, I think that the total
harmonic or intermodulation distortion, which is frequently specified in the makers
data sheets, is less important in determining the tonal quality of the system, provided
that it is better than 0.05%, than its behaviour under transient conditions, when tested
with typical (or accurately simulated) reactive loads, performance in which is seldom
or never quoted. Where appropriate, in the following text, I will try to show where
various benefits are obtained at the cost of some other potential drawbacks.
JLH 1996.
CHAPTER 1
ACTIVE C O M P O N E N T S
Electronic amplifiers are built up from combinations of active and passive
components. The active ones are those, like valves or transistors or integrated circuits,
that draw electrical current from suitable voltage supply lines and then use it to
generate or modify some electrical signal. The passive components are those, like
capacitors, resistors, inductors, potentiometers or switches, which introduce no
additional energy into the circuit, but which act upon the input or output voltages and
currents of the active devices in order to control the way they operate. Of these, the
active components are much more fun, so I will start with these.
Although the bulk of modem electronic circuitry is based on 'solid state' components,
for very good engineering reasons - one could not, for example, build a compact disc
player using valves, and still have room in one's house to sit down and listen to i t - all
the early audio amplifiers were based on valves, and it is useful to know how these
worked, and what the design problems and circuit options were, in order to get a better
understanding of the technology. Also, there is still an interest on the part of some 'HiFi' enthusiasts in the construction and use of valve operated audio amplifiers, and
additional information on valve based circuitry may be welcomed by them.
Valves or Vacuum Tubes
The term thermionic valve (or valve for short) was given, by its inventor, Sir Ambrose
Fleming, to the earliest of these devices, a rectifying diode. Fleming chose the name
because of the similarity of its action, in allowing only a one-way flow of current, to
that of a one-way air valve on an inflatable tyre, and the way it operated was by
controlling the intemal flow of thermally generated electrons, which he called
'thermions', hence the term thermionic valve. In the USA they are called 'vacuum
tubes'. These devices consist of a heated cathode, mounted, in vacuum, inside a sealed
glass or metal tube. Other electrodes, such as anodes or grids are then arranged around
the cathode, so that various different functions can be performed.
The descriptive names given to the various types of valve are based on the number of
its internal electrodes, so that a valve with two electrodes (a cathode and an anode) will
be called a 'diode', one with three electrodes (a cathode, a grid and an anode) will be
called a 'triode', one with four (a cathode, two grids and an anode) will be called a
'tetrode', and so on.
2
Valve and Transistor Audio Amplifiers
It helps to understand the way in which valves work, and how to get the best
performance from them, if one understands the functions of these internal electrodes,
and the way in which different groupings of them affect the characteristics of the
valve, so, to this end, I have listed them, and examined their functions separately.
The cathode
This component is at the heart of any valve, and is the source of the electrons with
which it operates. It is made in one or other of two forms: either a short length of
resistor wire, made of nickel, folded into a 'V' shape, and supported between a pair of
stiff wires at its base and a light tension spring at its top, as shown in Figure 1. l a, or
a metallic tube, usually made of nickel, with a bundle of nickel or tungsten heater
wires gathered inside it, as shown in Figure 1.lb. Whether the cathode is a directly
heated 'filament' or an indirectly heated metal cylinder, its function and method of
operation is the same, though, other things being equal, the directly heated filament is
much more efficient, in terms of the available electron emission from the cathode in
relation to the amount of power required to heat it to its required operating temperature
(about 775~ for one having an oxide coated construction).
(a) Directly heated
(b) Indirectly heated
Cathode
connection
Figure 1.1
Valve cathode styles
It is possible to use a plain tungsten filament as a cathode, but it needs to be heated to
some 2500~ to be usable, and this requires quite a substantial amount of power, and
leads to other problems such as fragility. Virtually all contemporary low to medium
power valves use oxide coated cathodes, which are made from a mixture of the oxides
of calcium, barium, and strontium deposited on a nickel substrate.
In the manufacture of the valve these chemicals are applied to the cathode as a paste
composed of a binding agent, the metals in the form of their carbonates, and some
small quantities of doping agents, typically of rare-earth origin. The metal carbonates
Active Components
3
are then reduced to their oxides by subsequent heating during the last stage in the
process of evacuating the air from the valve envelope.
In use, a chemical reaction occurs between the oxide coating and the heated nickel
cathode tube (or the directly heated filament) which causes the alkali metal oxides to
be locally reduced to the free metal, which then slowly diffuses out to the cathode
surface to form the electron emitting layer. The extent of electronic emission from the
cathode depends critically upon its temperature, and the value chosen for this in
practice is a compromise between performance and life expectancy, since higher
cathode temperatures lead to shorter cathode life, due to the loss through evaporation
of the active cathode metals, while a lower limit to the working temperature is set by
the need to have an adequate level of electron emission.
When hot, the cathode will emit electrons, which form a cloud around it, a situation in
which the thermal agitation of the electrons in the cathode body, which causes
electrons to escape from its surface, is balanced by the growing positive charge which
the cathode has acquired as the result of the loss of these electrons. This electron cloud
is called the 'space charge', and it plays an important part in the operation of the valve;
a matter which is discussed later.
The anode
In the simplest form of valve, the diode, the cathode is surrounded by a metal tube or
box, called the anode or plate. This is usually made of nickel, and it will attract
electrons from the space charge if it is made positive with regard to the cathode. The
amount of current which will flow depends on the closeness of the anode box to the
cathode, the effective area of the cathode, the voltage on the anode, and the cathode
temperature. For a fixed cathode temperature and anode voltage the ratio of anode
voltage to current flow determines the anode current resistance, R a, which is measured
by the current flow for a given applied voltage - as shown in the equation
R a = dVa/dI a
Because the anode is bombarded by electrons accelerated towards it by the applied
anode voltage, when they collide with the anode their kinetic energy is converted into
heat, which raises the anode temperature. This heat evolution is normally unimportant,
except in the case of power rectifiers or power output valves, when care should be
taken to ensure that the makers' current and voltage ratings are not exceeded. In
particular, there is an inherent problem that if the anode becomes too hot, any gases
which have been trapped in pores within its structure will be released, and this will
impair the vacuum within the valve, which can lead to other problems.
The control grid
If the cathode is surrounded by a wire grid or mesh - in practice, this will usually take
the form of a spiral coil, spot-welded between two stiff supporting wires, of the form
shown in Figure 1.2 - the current flow from the cathode to the anode can be controlled
by the voltage applied to the grid, such that if the grid is made positive, more
4
Valve and Transistor Audio Amplifiers
negatively charged electrons will be attracted away from the cathode and encouraged
to continue on their way to the anode. On the other hand, if the grid is made negative,
it will repel the electrons emitted by the cathode, and reduce the current flow to the
anode.
Support rods
Support rods
Figure 1.2
Control grid construction
It is this quality which is the most useful aspect of a valve, in that a quite large anode
current flow can be controlled by a relatively small voltage applied to the grid, and so
long as the grid is not allowed to swing positive with respect to the cathode, no current
will flow in the grid circuit, and its effective input impedance at low frequencies will
be almost infinite. This ability to regulate a large current at a high voltage by a much
smaller control voltage allows the valve to amplify small electrical signals, and since
the relationship between grid voltage and anode current is relatively linear, as shown
in Figure 1.3, this amplification will cause relatively little distortion in the amplified
signal. The theoretical amplification factor of a valve, operating into an infinitely high
impedance anode load, is denoted by the Greek symbol ~t.
Although in more complex valves there may be several grids between the cathode and
the anode, the grid which is closest to the cathode will have the greatest influence on
the anode current flow, and this is therefore usually called the control grid.
The effectiveness of the grid in regulating the anode current depends on the relative
proximity of the grid and the anode to the cathode, in that, if the grid is close to the
cathode, but the anode is relatively remote, the effectiveness of the grid in determining
the anode current will be much greater, and will therefore give a higher value of ~t than
if the anode is closer to the grid and cathode. Unfortunately, there is a snag in that the
anode current resistance of the valve, Ra, is also related to the anode/cathode spacing,
and becomes higher as the anode/cathode spacing is increased. The closeness of the
pitch of the wire spiral which forms the grid also affects the anode current resistance
in that a close spacing will lead to a high Ra, and vice versa.
The stage gain (M) of a simple valve amplifier, of the kind shown in Figure 1.4, is
given by the equation
Active Components
5
M = laR/(R + Ra)
so that a low impedance valve, such as a 6SN7 (typical I a = 9mA, R a = 7.7k, ~t = 20),
which has close anode-grid and grid-cathode spacings, and a relatively open pitch in
the grid wire spiral, will have a high possible anode current but a low amplification
factor, while a high impedance valve such as a 6SL7 (typical I a = 2.3mA, R a = 44k,
~t = 70) will have a low stage gain unless the circuit used has a high value o f anode
load resistance (R), and this, in turn, will demand a high value of HT voltage.
la (mA)
-5
!/
4
_3
_2
-1
-vg
+ Vg
-.5
i
-4
-3
-2
-1
0
~1
i
Figure 1.3
Triode valve characteristics
The space charge
Although a cloud o f electrons will surround any heated cathode mounted in a vacuum,
and will act as a reservoir of electrons when these are drawn off as anode current, their
E out
Ein
Figure 1.4
Simple valve amplifier
0V
6
Valve and Transistor Audio Amplifiers
presence becomes of particular importance when a negatively charged control grid is
introduced into the system, in that the electron cloud will effectively fill the space
between the cathode and the grid, and will act as the principal source of electrons.
The presence of this electron c l o u d - known as the space charge - has several
important operational advantages. Of these the first is that, by acting as an electron
reservoir, it allows larger, brief duration, current flows than would be available from
the cathode on its own, and that it acts as a measure of protection to the cathode against
the impacts of positive ions created by electronic collisions with the residual gases in
the envelope, since these ions will be attracted towards the more negatively charged
cathode. Finally, left to itself, the electronic emission from the cathode suffers from
both 'shot' and 'flicker' noise, a current fluctuation which is averaged out if the anode
current is drawn from the space charge.
This random emission of electrons from a space charge depleted cathode is used to
advantage in a 'noise diode', a wide-band noise source which consists of a valve in
which the cathode is deliberately operated at a low temperature to prevent a space
charge from forming, so that a resultant noisy current can be drawn off by the anode.
In the case of a triode used as an output power valve, where large anode currents are
needed, the grid mesh must be coarse, and the grid-cathode spacing must be close.
This limits the formation of an adequate space charge in the grid-cathode gap, and, in
its absence, the cathode must have a higher emission efficiency than would be
practicable with an indirectly heated system, and this means that a directly heated
filament must be used instead. Usually, the filament voltage will be low to minimise
cathode induced 'hum', and the filament current will be high, because of the size of
the filament (2.5A at 2.5V in the case of the 2A3 valve).
Directly heated cathodes are also commonly used in valve HT rectifiers, such as the
5U4 or the 5Y3, because the higher cathode emission reduces the voltage drop across
the valve and increases the available HT output voltage by comparison with a similar
power supply using an indirectly heated cathode type.
Tetrodes and pentodes
Although the triode valve has a number of advantages as an amplifier- such as a low
noise and low distortion factor- it suffers from the snag that there will be a significant
capacitance, typically of the order of 2.5pF, between the grid and the anode. In itself,
this latter capacitance would seem to be too small to be troublesome, but, in an
amplifying stage with a gain of, say, 100, the Miller effect will increase the
capacitance by a factor of 101, increasing the effective input capacitance to 252.5pF,
which could influence the performance of the stage.
When triode valves were used as RF amplifiers, in the early years of radio, this
anode-grid capacitance caused unwanted RF instability, and the solution which was
adopted was the introduction of a 'screening' grid between the triode control grid and
its anode, which reduced this anode-grid capacitance, in the case of a screened grid or
tetrode valve, to some 0.025pF.
Active Components
7
A further effect that the inclusion of a screening grid had upon the valve characteristics
was to make the anode current, in its linear region, almost independent of the anode
voltage, which led to very high values for R a and la. Unfortunately, the presence of this
grid caused a problem that when the anode voltage fell, during dynamic conditions, to
less than that of the screening grid, electrons hitting the anode could cause secondary
electrons to be ejected from its surface - especially if the anode was hot or its surface
had been contaminated by cathode material - and these would be collected by the
screening grid, which would cause a kink in the anode current/voltage characteristics.
While this might not matter much in an RF amplifier, it would cause an unacceptable
level of distortion if used in an audio amplifier stage.
Two solutions were found for this problem, of which the simplest was to interpose an
additional, open mesh, grid between the anode and the screening grid. This grid will
normally be connected to the cathode, either externally or within the valve envelope,
and is called the suppressor grid because it acts to suppress the emission of secondary
electrons from the anode.
Since this type of valve had five electrodes it was called a 'pentode'. A typical smallsignal pentode designed specifically for use in audio systems is the EF86, in which
steps have also been taken to reduce the problem of microphony when the valve is
used in the early stages of an amplifying system. The EF86 also has a wire mesh
screen inside the glass envelope, and surrounding the whole of the electrode structure.
This is connected to pins 2 and 7, and is intended to lessen the influence of external
voltage fields on the electron flow between the valve electrodes.
In use, a small-signal pentode amplifying stage will give a much higher stage gain than
a medium impedance triode valve (250" in comparison with, say, 30"). It will also have
a better HF gain due to its lower effective anode-grid capacitance. However, a triode
gain stage will probably have a distortion figure, other things being equal, which is
about half that of a pentode.
The second solution to the problem of anode current non-linearity in tetrodes,
particularly suited to the output stages of audio amplifiers, was the alignment of the
wires of the control grid and screening grid so that they constrained the electron flow
into a series of beams, which served to sweep any secondary electrons back towards
the anode- a process which was helped by the inclusion within the anode box of a pair
of 'beam confining electrodes', which modified the internal electrostatic field pattern.
These are internally connected to the cathode, and take the form shown in Figure 1.5.
These valves were called beam-tetrodes or kinkless tetrodes, and had a lower
distortion than output pentodes. Valves of this type, such as the 6L6, the 807, the
KT66 and KT88, were widely employed in the output stages of the high quality audio
amplifiers of the 1950s and early 1960s.
Both pentodes and beam-tetrodes can be used with their screen grids connected to their
anodes. In this mode their characteristics will resemble a triode having a similar
grid-cathode and grid-anode spacings to the grid-cathode and grid-screen grid
spacings of the pentode. The most common use of this form of connection is in power
8
Valve and Transistor Audio Amplifiers
output stages, where a triode connected beam tetrode will behave much like a power
triode, without the need for a directly heated (and hum-inducing) cathode.
Figure 1.5
Construction of beam-tetrode (by courtesy of RCA)
Valve parameters
In addition to the anode current resistance, R a, and the amplification factor, p,
mentioned above, there is also the valve slope or mutual conductance (gm) which is a
measure of the extent to which the anode current will be changed by a change in grid
voltage. Traditionally this would be quoted in milliamperes per volt (mA/V or milliSiemens, written as mS), and would be a useful indication of the likely stage gain
which would be given by the valve in an amplifying circuit.
This would be particularly helpful in the case of a pentode amplifying stage, where the
value of R a would probably be very high in comparison with the likely value of load
resistance. (For example, in the case of the EF86, R a is quoted as 2.5M~ and the gm
is 2mA/V.) In this case, the stage gain (M) can be determined, approximately, by the
relationship M ---gm.RL, which, for a 10Ok anode load would be ~--200".
The various valve characteristics are defined mathematically as,
R a = dVa/dI a
gm = -dla/dVg
p = -dVa/dVg
at a constant grid voltage,
at a constant anode voltage, and
at a constant anode current.
Active Components
9
In these equations the negative sign takes account of the phase inversion of the signal.
These parameters are related to one another by the further equation,
gm = lafRa or ~t-- gmRa
Gettering
The preservation of a high vacuum within its envelope is essential to the life
expectancy and proper operation of the valve. However, it is difficult to remove all
traces of residual gas on the initial pumping out of the envelope, quite apart from the
small but continuing gas evolution from the cathode, or any other electrodes which
may become hot in use. The solution to this problem is the inclusion of a small
container, known as a boat, mounted somewhere within the envelope, but facing away
from the valve electrodes, which contains a small quantity of reactive material, such
as metallic calcium and magnesium.
The boat is positioned so that after the pumping out of the envelope has been
completed, and the valve had been sealed off, the getter could be caused to evaporate
on to the inner face of the envelope by heating the boat with an induction heating coil.
Care is taken to ensure that as little as possible of the getter material finds its way on
to the inner faces of the valve electrodes, where it may cause secondary emission, or
on to the mica spacers where it may cause leakage currents between the electrodes.
While this technique is reasonably effective in cleaning up the gas traces which arise
during the use of the valve, the vacuum is never absolute, and evidence of the residual
gas can sometimes be seen as a faint, deep blue glow in the space within the anode
envelope of a power output valve. If, however, there is a crack in the glass envelope,
or some other cause of significant air leakage into the valve interior, this will become
apparent because of a whitening of the edges of the normally dark, mirror-like surface
of the getter deposit on the inside of the valve envelope. A further sign of the ingress
of air into the valve envelope is the presence of a pinkish-violet glow which extends
beyond the confines of the anode box. By this time the valve must be removed and
discarded, to prevent damage to other circuit components through an increasing and
uncontrolled current flow.
Cathode and heater ratings
For optimum performance, the cathode temperature should be maintained, when in
use, at its optimum value, and this requires that the heater or filament voltages should
be set at the correct levels. Since the voltage of the domestic AC power supply is not
constant, the design ratings for the heater or filament supply must take account of this.
However, this is not as difficult to do as it might appear. For example, Brimar, a wellknown valve manufacturer, make the following recommendations in their Valve and
Teletube Manual:'the heater supply voltages should be within +5% of the rated value
when the heater transformer is fed with its nominal input voltage, provided that the
mains power supply is within +10% of its declared value'.
10
Valve and Transistor Audio Amplifiers
An additional requirement is that, because of inevitable cathode-heater leakage
currents, the voltages between these electrodes should be kept as low as possible, and
should not exceed 200V. Moreover, there must always be a resistive path, not
exceeding 250k.Q, between the cathode and heater circuits. (As can be seen from some
of the audio amplifier circuits shown in Chapter 6, electronic circuit designers often
neglected this particular piece of advice.)
As a practical point, the wiring of the heater circuit, which is usually operated at 6.3V
AC, will normally be installed as a twisted pair to minimise the induction of mains
hum into sensitive parts of the system, as will the heater wiring inside the cathode tube
of low noise valves, such as the EF86. With modem components, such as silicon
diodes and low-cost regulator ICs, there is no good reason why the heater supplies to
high quality valve amplifiers should not be derived from smoothed and stabilised DC
sources.
It has been suggested that the cathodes of valves can be damaged by reverse direction
ionic bombardment if the HT voltage is applied before the cathode has had a chance
to warm up and form a space charge, and that the valve heaters should be left on to
avoid this problem. In practice, this problem does not arise because gaseous ions are
only formed by collisions between residual gas molecules and the electrons in the
anode current stream. If the cathode has not reached operating temperature there will
be little or no anode current, and, consequently, no gaseous ions produced as a result
of it. Brimar specifically warn against leaving the cathode heated, in the absence of
anode current, in that this may lead to cathode poisoning, because of chemical
reactions occurring between the exposed reactive metal of the cathode surface and any
gaseous contaminants present within the envelope. Unfortunately, the loss of electron
emissivity as the cathode temperature is reduced occurs more rapidly than the
reduction in the chemical reactivity of the cathode metals.
Indirectly heated HT rectifier valves have been used, in spite of their lower operating
efficiency, to ensure that the full HT voltage was not applied to the equipment before
the other valves had warmed up, but this was done to avoid the HT rail over-voltage
surge which would otherwise occur, and allow the safe use of lower working voltage,
and less expensive, components such as HT reservoir, smoothing, or inter-valve
coupling capacitors.
Microphony
Any physical vibration of the grid (or filament, in the case of a directly heated cathode)
will, by altering the grid-cathode spacing, cause a fluctuation of the anode current, and
this will cause an audible tinging sound when the envelope is tapped- an effect known
as microphony in the case of a valve used in audio circuitry. Great care must therefore
be taken in the manufacture of valves to maintain the firmness of the mounting of the
grids and other electrodes. This is done by the use of rigid supporting struts whose
ends are located in holes punched in stiff mica disc shaped spacers which, in tum, are
a tight fit within the valve envelope.
Active
Components
11
Since a microphonic valve will pick up vibration from any sound source, such as a
loudspeaker system in proximity to it, and convert these sounds into (inevitably
distorted) electrical signals which will be added to the amplifier output, this can be a
significant, but unsuspected, source of signal distortion which will not be revealed
during laboratory testing on a resistive dummy load. Since it is difficult to avoid valve
microphony completely, and it is equally difficult to sound proof amplifiers, this type
of distortion will always occur unless such valve amplifier systems are operated at a
low volume level or the amplifier is located in a room remote from the loudspeakers.
Solid State Devices
Bipolar junction transistors
'N'- and 'P'- type materials
Most materials can be grouped in one or other of three classes, insulators,
semiconductors or conductors, depending on the ease or difficulty with which
electrons can pass through them. In insulators, all of the electrons associated with the
atomic structure will be firmly bound in the valency bands of the material, while in
good, usually metallic, conductors many of the atomic electrons will only be loosely
bound, and will be free to move within the body of the material.
In semiconductors, at temperatures above absolute zero (0~ or-273.15~
electrons
will exist in both the valency levels where they are not free to leave the atoms with
which they are associated, and in the conduction band, in which they are free to travel
within the body of the material. This characteristic is greatly influenced by the
'doping' of the material, which is normally done, during the manufacture of the
semiconductor material, by introducing carefully controlled amounts of specific
impurities into the molten mass from which the single semiconductor crystal is grown.
The most common semiconductor material in normal use is silicon, because it is cheap
and readily available, and has good thermal properties. Germanium, the material from
which all early transistors were made, has electrical characteristics which are greatly
influenced by its temperature, which is inconvenient in use. Also it does not lend itself
at all well to contemporary mass-production techniques.
In the case of silicon, which has very little conductivity in its undoped 'intrinsic' form,
the most common dopants are boron or aluminium which give rise to a semiconductor
with a deficiency of valency electrons, usually referred to as holes- called a 'P'- type
material- or phosphorus, which will cause the silicon to have a surplus of valency
electrons, which forces some of them into the conduction band. Such a semiconductor
material would be termed 'N' type. Both P- type and N- type silicon can be quite
highly conductive, depending on the doping levels used.
Fermi levels
The electron energy distribution in single-crystal P- and N- type materials is shown in
Figure 1.6, and the mean electron energy levels, known as the Fermi levels, are shown
12
Valve and Transistor Audio Amplifiers
as dotted lines on the diagrams. If two such differently doped semiconductor materials
are in intimate contact with one another, electrons will diffuse across the junction so
that the Fermi level is the same on both sides. This will cause a shift in the relative
electrical potential of the two doped regions as is shown in Figure 1.7, and a number
of interesting effects arise from this.
Low temperature
or "P' type
FnrhiddAn hand
(energy gap)
.....
'
"
High temperature or
"N' type
Conduction
?o i ~ # o n
level
/ / / / / / -/, / /
...............
Valance
level
//////////,.v,
a
Figure 1.6
Fenrni
level
Valance
b
Average electron energy levels in semi-conductors as a function of temperature or
doping level
- v~
Conduction
level'x~ ~al
Forbidden
energylevel
,,..~ Eb Potentialbarrier
Conduction
-level
Fermi
Level (EF)
"P'type i r ~
I
I
"N' type
Valency
level
L
Depletiontayer
Figure 1.7
Mechanism by which potential barrier arises at P-N junction
(In practice, the only way by which such an intimate contact could be achieved within
a single crystal of a semiconductor material is by diffusing, say, a P-type impurity into
a wafer of N-type material, so that where these doped regions came into contact there
would be a predominance of P-type silicon on one side of the boundary and of N-type
silicon on the other.)
The P-N potential barrier and the depletion zone
The first of the phenomena which occur at the P-N junction is that the electron
conduction bands are displaced, so that electrical current (which consists, in the
philosophy of this book, of a movement of electrons from one place to another) cannot
flow through a semiconductor junction, even in the forward direction, from the
electron rich N-type zone into the electron deficient P-type one, until a high enough
potential exists to overcome the potential barrier (the voltage gap between the two
Active Components
13
conduction bands). This is about 0.58V in a silicon junction at room temperature, and
decreases as the junction temperature is increased.
The second effect, shown graphically in Figure 1.8, is the creation of a depletion zone,
in which the diffusion of electrons across the junction from the electron rich N region
into the electron deficient P region fills in the notional holes (labelled with '+' signs
in the drawing) where electrons should have been but were not. This leaves a region
where there are no electrons at all, and current will not flow through it. The effect on
the width of the depletion zone of the voltage across the junction is shown,
schematically, in Figure 1.9.
"P' type
eee
( - ve)
(+ ve)
!
"N' type
'
@~'~ I
@ @l,OJl .
__
I
-_
I-,-
.
Depletion
Figure 1.8
Growth of depletion layer on either side of a P-N junction
P
a
Reverse b~as wide
depletion band,
no current
~
Anode ( ~
b No bias, normal depletion
band, no current
N
c Small forward bias.
narrow depletion band.
Little or no current
Cathode
d
Large forward bias, no
depletion band, large
current
Figure 1.9
Influence of external potential on width of depletion zone in junction 'diode'
The semiconductor diode
The effect of the voltage across a semiconductor junction,
current flow is illustrated in Figure 1.10. When a forward
voltage is applied across such a P-N junction, little current
exceeds the potential barrier, when the current flow will
in this case silicon, on its
(i.e. conduction direction)
will flow until the voltage
increase rapidly until any
14
Valve and Transistor Audio Amplifiers
further increase is limited by the conducting resistance of the semiconductor material
or of the external circuit.
In the reverse or non-conducting direction there will always be a small leakage current
which will gradually increase as the reverse potential is increased, up to the voltage at
which the junction breaks down. This breakdown is due to what is termed an
avalanche effect, in which the electrons which form the reverse leakage current,
accelerated by the reverse potential, reach a high enough kinetic energy to cause
ionisation in the junction material. This ionisation releases further electrons which, in
turn, will be involved in further collisions, increasing yet further the leakage current.
In general, such reverse breakdown effects are to be avoided because the heat involved
causes a temperature rise at the junction which can permanently damage the diode.
However, some diodes are deliberately manufactured, by the use of very high doping
levels, so that reverse breakdown occurs at voltages which are low enough for thermal
damage to be avoided. These are commonly called Zener diodes, though this term is
really only appropriate for diodes having a reverse breakdown voltage below about
5.5V.
+i
Si
b At origin
-V~
=§
Figure 1.10
Normal forward and reverse characteristics of silicon and germanium junction
diodes
Junction capacitance
An additional feature of the semiconductor junction is its capacitance. If the junction
is reverse biased this capacitance is relatively small, and has the characteristics shown,
approximately, by the relationship
C=k/4V
where V is the voltage across the junction, and k is some constant which depends on
the doping level of the material, and the area and temperature of the junction. The
capacitance of a forward biased junction is much larger, and much more strongly
dependent on the doping level, the junction temperature and the forward current.
Active Components
15
Typical values of capacitance for a reverse biased small-signal diode will be of the
order of 2-15pF, while in the case of a forward biased junction the capacitance could
well be some ten times this value, depending on the diode current. (In the case of
transistors, where the relationship is much more complicated, the manufacturers
seldom attempt to specify the input (base-emitter) capacitance, but lump this factor,
along with other things which affect the transistor HF performance, in a general term,
called the HF transition frequency (fT) which is the frequency at which the transistor
current gain has fallen to unity in value.)
Transistor action
If the simple P-N semiconductor junction is elaborated so that it has three layers, for
example to make an NPN structure, of the kind shown schematically in Figure 1.11,
this can be made to act as an amplifying device, known as a junction transistor. In
practice, one of the N-type layers will be only lightly doped, and this region, because
of its function, will be called the collector. The other N-type region will be heavily
doped, to reduce its conducting resistance, and since it is the source of the current flow
it is termed the emitter. The doping levels chosen by the manufacturer for the
intermediate P-type region, known as the base, will mainly be determined by the
intended use of the device.
.•C
J
N
..
~
P
P
JzOne
Delolet~n
E -----J
_
~
N
zonll
E .----..J
Figure 1.11
Single crystal junction transistor structures
If a reverse voltage is applied across the collector-base junction, no significant current
will flow so long as the applied voltage is less than the breakdown voltage of the
junction. If a small forward voltage is applied across the base-emitter junction, once
again no current will flow until the forward voltage reaches the potential barrier level.
However, the differences in doping types and the existence of potential differences
across the junction will cause depletion zones to form, within the material, on either
side of the base-emitter and base-collector junctions, so that, if the base layer is thin,
the depletion zone may extend throughout its entire thickness.
Referring to the schematic drawing of Figure 1.11, if the base is made more positive,
electrons will flow from the emitter into the base region, but they will not necessarily
flow into the base circuit since, depending on the geometry of the junctions and the
positive potential on the collector, a large proportion of the electrons which set out
16
Valve and Transistor Audio Amplifiers
from the emitter will be swept straight through the base region and into the collector.
This leads to a base voltage vs. collector current relationship of the kind shown in
Figure 1.12, and allows the device to control a relatively large collector current by a
relatively low base voltage.
Vb = 0.59 V
Vb = 0.57
Vb - .056
Vb = 0.54
I
5
I
10
I
15
Figure 1.12
Relationship between collector current and collector voltage for small signal silicon
transistor
In this respect, the bipolar junction transistor (BJT) behaves in a similar manner to the
thermionic valve, except that the grid of a valve is essentially an open-circuit
electrode, whereas the base region of the junction transistor has a relatively low
resistance p a t h - whose value depends on the emitter current - to the emitter. Also,
while in a valve the anode current is controlled by its grid voltage, in a junction
transistor it is more correct to regard the collector or emitter current as being
controlled by the current flowing in the base circuit, which, as shown in Figure 1.10,
has a very non-linear relationship with the applied voltage.
Junction transistors are available in a range of packages, suitable for a very wide range
of collector voltages, currents and power dissipations. Provided that their dissipations
are not exceeded and the integrity of the encapsulation has not failed, all modem
semiconductor devices have a virtually indefinite life.
The mutual conductance of a bipolar junction transistor is very high, and can be
determined from the relationship
gm= Ic(q/kT)
where q is the charge on the electron (1.6 " 10-'~), k is Boltzmann's constant, and T is
the absolute temperature.
This means that the mutual conductance is dependent principally upon the collector
current, and for a junction temperature of 25~ the slope will be about 39S/A. For a
small-signal transistor, at l mA, a practical value of gm could be 40mS (mA/V). This
allows high stage gains with relatively low collector load resistances.
Active Components
17
PNP and NPN transistors
A major practical advantage of the junction transistor is that, because P-type and Ntype materials can be made with equal facility, it is possible to make PNP (negative
collector voltage) transistors almost as easily as NPN (positive collector rail voltage)
ones, by the simple substitution of differently doped materials. The availability of
these two types of device is of great convenience to the innovative circuit designer.
The schematic symbols used for these, and other discrete semiconductor devices, are
shown in Figure 1.13.
..~1 PN ~PN P
Junction
N-channel
transistors
Enhancement
MOSFETs
P-channel
Junction FETs
Depletion MOSFETs
Figure 1.13
Transistor circuit symbols
It should be understood, however, that these complementary types of transistor are not
identical, since there is a basic difference in the way in which current will pass through
N- and P-type materials. In an N-type region, where there is a surplus of electrons,
current flow is due to the movement of these electrons, exactly as in a metallic
conductor. On the other hand, in a P-type material, the shortage of electrons results
in vacancies occurring in the valency bands in places where electrons should
normally be.
In the semiconductor engineer's parlance, these absent electrons are referred to as
holes, and behave in much the same way as positively charged electrons. The
difference, though, is in their speed of flow, because the apparent movement of a hole
occurs when it is filled by an electron which has moved, in response to some electrical
field, and has left a hole where it had been. This hole will, in turn, be filled by another
electron which will leave yet another hole, and so on, like marbles on a solitaire board.
The effect of this is that current flow in a P-type region is both slower and more noisy
than in an N-type material.
A practical consideration which follows from this difference is that small-signal PNP
transistors, which have an N-type base region, have a slightly lower noise factor than
18
Valve and Transistor Audio Amplifiers
their equivalent NPN versions, though, in modem devices, this difference is slight. On
the other hand, the greater mobility of the electrons in the relatively broad emitter and
collector regions means that the HF performance of an NPN transistor will be better,
other things being equal, than a PNP one. This is a factor which should be remembered
in relation to power output transistors.
Junction field effect transistors
It has been seen that the presence of a reverse potential across a semiconductor
junction will increase the width of the depletion zone on either side of the junction,
and vice versa. Field effect transistors, normally called FETs, make use of this effect
to allow a voltage applied to a reverse biased P-N junction to control the flow of
current along a strip of suitably doped semiconductor material, called the channel,
from an input connection - called the source - at one end of the channel to an output
connection- called the drain- at the other end, as shown in Figure 1.14a. This control
electrode is called the gate, and behaves like the control grid in a valve. Usually there
will be two such parallel connected gate regions diffused into either side of the channel
to increase the effectiveness of this electrical control action.
If there is a voltage difference between the two ends of the channel, as will normally
be the case, this will have the effect of making the depletion zone somewhat
unsymmetrical, as shown in Figure 1.14b. As the drain voltage is increased, in respect
to the gate, the two depletion zones will move closer together until the channel is
pinched off entirely, and current flow through this depleted region, still controlled by
the gate voltage, is due to quantum-mechanical tunnelling. In this condition, which is
'ok
:
--7-
a
N O~
-)
.....
b
Figure 1.14a
Schematic construction of N- and P-channel junction FET
c,
Figure 1.14b
Effect of pinch off in junction FET
~(+re)
Active Components
19
that of normal operation, the drain current is almost completely independent of the
drain voltage, as shown in Figure 1.15. The relationship between gate voltage and
drain current is shown in Figure 1.16, and is much more linear than for a bipolar
junction transistor.
10--
~
5
-Vg = 0
=0.5
=1
-Vg =, 1.5
_
ol
'
''
,f=%M
I'
lO
5
I
Figure 1.15
Junction FET drain current characteristics at voltages above pinch off
lm~
7 l~
-- 5
I
-3
I
-2
I
-I
0
I
+1
Figure 1.16
Conduction characteristics of typical junction FET
Breakdown
potentials
and operating
characteristics
In use, the junction FET behaves very much like a thermionic valve, in that the gate is
effectively an open-circuit electrode so long as the gate-source potential is reverse
biased. If it becomes forward biased the gate-channel junction will be effectively the
same as a forward biased junction diode. It is commonly supposed that all FETs are
sensitive to, and can be destroyed by, the high voltages associated with inadvertent
electrostatic discharges, but junction FETs are, in reality, no more fragile in this
respect than any other small-signal diode.
By comparison with the bipolar junction transistor, FETs are relatively low voltage,
low power devices, with maximum drain-gate voltages typically restricted to about
25--40V, and free-air dissipations of about 400mW. Although normal mutual
20
Valve and Transistor Audio Amplifiers
conductance figures lie in the range of 2-10mS (mA/V), the in-circuit stage gain is
limited by the fact that the low drain-gate breakdown voltage precludes the use of high
drain load resistance values, unless very small drain currents are acceptable. Once
again, the stage gain (M) can be calculated, approximately, from the equation
M = gmRL
The input and drain-gate capacitances of an FET are similar, at about 2pF, to those of
a small power triode valve. Neither FETs nor junction transistors are at all
microphonic unless they are mounted at the end of long, flexible leads in proximity to
some charged body. Modern junction FETs are capable of very low voltage noise
figures, except that their high input impedances (typically some 10 '2 ohms) mean that
a high impedance input circuit can generate high values of Johnson (thermal) noise.
Insulated gate field effect transistors
These devices, known as IGFETs or MOSFETs (metal oxide/silicon field effect
transistors) have become the most widely used, in the greatest range of styles, of all
the field effect transistors. Their method of operation is exceedingly simple, and an
attempt to make a device of this type was made by Shockley in the early 1950s. (His
efforts were frustrated by his inability to obtain sufficiently pure semiconductor
material.)
A simple MOSFET is shown, schematically, in Figure 1.17, and consists, in principle,
of a strip of very lightly doped P-type material, into which, at either end, an N-type
region has been diffused to allow electrical connections to be made to the strip. As in
the case of the junction FET, these parts of the MOSFET are called the source, the
drain and the channel. In manufacture the emitter contact pad (an area of vacuum
deposited aluminium) is arranged to overlap both the source diffused zone and the
'P-' (a style of nomenclature which means lightly doped P-type) channel. This makes
the device behave just like a very low gain NPN transistor with its base connected
permanently to its emitter, a condition in which no collector current will flow.
Source + Substrate (OV)
a
-
Gate (or) S,O.. Drain (+ve)
P-
P§
l/lib
r
Source + Substrate (OV) Gate(+ve) S,O.. Drain (+ve)
b
Figure 1.17
Simple insulated gate FET
Active Components
21
If, however, a thin insulating layer (typically silica or silicon nitride in the case of a
MOSFET made from silicon) is formed so that it covers the channel region, and a
conducting layer (most commonly polycrystalline silicon) is formed on top of this
insulating surface, then if a positive potential is applied to this conducting surface
layer (also called the gate) it will induce a film of negative charges (electrons) across
the gap between the source and the drain.
Since there is no difference between electrons produced by electrostatic induction and
those due to any other cause, if a voltage is now applied between drain and source, a
current will flow, and this current will be controlled by the gate voltage. This gate
voltage vs. drain current relationship, shown in Figure 1.18, can be made very linear
(leading to a low distortion in an amplifying circuit) by suitable design of the
MOSFET structure (see Siliconix Technical Article, TA 82-3).
The style of MOSFET described above would be called an N-channel enhancement
type, because the drain current, at zero gate voltage, would be exceedingly small, but
would increase as the gate voltage was made more positive. Its method of construction
is shown schematically in Figure 1.19.
There is, however, a further type of MOSFET called an N-channel depletion type, in
which there is a significant current flow at zero gate-source voltage, but which will be
10-.-
(rr~)
S--
I
I
I
2
I
I
3
1
4
5
I
6
Figure 1.18
Conduction characteristics of small signal MOSFET
~rcs
§ ~l~ln~
_
.
Figure 1.19
Construction of lateral P-channel enhancement MOSFET
)
22
Valve and Transistor Audio Amplifiers
reduced as the gate is made negative. The characteristics of this type of MOSFET are
shown in Figure 1.20, and its method of construction is shown in Figure 1.21. In this,
the device fabrication has been modified so that there is a thin conductive N-type layer
connecting the source to the drain, underneath the gate electrode. When a positive
voltage is applied between the drain and the source a current will flow, which will be
reduced as the gate voltage is made more negative. In this respect the N-channel
depletion MOSFET behaves in a manner which is almost identical to the thermionic
triode, but without the valve's problems of microphony, fragility and loss of emission
during use.
10
-4
-8
--2
-1
0
1
2
Figure 1.20
Conduction characteristics of N-channel depletion MOSFET
+ ~l~lrWt.
.
)
Pp.
.
Figure 1.21
Depletion type MOSFET
A fundamental problem with the MOSFET, of whatever type, is that the channel will
usually be long, and the channel conductivity will be low. In low power applications
these characteristics will not be particularly inconvenient, but in higher current or
higher power usage they would be undesirable. Two methods are employed to lessen
these difficulties, firstly to construct the MOSFET so that the channels are vertical,
when the normal fabrication methods can make them short, and secondly to arrange
the layout of the device so that there are a multiplicity of channels operating in
parallel.
This method of construction allows very high values of mutual conductance to be
obtained: 2-5S/A for a vertically diffused power MOSFET, as compared with 2-10mS
Active Components
23
for a small-signal lateral device, having an ID of, say, 5mA. Similarly, while channel
resistances of 500-3k~ are normal for a small-signal MOSFET, on resistances (called
RDS on) of a fraction of an ohm are common for power devices. The penalty incurred
by this multiple-channel construction is that the gate-source capacitance is very high
- of the order of 1-2nF for an N-channel power MOSFET, and somewhat higher than
this for an equivalent P-channel device. Because of the method of construction, the
gate-drain capacitance will be smaller than this.
Because the electrostatic induction of a charge in the channel in response to the
presence of an electrical potential on the gate is an almost instantaneous effect,
MOSFETs have an excellent HF response, and this makes them prone to very high
frequency oscillation (which may be too high to be seen on the monitor oscilloscope)
if the physical layout of the circuit in which they are used is poorly arranged. D-MOS
and T-MOS devices normally have a somewhat lower F T than V-MOS or U-MOS
forms, and the avoidance of parasitic oscillation is consequently rather easier to
achieve in the D or T types.
Because of their very high values of F T, it is easier to obtain low values of harmonic
distortion and phase error by the use of overall negative feedback in power MOSFET
based audio amplifiers than in similar circuits using bipolar junction transistors in their
output stages. Also, in practice, power MOSFETs are more robust than similar bipolar
junction transistors. These advantages may justify the use of the rather more costly
power MOSFETs in high quality amplifier designs.
Since in RF amplifier use, the relatively high drain-gate capacitance of the MOSFET
would lead to the same type of problem that exists in the triode valve, a solid state
equivalent of the screened-grid valve, called a dual-gate MOSFET, is made using the
type of construction shown in Figure 1.22. These transistors are suitable for use as RF
amplifiers, with the drain-gate #1 capacitance reduced to about 0.0 l pF.
Source + Substrate G1
G,,
SiO-,
Drain
p
p+
"'
Figure 1.22
Lateral dual gate N-channel depletion MOSFET
Noise levels
Because junction transistors are fundamentally low input impedance devices, they can
also offer very low levels of thermal noise, in good quality components in
well-designed circuitry. Allowing for their higher input impedances, junction FETs
are also available in very low noise versions. Small signal MOSFETs, however, tend
to be relatively noisy, because of the mechanism by which the drain current is induced.
This is particularly true for P-channel MOSFETs because the induced current carrying
24
Valve and Transistor Audio Amplifiers
zone is composed of holes, whose action in tumbling over one another when carrying
current leads to higher values of flicker and shot noise than in the N-channel
versions 4.
Integrated circuits (ICs)
These are multiple-component devices which can be made by any type, or
combination of types, of semiconductor technology. In these, a range of active devices
are combined with other passive components, within a single package, to produce a
complete circuit module. In general these will only require the provision of input and
output connections, and either a single, or a pair of, power supply rails, to carry out
some specific electronic function. These were originally manufactured to carry out
various logic functions in which the very small physical sizes of the devices would
allow both high operating speeds and a large number of internally interconnected
gates. However, this has become the area of the most rapid growth in the whole of the
electronic component field, and ICs are now made for an enormous variety of
applications, from digital microprocessors to low-noise, high-gain linear operational
amplifiers (op amps).
There are a number of incentives to the audio circuit designer to use linear ICs, such
as op. amps, wherever their use would be satisfactory or appropriate. These are their
high reliability (an IC containing a hundred transistors and a score of resistors or
capacitors will be just as reliable, in general, as any single component of that type, and
much more reliable than the same arrangement built up from individual resistors and
transistors), their small size and their relatively low cost. In addition, since IC
manufacture is an exceedingly competitive business, the internal circuitry used in the
ICs will reflect the skills of some of the most gifted engineers in the field. In particular,
op amps are now capable of performances, as general purpose audio gain blocks,
which can hardly be matched, let alone bettered, by discrete component layouts.
Further reading
Since the principal purpose of this book is to explore the circuit designs employed in
audio amplifiers, both valve based and solid-state, I have tried to restrict my analysis
of semiconductor behaviour to that needed by the reader to understand what is going
on. If a more comprehensive explanation of the function and construction of solidstate devices is wanted, I would recommend the reader to read Chapters 4 and 5 of my
book The Art of Linear Electronics (Butterworth-Heinemann, 1993,).
CHAPTER 2
PASSIVE COMPONENTS
Inductors and Transformers
All conductors have inductance, and the longer the conductor, the greater this will be.
Although in the frequency range of interest in audio the amount of inadvertent
inductance due to things like connecting leads are not likely to be significant, they may
lead to occasional, unexpected, and generally unwanted, high frequency phenomenasuch as HF parasitic oscillation - which can spoil amplifier performance.
If a conductor is wound into the form of a coil, its inductance will be increased, and if
this coil is wound around a core of some ferromagnetic material its inductance will be
increased still further, in proportion to the permeability (p) of the core material.
If two such coils are placed in proximity to one another, so that their magnetic fields
will interact, then any change in the current flowing through one of them will cause a
voltage to be induced in the other. In general, the shape of the core of a transformer
will be chosen to give the highest practicable amount of electromagnetic coupling
between the windings, a process which is helped by the use of high permeability core
materials.
Transformers have the useful property of converting a small current flow, at a high
impedance, through one of the windings, into a larger current at a lower impedance,
through another winding, and vice versa. Also, because the conducting coils which
form the windings on a transformer can be electrically isolated from one another, so
far as static DC potentials are concerned, they allow signals or other AC voltages and
currents to be transferred from one circuit to another, even when these circuits operate
at widely different DC voltage levels.
Mains transformers are used in every piece of audio equipment, except those powered
from batteries, to generate the DC power rails used to operate it, and output
transformers are still used in every valve operated audio amplifier to couple the high
impedance level of the valve output stages to the relatively low impedance levels of
the loudspeaker circuit. In all of these applications, the problems are the same, and
require the same measures to reduce their unwanted effects.
26
Valve and Transistor Audio Amplifiers
Problems with transformers
Ideally, the changes in magnetic flux, which occur when there is a change in the
current flowing through one winding of a transformer, should interact completely with
the other windings on that transformer, and not at all with any other external circuits.
The use of a ferromagnetic core material - the term ferromagnetic means a type of
material which will increase the inductance of a coil in which it is placed- will help
to confine the magnetic flux within the transformer, and increase the effectiveness of
coupling from one winding to another. The higher the magnetic permeability of the
core material and the shorter the magnetic path in its core, the better this will be done.
These materials will have a relationship between magnetic flux and applied
magnetising field, known as the B-H curve, of the kind shown in Figure 2.1, and the
difference (X-Y) between the paths of the magnetisation and demagnetisation curves
shown in this graph indicates the extent of residual magnetisation of the core, and is
known as the magnetic hysteresis of the material. The value of the magnetic flux (B)
for a given magnetising field (H) is a measure of the permeability of the core material.
For comparison, the B-H curve for an air (or vacuum) cored system is also shown in
Figure 2.1.
+e
IRON
AIR
-H
+H
-B
Figure 2.1
Magnetic hysteresis curve
There are two broad categories of ferromagnetic material, those which have a high
value of magnetic retentivity, such as cobalt-steel, known as permanent or hard
magnetic materials, and those of low retentivity or soft magnetic materials, such as
iron with low carbon content.
Passive Components
27
For iron cored inductors or transformers it is essential to use only those materials
which have very low levels of magnetic retentivity, or otherwise the hysteresis loss
which will accompany every flux reversal in the core magnetisation will result in an
absorption of input energy which will be released as heat. Also, even in low retentivity
materials, the decay of the magnetic flux, on removal of the magnetic field, is not
instantaneous, so there will always be an increase in core losses at higher signal
frequencies.
The final major source of energy loss is that due to eddy current flow within the core.
This arises because the core will generally be made of an electrically conductive
metal, such as silicon steel (which has a permeability some 8000 times greater than
air), and any electrically conductive paths in this will look like the short-circuited turns
of a series of secondary windings. This eddy current loss can be made smaller by
constructing the core from a stack of thin sheet stampings, coated with some insulating
material on one or both faces, known as laminations. These can then be arranged to
divide the core block at fight-angles to the direction of the transformer windings and
of the current flow within them. Thin laminations of high permeability core material
give better transformer performance and lower losses, but they are expensive and
fiddly to assemble within the former on which the coils have been wound. Like
hysteresis losses, eddy current losses increase rapidly with operating frequency.
In the case of eddy current losses, T.H. Collins, Analog Electronics Handbook, pp.
292-293 (Prentice Hall, 1989), quotes the relationship
Pe = af2Bm2dz
where Pe is the power loss, a is a constant, f is the operating frequency, B m is the
maximum core flux density, and d is the thickness of the laminations.
Toroidal and 'C' core constructions
These types of core are shown in Figure 2.2, and allow the practical use of very thin
lamination material. In the manufacture of a C core, shown in Figure 2.2a, the core
material is wound into the required shape, and then solidly locked in this shape by
impregnation with epoxy resin, before being sawn into two sections to allow the
windings, on a suitable former, to be set in place. If the sawn faces are ground fiat, at
the correct angle, the two transformer core sections can be reassembled, and clamped
in place, with only a very small residual gap at their junction. In the case of the toroidal
construction shown in Figure 2.2b, the core is first wound into a ring form, and
impregnated, and the windings are then wound, in situ, by a suitably designed coil
winding machine. Both of these constructions allow low core losses, low external
radiation fields and low levels of unwanted currents induced in the secondary
windings by extemal sources.
Toroidal types are widely used as the mains power transformers in high quality audio
amplifiers because their low extemal 50/60Hz field greatly lessens the extent to which
hum will be induced in nearby low signal level circuitry. Their high electromagnetic
28
Valve and Transistor Audio Amplifiers
efficiency also provides proportionately higher power outputs, and lower winding
(copper) losses than the older (but less expensive) designs based on square stacks of
laminations.
b Tocoidalwinding
m"C" Ca:~e
Figure 2,2
Waveform distortion in coupling transformers
This arises both because of the non-linearity of the core magnetisation curve, and, in
amplifiers, because of core saturation and input overload due to inadequate primary
inductance at low frequencies. At high frequencies, the difficulty in specifying the
transformer behaviour and its transformation ratio becomes much greater, because of
the presence of parallel and inter-winding capacitances and inductances, as the
transformer equivalent circuit shown in Figure 2.3 will make clear. In this diagram, Lp
and L s are the primary and secondary winding inductances, LL,P and LL,s are the
primary and secondary leakage inductances, Rp and R s are the primary and secondary
winding resistances and Cp and C s are the internal capacitances of the primary and
secondary windings.
The inductance of any coil is determined by a number of factors, as shown in the
equation
L = kN2pa/1
where L is the inductance, k is a constant approximately equal to 8.3 " 1 0 8 when
dimensions are in centimetres, la is the permeability of the core material, N is the
number of turns of the winding, a is the cross-sectional area of the core and 1 is the
magnetic path length. By far the easiest way of increasing the inductance of any coil
Rp
Ll'p
R'p
Lis
Cp
L
Ls
Rs
c~
Figure 2.3
Spurious electrical components within normal two-winding transformer
Passive Components
29
is by using a ferromagnetic core material having a high permeability, such as
Permalloy (la = 20, 0 0 0 - 100, 000) or Mumetal ( p - 30, 0 0 0 - 150, 000), but all
ferromagnetic core materials cause waveform distortion, and their permeability
decreases with increasing operating frequency.
A dramatic demonstration of the problems caused by non-linearity in the 'B-H'
characteristics of ferromagnetic materials can be given by placing a ferrite bead over
the output lead between a high quality audio amplifier and its load. The immediate
effect of this is that the distortion in the amplifier output will be greatly increased and
changed from a smooth, low-order harmonic type to a spiky waveform rich in highorder harmonic components - distortion which disappears again if the ferrite bead is
removed.
To avoid the distortion caused by the core, in those applications where relatively low
values of inductance are required, as, for example, in the inductors and transformers
used in low impedance loudspeaker crossover networks, air-cored coils are invariably
employed, in spite of their greater physical size.
For good audio performance, in transformer coupled audio amplifiers, it is necessary
to employ negative feedback (NFB) in the amplifier design, and, in view of the
distortion which can arise in the loudspeaker coupling transformer, it is essential that
this component shall also be included in the NFB loop. For this to be practicable
without causing NFB loop instability, the low frequency performance of the
transformer- principally determined by its primary inductance- and its high
frequency characteristics- mainly resulting from the primary/secondary leakage
inductance and the inter-winding capacitances- must also be controlled within limits
set by the gain/phase relationships of the remainder of the amplifier.
An intrinsic difficulty in the use of any ferromagnetic material for the core of an
inductor or transformer is that the permeability of the core material, and hence the
inductance of the coil, is dependent on the magnetic flux density generated by the
current flowing in the windings. This leads to the type of relationship between core
inductance and AC excitation voltage shown in Figure 2.4. To ensure that the
transformer performance is equally good at low signal levels as at medium levels, this
Petrneabililv
|m)
IK
log
Figure 2.4
. . . . .
10
l
100
I
lk
I
1Ok
~
Ru~de~V
~(B)
Typical relationship between permeability and flux density in silicon steel core
material
30
Valve and Transistor Audio Amplifiers
minimum field inductance, known as the incremental inductance, should be the value
specified. The rapid decrease in core inductance at high excitation levels, also shown
in Figure 2.4, is a frequent cause of high power waveform distortion.
Where an inductor was to be used as a unit of impedance, as for example as a
smoothing choke in a power supply circuit, where its high AF impedance could be
used, in conjunction with a large value output capacitor, to assist in smoothing out the
residual AC tipple from a simple rectifier/reservoir capacitor power supply, it was
common practice to leave a deliberate air gap in the core to lessen the inductance loss
due to core saturation by the DC current flow. This was also normal practice if such
an inductor was used as a swinging choke, connected between the rectifier and the
output of a choke input type of power supply.
An excellent analysis of the design problems which the output transformer introduces,
and a practical approach to the solution of these, was given by D. T. N. Williamson
(Wireless WorM,April 1947, pp. 118-121) in his article describing a design for a high
quality audio amplifier. In his competitive amplifier system, P. J. Baxandall (Wireless
WorM, January 1948, pp. 2-6) described a method (mainly due to C. G. Mayo of the
BBC Research Dept.) which simplified the problems of overall NFB by taking the
feedback voltage from a tertiary transformer winding. In practice, however, this
technique gives less good results than if the NFB is derived from the LS output
terminals, and segmented and interleaved windings are used, as described by
Williamson, to reduce the inter-winding leakage inductance.
All in all, however, the output transformer of a valve audio amplifier is an awkward
component to live with, and it could be argued that one of the major advantages
inherent in low output impedance solid-state designs is that an output transformer is
unnecessary. In the past, AF chokes were occasionally used as the anode loads of
amplifier stages. These components would introduce similar problems, and are very
rarely found in any contemporary designs.
Capacitors
These components can be divided into two main categories - polar and non-polarand each of these capacitor types has its own set of advantages and snags. Polar
capacitors are those like aluminium and tantalum electrolytics, together with some
exotic ceramic-dielectric components, which are sensitive to the polarity of any direct
voltage applied across them, whereas the non-polar capacitors are not. All capacitors,
in their most basic form, consist of a pair of electrically conducting plates, or
electrodes, separated by a gap filled with air or some other material, known as the
dielectric. The differences between capacitor types relate to the nature and form of the
dielectric material and the type of electrode used. In electrolytic capacitors, very large
values of capacitance can be provided in relatively small package dimensions, but
their actual capacitance will seldom be specified with any degree of accuracy.
Because a current must flow into, or out of, a capacitor when it is charged or
discharged, if the external circuit resistance is sufficiently high that no significant
Passive Components
31
current flow can occur in the external circuit, and consequently no change in the state
of charge of the capacitor, then any change in the potential on one electrode of the
capacitor must cause an identical potential change on the other. This provides a very
useful method of transferring a voltage change from one circuit to another, when a
direct connection cannot be made between them because they are not at the same DC
potential. In this sense, it acts in a similar manner to an inter-stage coupling
transformer, but with very much less signal distortion or sensitivity to the frequency
of the signal.
By the same argument, if there is little current flow into the capacitor, then the
potential across it will not change to any great extent. This is the basis of the use of a
capacitor for AF bypass, or smoothing, or decoupling applications. The effectiveness
of a capacitor in these uses becomes greater if the turnover frequency (f0) due to the
value of the capacitor and the resistance of the external circuit (Rext) is made lower.
This frequency can be found from the relationship:
f0 = 1/(2/tCRext)
Electrolytic capacitors
The capacitance of a capacitor is related to the thickness (d) of the (insulating)
dielectric layer, and the relative dielectric constant (k) of that layer (as compared with
a vacuum) by the equation:
C = MkA/d
where M is some constant depending on the type of construction, and A is the area of
the electrodes. The dielectric constant is a measure, analogous to that of the
permeability of a magnetic core material, of the amount by which the capacitance of a
capacitor will be increased by the use of that particular insulating material instead of
a vacuum (or, for all practical purposes, air).
For a given electrode area the capacitance of the capacitor can be increased by making
the dielectric layer thinner, and by using a dielectric with a higher value of k. The
problem with making the dielectric layer thinner is that it may suffer an electrical
breakdown in use, when the capacitor will cease to be usable, unless some method
exists by which it can heal itself. Using a higher dielectric constant for the insulating
layer between the plates is an option which depends mainly on what is available in a
suitable physical form.
The electrolytic capacitor construction attempts to solve all of these problems by using
an electrochemically formed layer of insulation, such as aluminium oxide on an
aluminium foil electrode, or a tantalum oxide layer on a tantalum foil substrate. Not
only do these oxide layers have a high dielectric constant (k = 9 for A1203, k = 26 for
Ta2Os), but since it is arranged that some electrolyte will still be present within the
capacitor body, electrolytic processes can continue, so long as there is any voltage (of
the correct polarity) between the electrodes. By this mechanism, any points at which
electrical breakdown has occurred will be healed automatically.
32
Valve and Transistor Audio Amplifiers
A further advantage of the electrolytic is that when the oxide film is sufficiently thick
to prevent further current flow the film growth stops. This means that the dielectric
thickness is never greater than is needed to arrest the flow of current at the voltage at
which it was formed. Together with the high-k dielectric layer, this property means
that such capacitors will offer relatively large values of capacitance in very small
physical sizes. For example, a solid dielectric, epoxy resin encapsulated tantalum bead
capacitor, having a capacitance of 100~tF at a working voltage of 10V, can be
physically only the size of a pea.
Unfortunately, the oxide insulating film in an electrolytic capacitor will tend to
deteriorate during storage, or in use in applications where there is no voltage normally
present between the plates, as a result of which the leakage resistance between the
plates will decrease. If a voltage is then applied, a relatively large leakage current will
flow, which will decrease as the dielectric layer slowly heals its weak points. Tantalum
electrolytic capacitors are superior to aluminium based ones in this respect, and may
even tolerate short duration DC polarity reversals, up to one or two volts.
Aluminium electrolytic capacitors dominate the larger capacitance and higher working
voltage capacitor styles, with values in the range l~tF-470, 000~tF at working voltages
in the range of 4V-600V DC, though, of course, the larger capacitance values will
usually only be available at the lower working voltages. Unfortunately, leakage
currents are always present in electrolytic capacitors, and these currents are noisy. This
limits their use in inter-stage coupling applications, though leakage currents are
usually very low in the tantalum electrolytics. Aluminium bead capacitor types have
been introduced recently which are similar in physical size and capacitance values to
their more expensive tantalum based equivalents, but they are still not as good in their
tolerance of polarity reversals.
A further problem in electrolytic capacitors is that there is a significant amount of
resistance in series with the capacitor element, either due to the narrow internal strips
of foil which are used to make connection to the wound foil electrodes, in a spiral
wound capacitor, or due to the relatively poor conductivity of the electrolyte itself.
This inadvertent resistive element is of importance in capacitors used for power supply
applications, and is known as the equivalent series resistance or ESR, and should be
as low as possible in this use.
The other measure of the lack of perfection of a capacitor, its dielectric loss, also
known as dielectric hysteresis loss (roughly analogous to the core losses in a
transformer), is due to the residual hysteresis losses in the dielectric when the polarity
is reversed. Since this loss occurs each time the applied voltage is reversed the energy
loss is roughly proportional to signal frequency, and, in an equivalent circuit, of the
kind shown in Figure 2.5, would be represented by a further resistance in series with
the capacitance.
In Figure 2.5b, C is the capacitance, and R k is the dielectric loss, both of which will
depend somewhat on operating frequency, voltage and temperature, R s is the
equivalent series resistance, due to internal connections, L s is the series inductance, R E
Passive Components
33
is the leakage resistance of the component, and the CC terms refer to various internal
stray capacitances. The equivalent circuits of Figures 2.5c and 2.5d relate to
electrolytic and high-k ceramic capacitors, which have further problems of reverse
conduction and dielectric hysteresis, shown respectively as D and E e in the diagrams.
Although each of these spurious effects will be important in some circumstances, most
of them can be ignored for most of the time.
o
II
C
o
8
Rs
l
ilcc3
I
b
,,C
Rk
0
c
RL
R~
Ls
d
Figure 2.5
Spurious c o m p o n e n t s inherent in capacitors
If there is, in use, any leakage current between the capacitor electrodes, as there will
always be in electrolytic capacitors, this will appear as a resistance in parallel with the
capacitor in its electrical equivalent circuit, and will increase its effective equivalent
dielectric loss. This is normally measured as the tangent of the phase angle (8) between
the reactive and resistive components of an alternating current flow.
Values for tan 8, at lkHz, range from 0.02 for an electrolytic, down to 0.00008 for an
electrical grade polypropylene film. Losses increase with frequency, but, except in the
case of electrolytics, will not alter much throughout the normal audio frequency range.
Non-polar
capacitors
A variety of styles and dielectrics are used in these, mainly dependent on the working
voltages or capacitance values required. Where small, stable, but precisely known
values of capacitance are necessary, capacitors based on a stack of metallised sheets
34
Valve and Transistor Audio Amplifiers
of mica or on a Swiss roll type of construction using aluminium foil interleaved with
polystyrene film are preferred. These have low losses and capacitance values which
are stable with time.
Where very low values of series inductance are essential, as, for example, in RF
bypass applications, metallised ceramic dielectric types are preferred. These can offer
either large but imprecise capacitance values in a small size, or very small but
accurately defined values. All ceramic capacitors have capacitance values which are
dependent on temperature, usually specified by descriptions such as N750 or P100, or
simply high k (always N type), where the N or the P refer to their negative or positive
temperature coefficient of capacitance, and the number refers to this temperature
coefficient, in parts per million, per degree centigrade.
By far the most common types of non-polar capacitor are those made from a spiral
wound sandwich of an insulating thermoplastic film dielectric and a high conductivity
metal foil, usually made from high purity aluminium. A wide range of films have been
used for this purpose, of which the most commonly found are polycarbonate,
polyester, polypropylene, polysulphone or PTFE. The last two of these are mainly
found in high temperature and military applications, where their high cost is
acceptable. All wound film/foil capacitors will have some effective series inductance,
because of their construction, as indicated in Figure 2.5b. This is not usually an
important factor in the frequency range typical in audio applications, though some
manufacturers squash their wound structure into a more slab-like form, before
encapsulation, to reduce this inductance.
Self-healing properties can be built into capacitors using thin thermoplastic insulating
materials by using a vacuum metallised layer deposited on the surface of the dielectric
film, instead of a spiral length of metal foil. Then, in the event of an electrical puncture
occurring in the film dielectric, the resulting spark discharge will burn away the
metallised layer around the point of breakdown in the manner shown in Figure 2.6, and
the short-circuit between the electrodes will then be cleared. This type of capacitor
construction allows much thinner dielectric films to be used with safety, and
consequently relatively high values of capacitance in a small package size, but,
because of the thinness of the conductive layer, such capacitors will always have a
high ESR, and a relatively high value of tan 6.
Figure 2.6
Effect of self-healing of metallized film capacitor
Passive Components
35
Waxed paper/foil capacitors were the mainstay of valve amplifier circuitry, but they
always suffered from noisy leakage currents due to the ingress of moisture. Happily,
these capacitors are now almost unknown in small-signal circuitry.
In audio amplifier applications, the choice will mainly lie between polystyrene
(k = 2.5), polycarbonate (k = 2.8-3), polyester (k = 2.8-3.2) and polypropylene
(k = 2.2) film dielectric metal foil components, depending on the requirements of the
circuit application. Of these, polystyrene types will be used where low capacitance
components, having stable and accurate capacitance values, are n e e d e d - as in filter
circuitry or frequency response shaping networks. Polypropylene or polycarbonate
capacitors would be the component of choice where it is to be used in the signal line,
and low-cost polyester capacitors for other, non-critical applications.
Dielectric r e m a n e n c e
In certain dielectric materials the electrical dipoles within the dielectric, whose change
in orientation on the application of an electric field is the major source of the dielectric
constant of that material, can remain locked in their orientation even after the applied
field has been removed, giving rise to an effective stored charge. In this respect, their
behaviour is analogous to the persistence of orientation of the magnetic dipoles in a
permanent magnet, and this effect in an insulating film is called electret formation, and
is deliberately sought to provide materials for use in things such as electret
microphones or headphones. Electrets occur more readily in materials having a high
degree of rigidity in their structure, and are thus more readily formed in materials
having an essentially crystalline structure such as biaxially oriented polyester and
polypropylene films than in essentially limp and amorphous solution-cast films like
those of polystyrene or polycarbonate. The ill effects, if any, of such stored charges in
the audio field are not known, but their possible existence should be recognised.
A related point concems the observations of those people who specialise in the
assessment of the quality of audio systems from judgements formed during personal
listening trials, and who have, as a result of these trials, declared the superiority of one
type of component over another in respect of its sound quality. A particular instance
of this kind concerns polypropylene film/foil capacitors, which are alleged to 'sound
better' in audio systems than any other comparable type.
As it happens, I was employed for many years in the research laboratories of a largescale manufacturer of thermoplastic films, whose product range included several types
of polypropylene film, intended for use in both packaging and electrical applications.
Part of my duty was to provide technical support to the manufacturers of capacitors
using our electrical grade films, and we had a very well-equipped laboratory for
testing films, both in sheet form, and as wound capacitors. During my work, I visited
many capacitor manufacturing companies, and tested their capacitors for such things
as dielectric loss, breakdown voltage and electrical remanence (stored charge).
Part of my employer's product range was a biaxially oriented polypropylene film,
made from very high purity electrical grade polymer granules on machines which were
36
Valve and Transistor Audio Amplifiers
kept scrupulously clean and free from potentially deleterious contaminants. This film
was, of course, more expensive than the low purity, and relatively poor electrical
quality, packaging grade films, made on machines principally designed to provide a
high film output. The capacitor manufacturers noted this difference, but often used the
cheapest film they could get, either from my own company, or from competitive
suppliers with whose products I was familiar, because they said their own customers
could not tell the difference, and mainly bought those capacitors which were cheapest.
If there were significant differences in audio sound quality due to electrical
differences, these differences would, I thought, have been exposed by those who made
the pursuit of this quality their business, and would have observed that one make of
polypropylene capacitor sounded better than another.
Resistors
Like capacitors, these are made in a range of types from a variety of materials, and
have qualities which differ somewhat one from another.
Fixed resistors
For low power applications, where small size is generally sought, the most common
construction is that of a small ceramic rod, having a suitable coefficient of thermal
expansion, coated with a film of carbon, or metal, or tin oxide, or some other resistive
glaze. The resistivity and the thickness of the deposited layer, applied as the first stage
of manufacture, is then chosen to give a somewhat lower resistance to the composite
r o d - as measured between conductive metal caps attached at either end of the r o d than the target value, and a spiral groove is then cut through the resistive coating
material, to lengthen the surface conductive path and increase the resistance value to
the required figure. Connecting wires are then attached to the end caps, the resistor is
dipped in some tough protective lacquer, and finally, the resistor value, and the
tolerance (the manufacturers' permitted margin of error in the actual resistance), is
printed on the body, or, alternatively, these parameters may be shown by the use of an
internationally agreed colour code, as shown in Table 2.1.
For example, a 47k.Q, 5% tolerance resistor would be coded with the sequence of
colour bands, starting at one end of the body, of yellow, violet, orange, gold, where
the third band (orange) indicated the number of zeros (3) following the coded number
47, and the band of gold indicated a +5% tolerance value. Where higher accuracies are
required the number of colour bands is increased, so that a 47k5f2 (47, 500f~), 1%
tolerance resistor would be coded with yellow, violet, green, red and brown bands, to
indicate that there would be two zeros following the number 475, and the final brown
In principle, since the dielectric loss factor o f a capacitor is frequency dependent, and in the
case o f an electrolytic, also dependent on the applied voltage, it would seem probable that
capacitors could introduce waveform distortion into any audio line which contained them.
However, I have made many measurements o f this kind o f circuit, and my conclusions are that
the distortion levels due to this effect are too small to be easily identifiable by conventional test
procedures. Unfortunately, not finding any measurable defects does not guarantee that such
performance shortcomings,do not give rise to effects which may be audible to the perceptive
listener.
Passive Components
37
band would denote the i 1% tolerance specification. (This colour code system has also
been used to indicate the value of capacitors, measured in pF, especially radial lead
polyester block types.)
Table 2.1 The resistor colour code system
Colour
Black
Brown
Red
Orange
Yellow
Green
Blue
Violet
Grey
White
Gold
Silver
No colour band
Number
0
1
2
3
4
5
6
7
8
9
(" 0.1)
(" 0.01)
Tolerance
1%
2%
5%
10%
20%
Metal film resistors are available at dissipation ratings from 0.125 to 0.4 watts, while
carbon film types cover the range 0.125-2 watts. For higher power dissipations, up to,
say, 200W, wirewound resistors are available in a variety of forms.
Because semiconductor circuitry is generally associated with low voltage, low
dissipation devices, it is often forgotten that there will be a maximum voltage rating
as well as a maximum dissipation figure for a resistor, typically 200V for a 0.125W
metal film resistor, and up to 700V for a 2W carbon film one.
All resistors, even wirewound ones, are prone to some irreversible change in their
resistance value if their permitted heat dissipations are exceeded. This was a particular
problem with the wax impregnated carbon-composition rod types (which were
basically fired clay rods containing a proportion of graphite in their body), whose
actual value would not be known, until after the firing. This value would then be
moderately permanent, unless of course they were allowed to get hot in use, when they
would change in their value once more. Luckily, this type of resistor- widely used in
vintage audio systems - is no longer made, and only found in museum pieces.
All modem fixed value resistors from reputable manufacturers will have resistance
values which are very close to their marked figures, though these can be affected by
heat, so it is generally prudent to avoid heating the body excessively when soldering,
or to make solder joints too close to the resistor body.
38
Valve and Transistor Audio Amplifiers
,,
Noise
All circuits suffer from thermal or Johnson noise, proportional to the circuit resistance,
its temperature and the effective measurement bandwidth. In addition, there will be a
certain amount of excess or 1/f noise, as a result of the physical imperfections in the
manufacture of the constituent components. There are small differences in this excess
noise between the various types of modem resistors but these are usually insignificant,
except in very low signal level, low-frequency circuitry. For the purist tin oxide film,
metal glaze or wirewound types with welded end leads are the best. For low powers,
metal film resistors are the next best choice.
Inductance
Most film type resistors are virtually non-inductive. Wirewound types are inductive,
unless otherwise specified, but this is not likely to be important except in the power
output stages of audio amplifiers, or in wirewound variable resistors.
Potentiometers and variable resistors
It is a common requirement that it should be possible to alter the resistance between
two points in a circuit, or to select the proportion of a voltage occurring between
specific points. For this purpose a range of three-terminal potentiometers are made,
using many of the techniques used in the manufacture of fixed-value resistors, but with
the provision of a sliding contact which allows a connection to be made to the resistor
body at some chosen point along its length.
Predictably, the problem arises that any sliding contact will be less satisfactory than a
fixed one in respect of its noise component or its stability of resistance value. In high
quality systems the best quality potentiometers should be chosen, since the
imperfections in cheap and cheerful components can have a disproportionately large
effect on the system performance. In terms of quality, wirewound potentiometers are
probably the best, with cermet (resistive ceramic) or conductive plastic track types the
next best option. The mainstays of low cost potentiometers are those with carbon
composition tracks, with either graphite rod (better) or spoon-shaped metal (less good)
sliders as the means of making contact to the track.
There will always be some mechanical wear in the repeated use of any adjustable
component, usually made visible or audible by the slider contact becoming
intermittent, and electrically noisy in its track to slider connection, so the better quality
components should always be employed in those places such as amplifier gain controls
which are likely to experience the most use.
Potentiometers are also available in twin-gang types, to meet the needs of stereo
systems. The tracks of these will nearly always be of relatively low quality carbon
composition type, and there will often be noticeable differences in these between the
proportion of the resistance tapped off by the sliders in each of the two tracks. This is
even more conspicuously the case where each track of the twin-gang potentiometer is
required to have a logarithmic relationship between the tapped-off resistance and the
Passive Components
39
slider rotation. So, where a high standard of performance is sought, the style of
potentiometer is one of the areas where particular care should be taken in the choice
of components.
Switches and Electrical Contacts
These components are a matter of particular interest in audio amplifier design, because
the operation of any audio system will require the interconnection of many different
units, and the switching of many electrical signal paths within those units, and this can
open the way to a number of potential problems. In its simplest form, the difficulty is
that any conducting metallic surface which is exposed to the air will, in due course,
become coated with grease, dust, and other non-conducting contaminants. The surface
will also suffer from chemical degradation because of the effects of oxygen, sulphur
dioxide, and other corrosive gases in the atmosphere.
If little or no electrical current passes between such contacts there may not be enough
voltage across the contaminant layer to cause it to break down, so that an unsuspected
high or variable resistance may be introduced in the circuit. Higher contact currents
are obviously preferable from this point of view, but, on the other hand, chemical
changes to the conductor surface may form thin semiconducting inter-metallic layers
which are capable of passing quite high currents without disruption, but which will
distort the signal voltage waveform.
If the contacts are arranged to slide over one another, as in the case of a switch, the
wiping action will help to keep the contacting surfaces clean, but if there is much
current flow through these, electrical arcing may occur, which will erode the contact
surfaces. Overall, pure silver offers the best of the lower priced contact materials,
while, if cost is unimportant, gold-plated palladium/nickel alloy contacts offer the best
combination of performance with life expectancy.
Little can be done about such things as fuseholders in a loudspeaker circuit, which may
have to pass substantial currents, since the fuses themselves will be relatively crude,
low cost components, tin-plated at best. The only really satisfactory answer is to
choose some other type of loudspeaker protection, such as a high quality, inert gas
filled relay, with adequately sized, gold-plated contacts, or, alternatively, some purely
electronic means of protection.
Until comparatively recently, the interconnections between components in an audio
system were often regarded as being of minor importance. However, the present view
- which I think to be correct - is that the means of connection are things which should
be considered with equal care, in a high quality audio system, to any of the
components which they join. In particular, the tracks on the printed circuit boards, or
the extemal wiring, or the mechanical connectors which join those parts of the circuit
which can pass large currents, such as the leads to loudspeaker outputs, transformers,
capacitors or power supply rectifiers, should be of adequate gauge and of carefully
chosen route and type if signal impairment is to be avoided.
This Page Intentionally Left Blank
CHAPTER 3
VOLTAGE AMPLIFIER STAGES USING VALVES
The design of valve voltage amplifier stages is essentially a simple task by comparison
with much solid state linear circuit design, if only because there are not many circuit
options open to the designer of valve circuits. Valves are also relatively linear in their
input/output transfer characteristics, and therefore require less in the way of circuit
design elaboration to improve the circuit performance and reduce any residual
waveform distortion.
Circuit Configurations
For valve operated low frequency amplifier stages the only real choice is between
triodes and pentodes. Both of these valve types can be employed in one or other of the
five circuit arrangements shown in simplified form in Figure 3.1 - grounded cathode,
grounded grid, cascode, long-tailed pair and cathode follower, sometimes called
grounded anode. For simplicity, I have shown at this stage only the triode valve
versions of these circuits.
In the case of a normal (grounded cathode) triode voltage amplifier stage, of the kind
shown in Figure 3. l a, the major choices to be made are between the type of grid bias
system which is used and the type of anode load which is employed to convert the
changes in anode current, brought about by changes in the control grid voltage, into a
signal voltage output.
Grid Bias Systems
A graph showing the relationship between the anode current and the grid voltage in a
typical small-power triode amplifier valve (one half of an ECC83 double triode) is
shown in Figure 3.2. It is required, for the lowest possible amplitude distortion in the
output signal waveform, that the valve should be operated on the most linear part of
its Ia/Vg characteristic curve. This implies that the grid voltage will not be driven so
far negative, during the input voltage swing, that it runs into the curved region of this
characteristic, near I a cut-off, nor will it be allowed to become positive in relation to
the cathode voltage, when grid current would flow, and the grid would cease to offer
a high input impedance. An additional consideration is that the actual anode current
should be chosen- if there is, within the desired distortion criteria, a range of current
levels at which the valve could be operated equally well - to be the lowest current
level within this range. This should be done to minimise the waste of energy in
excessive anode dissipation, and to avoid any needless reduction of cathode life, a
thing which has an inverse relationship with anode current.
42
Valve and TransistorAudio Amplifiers
- +V0
~!
O
Output
VIB
Input
+
(out2)
-io
o
+V
put
bias
e _L-:
Vref
Input
O'--'- OV
~176
(b) Grounded grid
RL
RL -9V
~1~-~~+
r~~~____~V ef
put
I n ~
+ .$
(d) Long tailed pair
put
V1
Vl
Output
In~-""
+b4as
Input
ov
(a) Grounded cathode
T
(c) Cascode
RI~Q v
R1
-
'0
(e) Cathode follower
Figure 3.1
Valve gain stages
la (mA)
1/2 ECC83 Va. = 200v
Optimum bias voltage
/
4
Ii
1
-Vg i
-4
-3
Figure 3.2
Anode current vs. grid voltage
-2
-1
0
"~
+Vg
The static working voltage applied to the grid will normally be provided in one of
three ways, as shown in Figure 3.3. The simplest of these, illustrated in Figure 3.3a,
takes advantage of the fact that there will be random collisions between the electrons
in the 'space charge' surrounding the cathode and the control grid wires, even if the
control grid is slightly negative in respect to the cathode. If a grid resistor (R1) of 2
Voltage Amplifier Stages Using Valves
43
O+V
RL
Output
C1
/ ~ l V
+V
1
Input
Output
?
--
-
(a)
0 ov
_
C1
II
Input
0 ov
[
-O+v
(b)
O -ve Bias
0 output,
C1
Input
R1
T
00V
(c)
Figure 3.3
Valve biasing systems
megohms or greater is used, the grid current (perhaps 250pA) flowing through this
will cause the grid to acquire a small negative voltage (perhaps 0.5V) in respect to the
cathode, which will probably be adequate for a small-signal input stage. This
arrangement has a number of advantages in small-signal circuitry, in addition to its
simplicity, of which the major one is that it provides the highest gain figure for a given
anode load impedance. The low cathode circuit impedance of this layout also assists
in keeping 'hum' and noise to a low level.
This advantage is shared by the fixed bias arrangement shown in Figure 3.3b, in which
any convenient value of input resistor (R 1) can be used, provided that it is not so high
that grid-current biasing also occurs. This layout is invariably used in directly heated
(battery operated) valve circuitry, where all the cathodes (which will be directly heated
filaments) will be operated from a common supply, and is occasionally used in power
output stages because it will allow a slightly greater output power to be obtained for
44
Valve and Transistor Audio Amplifiers
the same distortion level than would be given by a similar output stage using cathode
bias. For example, Mullard quote an output from a push-pull pair of EL91 output
pentodes of 4 watts, at 3.3% THD, whereas 4.8 watts could be obtained, at the same
distortion figure, if fixed bias were used. A similar relationship in respect of the
maximum output power levels for fixed and cathode bias arrangements exists with the
KT66 and 6L6 output beam-tetrodes.
In practice, the circuit layout shown in Figure 3.3c is by far the most widely used
method of generating the required level of grid bias for an indirectly heated valve. In
this arrangement, a resistor (R3) is included in the cathode circuit so that the cathode
current will cause the cathode voltage to rise so that it is positive to the control g r i d which will be the same, in effect, as the grid being negative with reference to the
cathode.
If there is no bypass capacitor across R3 the presence of the cathode circuit resistor
will somewhat reduce the gain of the stage. This is due to the local negative feedback
caused by the appearance at the cathode of a small AF signal, which acts to reduce the
effective input signal voltage. Since the cathode resistor R3 is effectively in series with
the anode circuit resistance (R2 in parallel with Ra) the feedback factor (13) will be
I] = R3/(R2//R a)
(in this I have used the s y m b o l / / t o mean in parallel with).
If we take the case of the ECC83 triode referred to previously, at V a = 150V, then R a
= 47 000fL and the amplification factor (~t) will be 100. If we make R2 the same value
as R a, then the stage gain, without any cathode circuit feedback, called A ~ will be
given by the relationship:
Gain = ~tR2/(R2 + Ra) -- 100 47k/94k = A" - 50
However, if we make R3 a l k ~ resistor, then 13 will be
R3/23.5k - 0.042
The expression for gain with feedback (A') is
A "= A~
+ [3A~ = 23.8
which shows that the presence of an unbypassed cathode resistor will reduce the stage
gain to a significant extent. The normal technique employed to lessen this drawback is
to connect a capacitor (C2) in parallel with R3. At AC, the impedance of the capacitor
will be:
Z c = 1/(2rtfC)
where f is the operating frequency. For example, a capacitor of 220~tF will have an
Voltage Amplifier Stages Using Valves
45
impedance of 36~ at 20Hz, and the effect of this in parallel with the lk,Q cathode
circuit resistor will be to increase the stage gain to 46.5. This will still cause a small
reduction in the LF response, and, if this is important, a larger value could be chosen
for C2. At high audio frequencies the impedance of the bypass capacitor will be so low
that the effect of the cathode circuit components can be neglected.
A further disadvantage in the use of cathode bias arrangements in comparison with
fixed bias systems (where the cathode is tied to the 0V rail) is that the voltage
developed in the cathode circuit is effectively subtracted from the anode supply
voltage, and will reduce the possible undistorted anode voltage swing. This is a factor
which should be considered in valve amplifier output stage design.
Cathode Bias Resistor Calculations
If the chosen amplifying device is one half of a ECC83 double triode, the required
value of cathode bias resistor can be determined approximately by calculating the
value of anode load resistor across which the voltage drop will place the quiescent
anode voltage at about half the +V supply line potential. However, in order to
determine this one must first choose a level of anode current which will place the
operating point for the valve somewhere about the middle of the straight portion of the
Ia/Vg relationship shown by the manufacturers (2mA at-1.4V). If the +ve supply
voltage is 300V, and the anode load resistor is 75k.Q, the quiescent anode voltage will
be 150V, at this value of anode current. A cathode bias resistor of 700~ would then
be required to provide the r e q u i r e d - 1 . 4 V cathode bias. Unfortunately, the
characteristic Ia/Vg curve shown for the valve is that for a +200V anode voltage rather
than a + 150V one, as presumed in our calculations, and this lower anode voltage will
reduce the anode current flow, which will, in tum, require a slightly lower value of
both cathode bias and cathode bias resistor- say 620~ rather than 700s However,
the valve manufacturers will often quote a recommended value for the cathode bias
resistor for a given anode load resistor and supply voltage.
Anode Load Systems
Although the resistor/capacitor circuit shown in Figure 3.4a is by far the most
commonly used inter-stage coupling layout, in that it allows a wide gain bandwidth
and straightforward phase/frequency relationships, the circuit stage gain is still further
reduced in comparison with the simple model of Figure 3.3c because, assuming that
the impedance of C3 is small enough to be neglected, the anode load resistance seen
by V 1 is that of R a in parallel with both R2 and R4. It is also evident that there will be
a voltage drop across R2, which will reduce the anode current for a given level of grid
bias, the amplification factor (la) and the mutual conductance (gm) of the valve, since
all of these parameters are anode voltage dependent.
The use of an LF choke, L 1, as the anode circuit load, as shown in Figure 3.4b, offers
an increase in stage gain in comparison with the simple resistor coupled arrangement
because the DC winding resistance of the choke and the voltage drop across it will
probably be low enough to be neglected. Also, if the anode current of V 1 is modulated
so that it swings both below and above its quiescent level, the output voltage, as seen
46
Valve and Transistor Audio Amplifiers
at the anode of V 1, will swing both above and below the level of the +V rail, and, for
this reason, can probably give an output voltage swing rather more than double that of
the circuit layout shown in Figure 3.4a.
qV
R2
vl
Cl
J,
Input
Output
q,--O
I"' .C'Z/. i =
1
ov
0
(a)
)ut
Inp
(b)
Figure 3.4
Anode load systems
(c)
The major disadvantages of the choke/capacitor inter-stage coupling system are that
the phase/frequency characteristics of the choke impedance are very complex- due to
the characteristics of the core material, the leakage inductance of the windings, which
will either be in the form of segments or layers, and the inter-winding capacitances and these effects will impair both the distortion characteristics and the flatness of the
frequency response of the stage. This will limit the extent to which it is possible to use
overall negative feedback to improve the performance of a complete amplifier.
The use of an inter-stage transformer coupling, as shown in Figure 3.4c, suffers from
all the snags of choke/capacitor coupling, but will allow an even greater increase in
both stage gain and possible output voltage swing in comparison with Figure 3.4a.
Voltage Amplifier Stages Using Valves
47
This made it a very popular arrangement in the early years of electronic audio
amplifier designs in which it was sought to keep the number of valves used to as few
as practicable. Transformer designs of that period offered voltage step-up ratios from
1:2 to 1:7 (in general, the lower step-up ratios offered better audio performance) and
this could give, for example, a two valve amplifier design enough gain to drive a
loudspeaker from a piezo-electric gramophone pick-up input. A particular advantage
of an inter-stage coupling transformer was that, if the secondary winding was centretapped, the input stage could be made to drive a push-pull pair of output valves, which
offered much greater output stage efficiency and output power. This topic is examined
at greater detail in the chapter dealing with phase splitter arrangements.
Grounded Grid Stages
This type of circuit layout, shown in Figure 3.1 b, mainly found use as an RF amplifier
- in which it was effective up to very high frequencies - because the earthed control
grid would act as an electrostatic screen between the output and the input circuits,
which would avoid the problems due to unwanted signal feedback through the interelectrode capacitances.
The advantages of this type of stage are that its input/output transfer characteristics are
very linear when used with an input current source, or any other high impedance signal
source, and that, as already noted, there is almost complete isolation between the input
and output circuits. The main problem is that the input impedance (Rin) as seen at the
cathode, is very l o w - approximately 1/gm, for an impedance in 1~ if the gm is
measured in mA/V. This could be as low as 200~ for a valve with a mutual
conductance (gm) of 5mA/V. A further problem is that any current leakage between
the heater and cathode circuits will inject hum and noise into the input signal path.
Valves such as the E88CC were made especially to avoid this problem in grounded
grid and cascode layouts.
Since the anode (Ia) and cathode (Ik) currents will be substantially identical, the output
voltage (Vout) will be"
V out = IaR1 = IkR1
If the input impedance (1/gm) is low enough in comparison with the source resistance
(Rs) that it can be neglected, then the stage gain will be"
Vout
Ia R 1
Vin
Ik R s
= R1/Rs
In other words, the grounded grid stage will only provide more than unity gain if the
source impedance is less than the load impedance. The anode impedance (Ra) of the
grounded grid stage is very high, approximately PRin.
48
Valve and Transistor Audio Amplifiers
The Cascode Circuit
This circuit arrangement, shown in Figure 3.1c, has all of the advantages, such as
linearity and input/output circuit isolation, which are associated with the grounded
grid layout, but with the added benefit that the input impedance is very similar to that
of the conventional grounded cathode layout of Figure 3. l a. A small disadvantage of
the cascode layout is that an additional voltage supply to V 1B (shown in the diagram
as +V ref) is needed, and since this must allow V1A enough anode voltage (say 60V)
to operate satisfactorily (the V a of V1A will be typically Vref +l.5V) the supply
voltage available to V 1B will be reduced by this amount, with a consequent reduction
in the possible output voltage swing in comparison with the layout of Figure 3.1 a.
As is the case with the grounded grid amplifier, the anode resistance (Ra) of V 1B is
very high because its anode current must be the same as its cathode current, and this
is simply the anode current of VIA, which will be largely unaffected by the anode
voltage of V1B. Because of the passive operation of V1 B, the characteristics of the
cascode stage are determined by the operation of VIA, and that component of nonlinear distortion in a valve amplifier which is due to the effect of variations in anode
voltage on the anode current is very much reduced because the anode voltage of V1A
is clamped to a fixed level by the cathode follower action of V 1B"
As with the grounded grid layout, a valve type designed for cascode applications
should be employed, where possible, in this application to minimise the intrusions of
'hum' and noise from the heater supply line into the cathode circuit of V1B. With
modem technology, it would be easy to reduce the extent of this problem by operating
the heater circuit of V 1B from a stabilised DC supply line.
The Long-tailed Pair
The circuit layout shown in Figure 3.1d should perhaps be called a cathode coupled
amplifier, but the term long-tailed pair is so widely used that other descriptions seem
pedantic. As in the cascode layout, the amplifying valve operates as a grounded grid
stage, with its grid, in this case, being held at a fixed voltage by a low impedance DC
source (which could be the 0V rail if the tail resistor is taken to some suitable negative
supply line), and with the signal input being injected into its cathode circuit.
A practical disadvantage of this circuit is that the stage gain of this arrangement is
approximately half that of the simple grounded cathode layout of Figure 3.1a. This
comes about because both V 1 A and V 1B act as cathode followers in relation to the tail
resistor R1. While V1A seeks to transfer the input signal present at its grid to its
cathode rail, V 1B seeks to hold this cathode rail voltage at a fixed and constant voltage.
If both triodes are identical in their characteristics the result of these opposing
endeavours will be to produce a signal voltage change at their cathodes which is half
that of the input signal amplitude, at V1A grid.
A useful feature of this circuit arrangement is that, if the cathode tail resistor is
sufficiently high, and if both anode circuits have identical load resistors (for
Voltage Amplifier Stages Using Valves
49
simplicity, in Figure 3.1d, I have only shown one (R2) in V la's anode circuit) the
signal voltages developed at the two anodes will be very nearly equal in amplitude but
in phase opposition. This allows such a stage to be used as a phase splitter preceding
a push-pull output s t a g e - this offers a great improvement in performance in this
function in comparison with an inter-stage coupling transformer with a centre-tapped
secondary winding. It also allows the input to be directly connected to the grid of V 1A,
since the reference voltage at the grid of V1B can be adjusted to give the correct
balance between the two valves.
The Cathode Follower
This circuit layout is shown in Figure 3.1e, and normally gives a low impedance
output at a voltage gain which is slightly less than unity. The actual output voltage can
be calculated by treating the circuit as a simple amplifier with a gain A ~ and 100%
negative feedback (i.e. [3 = 1). In this mode, using half of an ECC83 double triode, the
gain with feedback (A') will be
A" = A~
+ I]A~ -- A~
+ A~
If A ~ = gR1/(R1 + Ra), and g = 100, R1 = 10k~, and R a -- 62k.Q, then
A ~ = 13.8 a n d A "= 0.93
and the output impedance will be approximately 1/gm -- 600f2.
An input DC bias voltage will normally be chosen so that an appropriate level of
cathode current will flow and will maintain the DC operating point of the valve at a
suitable position on its Ia/Vg curve. The main use of the cathode follower is to
transform an input signal at a high impedance into a virtually identical signal at a low
impedance, without the need for an iron cored transformer, which would introduce
unwanted distortion and phase/frequency errors.
+V
Output
Input
T
Figure 3,5
The H follower
50
Valve and Transistor Audio Amplifiers
The/J Follower
This circuit, shown in Figure 3.5, has been recently reintroduced as a way of getting
an increased stage gain from an amplifying valve (V1) by using a cathode follower
(V2) as an active anode load. The cathode follower circuit gives a low output
impedance, and as an active load it effectively increases the stage gain of V 1. The snag
is, as with all output load increasing systems, that the inter-electrode and other stray
capacitances associated with the grid and cathode circuit of V2 will then have an
increasingly important effect on the gain of the system, which will decrease as the
signal frequency is increased. Also, an upper limit to the available gain is imposed by
the shunting effect of the anode current impedance (Ra) of V 1. A small-signal pentode,
which will have a higher value of R a than a triode, would be better as the input valve,
V1.
Tetrodes, Beam-tetrodes and Pentodes
In the early days of valve radio receivers it was found that the triode valve was
unsatisfactory as an amplifier at high frequencies because the unavoidable internal
anode/grid capacitance of the valve would cause RF instability, particularly with
inductive input and output circuitry.
An engineering solution to this problem was found in the introduction of a fine mesh
grid, interposed between the control grid (G1) and the anode, connected to a low
impedance voltage source, and held at a positive potential with respect to the cathode.
This would act as an electrostatic screen between the anode and cathode and virtually
eliminate the anode/grid capacitance. Since this grid was positively charged it would
not significantly affect the cathode current of the valve.
The effect that this had upon the valve characteristics was to reduce the influence of
the anode voltage upon the anode current. This caused a great increase both in R a, to
perhaps several megohms, and also in the magnification factor (p), which could be as
high as 5000, though because this depends so much on other things it is not often
quoted by the manufacturers, and seldom used in stage gain calculations, for which the
relationship A ~ = gmR1 is normally employed rather than A ~ = ~tR1/(R1+ R a) Although
the tetrode, or screened grid valve, provided an effective answer to the problems of
instability in RF amplifier stages it was unsatisfactory for use as a small-signal audio
amplifier stage, except at low output signal voltage levels, because of the kink in the
tetrodes Ia/V a characteristics. This kink occurs for the following reason. In any valve
some secondary electrons will be emitted from the surface of the anode when high
velocity electrons (from the cathode) impinge on it, especially if its surface has
become contaminated with cathode metals. In a triode, this effect is unimportant
because there is nowhere that such electrons can go, apart from returning to the anode.
In a screened grid valve, however, these electrons will be attracted to the positively
charged screening grid, which will effectively reduce the electron flow to the anode,
and this will result in a downward dip in the anode current curve, as shown in Figure
3.6.
Voltage Amplifier Stages Using Valves
51
Vg2 = 120V
OV
i
IOOV
Va
!
200V
Figure 3.6
The 'tetrode kink'
Two solutions were found for this problem, of which the simplest and most widely
adopted was the introduction of an open-mesh suppressor grid between the anode and
the screening grid (G2). If this grid is connected to a point having a negative potential
with respect to the anode - normally the suppressor grid (G3) is connected, either
internally or externally, to the cathode - the decelerating potential between this
electrode and the anode will cause the secondary electrons to fall back upon the anode,
and the kink due to secondary emission from the anode disappears.
The second, more elegant and more expensive, solution to this problem was the socalled kinkless tetrode design, invented by the Marconi-Osram Valve Company. In
this arrangement, shown in Figure 3.7, a pair of beam-confining plates, connected to
the cathode, were interposed between the grids and the anode so that the electrostatic
field pattem due to their shape would discourage any electron flow back towards the
screening grid (G2). As a further improvement, the two sets of grid wires of the valve
(G 1 and G2) were aligned so that electrons from the cathode were formed into beams,
and those which had passed between the control grid wires would generally fly past
the screening grid wires as well, without collision.
The advantage of this structure was that it brought about a reduction in the current
flow to G2, by comparison with a pentode structure, and the G2 current is just wasted
energy, which contributes nothing to the design except heat. It also offered a lower
distortion than an equivalent output pentode, especially when G2 was connected to the
anode, to simulate an output power triode. However, beam tetrodes were more
expensive than pentodes, and although there were small-signal versions of these
valves, such as the KTW63 and the KTZ63, it was not generally thought that the extra
cost and complexity of this construction was justified, except for power output stages
where the KT66, KT77 and KT88 output beam tetrodes were the connoisseur' s choice.
Other output beam tetrodes were the 6V6, the 6L6 and the 807, all from the USA.
Beam tetrodes seem not to have been used in mainland Europe.
52
Valve and Transistor Audio Amplifiers
Figure 3.7
Construction of beam tetrode (courtesy of RCA)
As a small-signal amplifier stage the pentode offers a mixture of advantages and
drawbacks. On the credit side, it gives a much higher stage gain (say 200) in an
amplifier circuit of the type shown in Figure 3.8, in comparison with that of the triode
amplifier stage of the type shown in Figure 3.4a, for which a gain of 50 would be
typical. On the other hand the pentode valve gives a higher level of harmonic
distortion than an equivalent triode circuit, and it has a somewhat higher noise level though this can still be very low in a well-designed valve - than a triode because of
the existence of partition noise in the pentode. This defect arises because the
individual electrons which collectively make up the cathode current of the valve can
arrive at either the anode or G2 in a random manner, and this means that even without
an input signal the anode current will fluctuate about its mean level, an effect which is
heard as noise.
With suitable amendments to the circuit to provide the necessary G2 voltage, the
pentode can be used in any of the circuit layouts shown in Figures 3.1 and 3.4, though
unless a high stage gain is desirable a triode would be a more normal choice, if only
because small-signal triodes, such as the ECC83 or the 6SN7, are very readily
available as two valves having closely similar characteristics within a single envelope,
and this could save space on the chassis.
A final point to be considered in the use of a pentode amplifying valve is that the
screen grid (G2) current is modulated by the input signal voltage on the control grid
(G 1) in exactly the same manner as the anode current. It has been seen that if G2 is
connected to the anode then a pentode valve will behave as though it were a triode -
Voltage Amplifier Stages Using Valves
53
with a much lower stage gain. What is not generally appreciated is that an almost
identical behaviour is given if G2 is fed from the +V supply line through a sufficiently
high value load resistor that the screen and anode voltages move simultaneously in the
same manner. Once again, the stage gain of the valve is reduced to the level of a triode
stage.
Therefore, in order to obtain the full stage gain from a pentode amplifying valve, the
circuit must be chosen with an adequate value of G2 decoupling capacitor (C4 in
Figure 3.8) to ensure that the G2 voltage remains at a constant level. On the other
hand, it is possible to choose the values of the components associated with such an
amplifier stage so that the gain will rolloff with frequency, either at the HF or LF ends
of the frequency spectrum, where this gain reduction, and the phase changes
associated with it, may help in stabilising an amplifier using overall negative feedback.
+V
0
A
220k
.J
01uF I
22k
I
Output
0.47u F
Input
d~ 470k
0.47uF
OV
Figure 3.8
Pentode gain stage
Load Lines
If the valve manufacturers publish a full set of performance data it will be a
straightforward matter to calculate the stage gain from the equation
A o = laR1/(R1 + Ra)
However, the necessary data are not always published, and in this case it is necessary
either to use the Ia/V a data, if these are quoted, or to determine these for oneself. To
do this it is necessary to have a fixed high voltage source, a variable low voltage
(negative grid bias) source, a chosen value of load resistor, and a current meter. With
this one can obtain the family of curved lines, relating anode current to anode potential
for various grid bias voltages, shown in Figure 3.9 - those shown are published by
Mullard Ltd for one half of an ECC83 double triode.
54
Valve and
Transistor Audio Amplifiers
la (mA)
RI = lOOk
Gain = 100/1.5 = 66
Vs = 400V
-0.5V
//
/ 1 V _1.5v
////-2v.2.s v
-V
ECC83
Va(V)
o
Figure
1~)o
200
300
400
3.9
ECC83 load lines
The two points on the Ia and V a c u r v e s which can be defined are the anode voltage
with no anode current - in this case I have chosen a supply voltage of +400V - and
the anode current if the valve is turned fully on, which would be 400/100k (4mA) for
a 100k anode load resistor. If a straight line is drawn between these two points the
intercepts on the diagonal Ia/Vg curves will be a t - I V a n d - 2 . 5 V corresponding to
anode voltages of +200V and +300V. So, with this value of load resistor a change in
anode voltage of 100V will be caused by a - 1 . 5 V change in the grid bias voltage. The
gain of the stage (A~ for a stage without any added cathode circuit resistance, is
therefore -100/1.5 = -66 (which is the calculated stage gain for an ECC83 valve with
a 'p' value of 100 and an R a of 50k). The same graphical process can be carried out
for other values of load resistor or other values of supply voltage.
Similarly, this graphical approach can be used to determine the stage gain of a
pentode, as shown in Figure 3.10. Here, the slope of the Ia/W"a curve is very much
flatter than that of a triode - being almost parallel to the 0V axis - and this contributes
to the higher stage gain of the valve, by comparison with a triode equivalent. However,
the compression of the Ia/Va curves at more negative grid voltages indicates clearly
that the high gain pentode amplifier valve gives a higher distortion level than the lower
gain triode stage. It is also obvious from Figure 3.10 that the mutual conductance (gm)
of the EF86 pentode (the change in anode current for a fixed increment in grid voltage)
increases towards the higher anode current end of the anode current graph, so for
maximum stage gain for a given anode load resistor the valve should be operated at
the highest value of anode current which is compatible with avoidance of signal
waveform clipping on the negative peaks of anode current excursions.
The triode cascode of Figure 3.1 c - there is little benefit in the additional complexity
of a pentode valve as either V1A or V1B - has similar gain characteristics to the
pentode stage, with very much lower distortion. In this case, the simplest way of
V o l t a g e A m p l i f i e r Stages Using Valves
,
,
55
la (mA)
Gain = 1 0 0 / 0 . 8 = 125
Vs = 4 0 0 V
RI = lOOk
.,
__ - 1 . 5 V
-2V
3".
..
.
-2.5V
_
.
.,
~
i
,
o
~
~'oo
~
,
~oo
~
-3V
~
I ....
~_~.~v
~
;oo
9
-~v
- -
I-
,oo
~
E~
Va (V)
Figure 3.10
Load
lines for
pentode
gain
stage
determining the stage gain is to determine the gm of the lower valve at the chosen
anode voltage (gm - 1.25 for an ECC83, operating at V a = 100V, and I a = 0.5mA), and
then use the relationship A ~ = gmRl , so that an ECC83 cascode stage with a 100k anode
load resistor would have a stage gain of 150, which is rather better than that given by
the EF86. Since the voltage drop across the anode load will be 50V, a supply voltage
of, say, +220V would be quite adequate, and would allow an output voltage swing
approaching 100Vp_p. For a higher output voltage swing, a larger value of anode load
resistor and a higher +V supply line voltage would be required.
CHAPTER 4
VALVE AUDIO AMPLIFIER LAYOUTS
In its simplest form, shown in Figure 4.1, an audio amplifier consists of an input
voltage amplifier stage (A) whose gain can be varied to provide the desired output
signal level, an impedance converter stage (ZC) to adjust the output impedance of the
amplifier to suit the load, which could be a loudspeaker, a pair of headphones, or the
cutting head in a vinyl disc manufacturing machine.
o,
Input
-
!
---1,,
[,
I
Output
Figure 4.1
Audio amplifier block diagram
In the case of the headphones, their load impedance could be high enough for them to
be driven directly by the voltage amplifier stage without a serious impedance
mismatch but, with the other types of output load it will be necessary to interpose some
sort of impedance conversion device- in valve operated audio systems this is most
commonly an iron-cored audio frequency transformer. This is a difficult component to
incorporate within a high-fidelity system, and much thought must be given both to its
design and the way it is used in the circuit.
A very simple circuit layout embodying the structure outlined in Figure 4.1, using
directly heated (battery operated) valves, is shown in Figure 4.2. This is the type of
design which might have been built some fifty years ago by a technically minded
youngster who wanted some means of driving a loudspeaker from a simple piezoelectric gramophone pick-up.
For the maximum transfer of power from an amplifier to its load it is necessary that
both of these should have the same impedance, and since the anode resistance (Ra) of
the output valve is of the order of 10kD, and the most common speech coil impedance
of an inexpensive moving coil loudspeaker is 3D, there would be a drastic loss of
available power unless some impedance converting output transformer was employed.
58
Valve and Transistor Audio Amplifiers
The primary:secondary rums ratio of this component would need to be ~/(10k/3) =
58:1. It is difficult to design transformers having such high turns ratios without losses
in performance and, in consequence, when higher audio quality was required, the LS
manufacturers responded by making loudspeaker drive units with higher impedance
speech coils. Before the advent of transistor operated audio amplifiers the most
common LS driver impedance was 15D.
supply
+V
R2
22k
Output
t o I.S
C2
lOOuF
0.1uF
V2
o
....
Input
C1
IL _
-
KT2
0.1uF
o----i,
1MO
q
Gain control
+Vf
9
q
Figure
m
m
n
9.
.
.
. . . . . .
.
.
0 ov
-~
.
.
.
.
-0
-V
bias
4.2
Simple valve amplifier
With regard to the amplifier design shown, the input stage (V 1) uses a simple directly
heated triode, with grid-current bias developed across the 4M7D grid resistor, R1, as
shown in Figure 3.3a. This is resistor/capacitor coupled to V2, a small-power beam
tetrode or pentode, operated with fixed bias derived from an extemal DC voltage
source.
Because both V 1 and V2 will contribute some distortion to the signal- in the case of
V1, this will mainly be second harmonic, but in the output valve (V2) there will also
be substantial third harmonic component- the output signal will sound somewhat
shrill, due to the presence of these spurious high signal frequency components in the
output. The simplest and most commonly adopted remedy for this defect was to
connect a capacitor (C4) across the primary of the output transformer (TR1) to roll-off
the high frequency response of the amplifier as a whole, to give it the required mellow
sound. The HT line decoupling capacitor (C2) serves to reduce the amount of spurious
and distorted audio signal, present on the +V supply line, which will be added to the
Valve Audio Amplifier Layouts
59
wanted signal present on V2 grid. An amplifier of this type would have an output
power of, perhaps, 0.5 watts, a bandwidth, mainly depending on the quality of the
output transformer, which could be 150Hz- 6kHz, +6dB, and a harmonic distortion,
at l kHz and 0.4 watts output, of 10%.
The amplifier shown in Figure 4.2 uses a circuit of the kind which would allow
operation from batteries, and it was accepted that such designs would have a low
output power, and a relatively poor performance in respect of its audio quality: this
was the price paid for the low current drain on its power source. If, however, the
amplifier was to be powered from an AC mains supply, the constraints imposed by the
need to keep the total current demand low no longer applied, and this gave the circuit
designer much greater freedom. The other consideration in the progress towards
higher audio power outputs was the type of output stage layout, in that this influenced
the output stage efficiency, as examined below.
Single-ended vs. Push-pull Operation
These two options are shown schematically in Figure 4.3, in which Q 1 and Q2 are
notional amplifier blocks, simplified to the extent that they are only considered as
being either open-circuit (O/C) or short-circuit (S/C), but with some internal
resistance, shown as R3 (or R1 in the case of Figure 4.3b). I have also adopted the
convention that the current flow into the load resistor (R2) is deemed to be positive
when the amplifier circuit is feeding current (as a current source) into the load, and
negative when the amplifier is acting as a current sink and drawing current from R2
and its associated power supply. I have also labelled the voltage at the junction of these
+2V
+ 2V
r
Q2
b R1
i.
IL
i.
R2
V
--
,b~3
R2
~&AA
-u
12
OV
(a) Single-ended
Figure 4.3
Output arrangements
' -
13 4
12
OV
(b) Push-pull
-O
+V
60
Valve and Transistor Audio Amplifiers
three resistors as V x. The efficiency of the system can be considered as related to the
extent of the change in the current through R2 brought about by the change from o/c
to s/c in Q1 or Q2.
If we consider first the single-ended layout of Figure 4.3a, when Q 1 is O/C, the current
flow into R2 is only through R1. and i2 = V/(R1 + R2). If, however, Q1 is shortcircuited, S/C, then, from inspection,
i 3 = i 1 + i2
(1)
but i2 = ( V - Vx)/R2
(2)
i~ = ( 2 V - Vx)/R1
(3)
and i3 = Vx/R3
(4)
from (1), (2) and (3) we have
i3 = ( 2 V - Vx)/R1 + ( V - Vx)/R2
(5)
but it has been seen from (4) that i3 = Vx/R3
therefore (2V - Vx)/R1 + (V - Vx)/R2 = Vx/R3
(6)
If we insert the actual values for R1, R2 and R3, we can discover the difference in
output current flow in the load resistor (R2) between the o/c and s/c conditions of Q 1.
For example, if all resistors are 10~ in value, when Q 1 is s/c, V x will be equal to V,
and there will be no current flow in R2 and the change on making Q 1 o/c will be
(V/20)A. If R1 and R2 are 10f2 in value and R3 is 5fL then the current flow in R2,
when Q1 is o/c, will still be (V/20)A, whereas when Q1 is s/c, the current will be
(-0.25V/10)A and the change in current will be (3V/40)A. By comparison, for the
push-pull system of Figure 4.3b, the change in current through R2, when this is 10f2
and both R1 and R3 are 5~ in value, on the alteration in the conducting states of Q1
and Q2, will be (2V/15)A, which is nearly twice as large.
The increase in available output power from similar output valves when operated at
the same V+ line voltage in a push-pull, rather than in a single-ended, layout is the
major advantage of this arrangement, although if the output devices have similar
distortion characteristics, and the output transformer is well made, the even harmonic
distortion components will tend to cancel. Also, the magnetisation of the core of the
output transformer due to the valve anode currents flowing in the two halves of the
primary winding will be substantially reduced, because the induced fields will be in
opposition.
In addition, an increase in the drive voltage to the grids of the output valves, provided
that it is not large enough to drive them into grid current, will, by reducing their
equivalent series resistance (R1, R3 in the calculations above), increase the available
Valve Audio Amplifier Layouts
61
output power, whereas in the single-ended layout the dynamic drive current cannot be
increased beyond twice the quiescent level without running into waveform clipping.
However, there are other problems, which are discussed below.
Phase Splitters
In order to drive a pair of output valves in push-pull it is necessary to generate a pair
of AC control grid drive voltages which are equal in magnitude but in phase
opposition. The simplest way of doing this is to use a transformer as the anode load
for an amplifier stage, as shown in Figure 3.4c, but with a centre-tapped secondary
winding.
I have shown in Figure 4.4 a typical centre-tapped transformer coupled 20 watt audio
amplifier, of the kind which would have been common in the period spanning the late
1930s to early 1940s. Because there are two coupling transformers in the signal path
from the input and the LS output, which would cause substantial phase shifts at the
ends of the audio spectrum, it would be impractical to try to clean up the amplifier's
relatively poor performance by applying overall negative feedback (from the LS
output to V 1 cathode) to the system.
Some local negative feedback from anodes to grids in V2 and V3 is applied by way of
C4/R4 and C6/R7 in an attempt to reduce the third, and other odd-order, harmonic
distortion components generated by the output valves. Since the designer expected that
the output sound quality could still be somewhat shrill, a pair of 0.05~tF capacitors, C7
and C8, have been added across the two halves of the output transformer primary
windings to reduce the high frequency performance. These would also have the effect
of lessening the tendency of the output valves to flash over if the amplifier was driven
into an open-circuited LS load- an endemic problem in designs without the benefit of
overall negative feedback to stabilise the output voltage.
The anode voltage decoupling circuit (R3,C3), shown in Figure 4.4, is essential to
prevent the spurious signal voltages from the +V supply line to the output valves being
introduced to the output valve grid circuits. This would, in the absence of the supply
line decoupling circuit, cause the amplifier to oscillate continuously at some low
frequency- a problem which was called motor-boating, from the sound produced in
the loudspeakers.
Various circuit arrangements have been proposed as a means of generating a pair of
low distortion, low phase shift, push-pull drive voltages. Of these, the phase inverter
circuit of Figure 4.5 is the simplest, but does not offer a very high quality performance.
It is, in principle, a bad thing to attenuate and then to amplify again, as is done in this
arrangement, because this simply adds just another increment of waveform distortion,
due to V2, to that contributed by V 1.
A much more satisfactory arrangement is that shown in Figure 4.6, in which V2 is
operated as an anode follower, which like the cathode follower, employs 100%
negative feedback, though in this case derived from the anode. This stage contributes
62
Valve and Transistor Audio Amplifiers
very little waveform distortion. Also, because both valves operate as normal amplifier
stages, the available voltage from either output point will be largely unaffected by the
operation of the circuit. An additional advantage over the circuit shown in Figure 4.5
is that the two antiphase output voltages are equal in magnitude, without the need to
adjust the preset gain control, RV 1.
+300V. 100mA
R4
lOOk
LS output 3R
C1
/
/
' ~, ~:~
I
i
I
IOOR
cs
Input
TR2
O/P Trans
0.1uF
1.50V
C2
25uF
~
1
C3
25uF
lOOk
Figure 4.4
Simple 20 watt amplifier
--
+Vcc
27k
27k
C4
Output 1
C6
O.05uF
C3
O.05uF
Input
~
Output 2
O.05uF
V2
1/26SN7
0.1uF
R6
470R
1Ok
OV,
Figure 4.5
Simple phase inverter
0V
Valve Audio Amplifier Layouts
63
Another satisfactory circuit is that based on the long-tailed pair layout, in which,
provided that the tail resistor is large in relation to the cathode source resistance
(1/gm), the two antiphase anode currents will be closely similar in magnitude. The
advantage of this circuit is that it can be direct coupled (i.e. without the need for a DC
blocking coupling capacitor) to the output of the preceding stage, and this minimises
circuit phase shifts, especially at the LF end of the pass-band. By comparison with the
two preceding phase-splitter circuits, it has the disadvantage that the available AC
output swing, at either anode, is greatly reduced by the fact that the cathode voltages
of V2 and V3 are considerably positive in relation to the 0V line, and this will almost
certainly require an additional amplifier stage between its output and the input of any
succeeding triode or beam-tetrode output stage.
R3
27k
R4
-9L
C1
0.11/u
,, - -
" .., Y~.~ f ~ " T
Z ZUK
-
0 Output 1
~F---4t----o
ou,pu,~
O.OSuF
~I.._
.~V11126SN,'
. . . .
F
- "
.~?
_
--~'ok
C
+Vcc
27k
=l
Input
0
1
MC
-r
ov
~ .
.
.
.
.
.
~R2
:470R
.
.
.
.
.
L
.
_!
I
-~oov
Figure 4.6
Floating paraphase circuit
R4
;'&"
-
--
22k
-
" -
9
......
l
ov 0
Figure 4,7
Long-tailed pair circuit
c~
o.
c,
+Vcc
~
Output 2
R7
27k
1_
0
,~o
1
1
27k
-
~ Output 1
1C~uF
0
ov
64
Valve and Transistor Audio ....Amplifiers
This disadvantage is shared by the circuit layout shown in Figure 4.8, in which a direct
coupled triode amplifier is operated with identical value resistive loads in both its
anode and cathode circuits. Because of the very high level of negative feedback due to
the cathode resistor both the distortion and the unwanted phase shifts introduced by
this stage are very low. Significantly, this was the type of phase splitter adopted by
D.T.N. Williamson in his classic 15 watt audio amplifier design.
PA
22k
+Vcc
R5
27k
C6
0.05uF
Output 2
V2
1/26SN7
Input
0.1uF
Output 1
Ic3
125uF
~R6
C~P.N P
~ 27k
0V
Figure 4.8
Split load phase splitter
Output Stages
The basic choice of output valve will lie between a triode, a beam-tetrode or a pentode.
If large output powers are required- say, in excess of 2 w a t t s - triode output valves
are unsuitable because the physical spacing between control grid and anode must be
small, and the grid mesh must be relatively widely spaced, in order to achieve a low
anode current resistance and a high practicable anode current level. This closely
packed type of construction will lead to the almost complete stripping of the space
charge from the region between the cathode and the grid. Experience shows that the
life expectancy of cathodes operated under such conditions is short, and the only way
by which this problem can be avoided is by the use of a directly heated (filament type)
cathode construction, which is much more prolific as a source of electrons, and this
leads to other difficulties such as hum intrusion from the AC heater supplies, and the
awkwardness of arranging cathode bias systems.
So, if it is required to use a triode output stage, at anything greater than the 50mA
anode current obtainable from a parallel connected 6N7 double triode (the 6SN7 has
a smaller envelope and, in consequence, a lower permissible anode dissipation), a
directly heated valve such as the, now long obsolete, 6B4 or PX25 would need to be
found. Therefore, in practice, the choice for output valves will be between output
beam-tetrodes or pentodes. Although a fairly close simulation of a triode characteristic
can be obtained in both of these valve types if the anode and G2 are connected
together, this approach works better with a beam-tetrode than a power output pentode
Valve Audio Amplifier Layouts
65
because the presence of the suppressor grid in the pentode somewhat disturbs the
anode current flow.
The required grid drive voltage for typical pentode or beam-tetrode output valves, at
V a = 300V, will be in the range 20-50Vp__ for each output valve, and whether or not
the valve is triode connected has little ef-tYect on this requirement. Triode connection
does, however, greatly affect the anode current impedance, which is reduced, in the
case of the KT88, from 12k.Q to 670s and the need for a lower turns ratio greatly
simplifies the design of the necessary, load-matching, output transformer with low
half-primary to half-primary and primary to secondary leakage inductances.
Output (Load-matching) Transformer
This component is probably the most important factor in determining the quality of the
sound given by a valve operated audio amplifier and the performance of this
component is influenced by a number of factors, both mechanical and electrical, which
will become of critical importance if an attempt is made to apply negative feedback
(NFB) over the whole amplifier. However, for a low power system, such as might be
used as a headphone amplifier, it is possible to make a quite decent sounding system
without the need for much in the way of exotic components, circuit complexity or very
high quality output impedance-matching transformers, and I have sketched out in
Figure 4.9 a typical circuit for a two valve, 1W, headphone amplifier based on a pair
of 6SN7s or equivalents.
In this design the input pair of valves acts as a floating paraphase phase-splitter circuit,
which provides the drive for the output valves. Since the cathode currents from the two
.......
RIO
=, ,
~.A . . . . . . . . . .
-0
)OV
Vcc
Li 3'
R2
.......
'
'
o.4,uF
:E'M~
1,
/
v'~l
i Output trz'~ins
o,
Output
~R1
-.8
C:ou•
4
RV1
1 M0
0V
O
Figure
-
-- ==-----
4.9
1
_
0.15uF V2b
I
•
Simple headphone amplifier
1
1/26SN7 I
R6
820R
_
_
-
9 --.
/
-
.
.
.
.
r
OV
66
Valve and Transistor Audio Amplifiers
input valves are substantially identical, but opposite in phase, it is unnecessary to
provide a cathode bypass capacitor to avoid loss of stage gain. Also, since this cathode
resistor is common to both valves, it assists in reducing any differences between the
two output signals, because the arrangement acts, in part, as a long-tailed pair circuit.
Since the total harmonic distortion from a push-pull pair of triodes will probably be
less than 0.5%, and will decrease as the output power is reduced, provided a
reasonable quality output transformer is used, I have not included any overall NFB,
and this avoids any likely instability problems. To match the output impedances of
V2A and V2B to a notional load impedance of 100D a transformer turns ratio, from
total primary to secondary, of 12:1 is required.
In more ambitious systems, in which NFB is used to improve the performance of the
amplifier and reduce the distortion introduced by the output transformer, much more
care is needed in the design of the circuit. In particular, the phase shifts in the signal
which are introduced by the output transformer become very important if a voltage is
to be derived from its output, and fed back in antiphase to the input of the amplifier,
in that to avoid instability the total phase angle within the feedback loop must not
exceed 180 ~ at any frequency at which the loop gain is greater than unity. This
requirement can be met by both limiting the amount of NFB which is applied, which
would, of course, limit its effectiveness, and by controlling the gain/frequency
characteristics of the system.
Although there are a number of factors which determine the phase shifts within the
transformer, the two most important are the inductance of the primary winding and the
leakage inductance between primary and secondary, and a simple analysis of this
problem, based on an idealised, loss-free transformer, can be made by reference to
Figure 4.10. In this, R1 is the effective input resistance seen by the transformer, made
up of the anode current resistance of the valve, in parallel with the effective load
resistance, and L1 is the inductance of the transformer primary winding. When the
signal frequency is lowered, a frequency will be reached at which there will be an
attenuation of 3dB and a phase shift of 60 ~ This will occur when R1 = jmL 1, where
m is the frequency in radians/sec.
L2
R1
C
A
,
,
Out
In
ClL
T_
R2
L1
,
_
~:~
Figure 4.10
Equivalent circuits of idealised coupling transformer
R2 is the secondary load resistance, which is the sum of the resistance reflected
through the transformer and the anode resistance, and L2 is the primary leakage
inductance- a term which denotes the lack of total inductive coupling between
Valve Audio Amplifier Layouts
67
the primary and secondary windings - which behaves like an inductance between the
output and the load, and introduces an attenuation, and associated phase shift, at the
HF end of the passband. The HF -3dB gain point, at which the phase shift will be 60 ~
will occur at a frequency at which R2 = jwL2.
To see what these figures mean, consider the case of a 15D resistive load, driven by a
triode connected KT66 which has an anode current resistance of 1000 ohms. Let us
assume that, in order to achieve a low anode current distortion figure, it has been
decided to provide an anode load of 5000 ohms. The turns ratio required will be
~/(5000/15) = 18.25:1 and the effective input resistance (R1) due to the output load
reflected through the transformer will be 833 ohms. If it is decided that the transformer
shall have an L F - 3 d B point at 10Hz, then the primary inductance would need to be
833/2~10 = 833/62.8 = 13.26H. If it is also decided that the HF -3dB point is to be
50kHz, then the leakage inductance must be 833/27t50 000 = 2.7mH. The interesting
feature here is that if an output pentode is used, which has a much higher value of R a
than a triode, not only will a higher primary inductance be required, but the leakage
inductance can also be higher for the same HF phase error.
Unfortunately, there are a number of other factors which affect the performance of the
transformer. The first of these is the dependence of the permeability of the core
material on the magnetising flux density, as I have shown in Figure 4.11. Since the
current through the windings in any audio application is continually changing, so
therefore is the permeability- and with it the winding inductances and the phase errors
introduced into the feedback loop. Williamson urged that, for good LF stability, the
value of permeability, ~t, for low values of B should be used for primary inductance
calculations.
Permeability (/J)
1Ok
lk
J
J
J
J
100
Flux density (B)
10
i
10
Figure 4.11
Magnetisation curve
100
I
lk
I
1Ok
,v
68
Valve and Transistor Audio Amplifiers
Secondly, this change in inductance, as a function of current in the windings, is a
source of transformer waveform distortion, as are - especially at high frequencies the magnetic hysteresis of the core material and the eddy current losses in the core.
These problems are exacerbated by the inevitable DC resistance of the windings, and
provide another reason, in addition to that of improved efficiency, for keeping the
winding resistance as low as possible.
The third problem is that the permeability of the core material falls dramatically, as
seen in Figure 4.11, if the magnetisation force exceeds some effective core saturation
level. This means that the cross-sectional area of the core (and the size and weight of
the transformer) must be adequate if a distortion generating collapse in the transformer
output voltage is not to occur at high signal levels. The calculations here are essentially
the same as those made to determine the minimum turns per volt figure permissible for
the windings of a power transformer (see Linsley Hood, J., The Art of Linear
Electronics, Butterworth-Heinemann, 1993, pp. 29-31).
In practical terms, the requirements of high primary inductance and low leakage
inductance are conflicting, and require that the primary winding is divided into a
number of sections between which portions of the secondary winding are interleaved.
Williamson proposed that eight secondary segments should be placed in the gaps left
between ten primary windings. This increases the stray capacitance, C1, across the
primary winding and between primary and secondary coils. However, the HF phase
errors introduced by these will probably be unimportant within the design frequency
spectrum.
Effect
of Output
Load
Impedance
This is yet another area in which there is a conflict in design requirements, between
output power and output stage distortion. In Figure 4.12a, I have shown the output
power given for 1% and 2% THD values by a push-pull pair of U/L connected KT88s
Power output (watts)
40-
2%
KT66 Va = Vg2 = 400V U/L connected
-//
1%
20-
1
0
Figure 4.12a
Power output vs. THD
4k
------'--~
!
!
r
8k
12k
16k
Load resistance a-a (ohms)
I
20k
Valve Audio Amplifier Layouts
69
in relation to the anode to anode load impedance chosen by the designer. This data is
by courtesy of the GEC (Audio Amplifier Design, 1957, p. 41). Since the distortion can
also alter in its form as a function of load impedance, I have shown in Figure 4.12b
the way these circuit characteristics change as the load resistance changes. The figures
given for an single-ended 6AK6 output pentode are due to Langford-Smith (Radio
Designers Handbook, 4th Edition, 1954, p. 566).
Power output (watts)
6AK6 va = 180V
1.2-
~ ~ - ~ ' ~Po~.
~r~
/
0""
--~
1
0
o~~-~'"
1
4k
~.-12
t o
~-1
8k
12k
Load resistance (ohms)
Figure 4.12b
Power output curve
~-6
16k
Available Output Power
The power available from an audio amplifier, for a given THD figure, is an important
aspect of the design. Although there are a number of factors which will influence this,
such as the maximum permitted anode voltage or the maximum allowable cathode
Dissipation
\
Limit
\
..,
c,_,.~
Load Lin
/
\ 'v~
/
/
u
~
0
e-
//(ff
"
Anode voltage
Figure 4.13
Anode dissipation curve
70
Valve and Transistor Audio Amplifiers
current, the first of these which must be considered is the permissible thermal
dissipation of the anode of the valve. These limiting values will be quoted in the
manufacturers' handbooks, and from these it is possible to draw a graph of the kind I
have shown in Figure 4.13, where the maximum permitted combinations of anode
current and anode voltage result in the curved (dashed) line indicate the dissipation
limits for the valve, and the load line for its particular operating conditions can then
be superimposed on this graph to confirm that the proposed working conditions will
be within these thermal limits.
CHAPTER 5
NEGATIVE FEEDBACK
The concept of feeding back into its input some part of the output signal from an
amplifying block was invented, like so many other useful ideas in the field of
electronics, by Major Edwin Armstrong, of the US Army Air Corps. His initial
intention was to use positive (i.e. signal level enhancing) feedback (PFB) to make the
electrical oscillators which were required as signal sources for radio broadcasts, but it
was soon discovered that the feeding back of an antiphase signal - signal diminishing
or negative feedback (NFB) - from the amplifier output into its input circuit also
conveyed some valuable advantages. In particular, simple NFB can reduce the extent
of any waveform distortion or noise introduced by the amplifier block, and can help to
make the frequency response of the system more uniform.
I describe this usage as simple NFB because there are many other circuit possibilities
using NFB in which the feedback is used to cause specific modifications to the
frequency response characteristics of a circuit block, as, for example, in the
construction of frequency response correction circuitry or notch or bandpass filters.
These will be considered later.
The effects caused by the normal - flat frequency response - type of voltage feedback
are most easily explored by reference to the simple schematic layout shown in Figure
5.1. In this a circuit block having separate, and isolated, input and output connections,
m-n, p-q, and a gain of A ~ is connected so that a proportion of the output voltage is
fed back, in series with the input signal, E i, through a network, FB, having a gain of
13. If we define the signal actually appearing between the input points m-n as e i, and
Ei
t
rn
t
Ao
ei
_
p
n
q
FB
F i g u r e 5.1
Layout of feedback system
Eo
72
Valve and Transistor Audio Amplifiers
that appearing across the output points p-q as E o, then the stage gain of the amplifying
block, A ~ is
A ~ = Eo/e i
(1)
which is called the open loop g a i n - i.e. the gain before any feedback is employed.
However, if the electrical network, FB, connected between the output and input of this
gain block causes a proportion 13 of the output signal to be inserted in series with the
input voltage Ei, then the actual input applied to the gain block will be changed, so that
e i will actually be E i + ~Eo, or
E i = e i - 13Eo
(2)
These equations allow us to determine the effect of feedback on the stage gain, bearing
in mind that in the absence of any feedback signal, E i = e i.
The closed loop stage gain, i.e. that when feedback is applied, is normally called A',
and is defined by the equation
A "=
Eo/E i
(3)
Combining equations (2) and (3)
A "=
Eo/(e i - ~Eo), but e i - E o / A 0
therefore
A" =
Eo
Eo/A~ - ~Eo
from this A'/A ~ -
=
1
(1/A ~
1/(1 - I~A~
[3)
Ao
or
1 -
[3A~
(4)
One of the effects which can be seen immediately from equation (4) is that, if the
feedback is positive, then if ]3A~ is unity, the stage gain will be infinite. This
consideration is important for two reasons:
9 that if there is some mechanism in an electrical circuit which limits the gain once
a certain output amplitude is reached, then the use of positive feedback can be
made to provide a continuous and stable amplitude output signal from the circuit
without any input signal being required. In general, some frequency determining
network will be employed to ensure that this output oscillation occurs at some
desired and specified frequency;
9 that if there is some characteristic of the layout or the components used in the gain
block which tends to cause a phase shift in the fed back signal, so that, although
the feedback factor [3 is intended to be negative, it is actually positive at some
frequency, then continuous oscillation may occur at that frequency if the closed
Negative Feedback
73
loop gain reaches unity. Not only that, but the stage gain will tend to increase, in
any amplifier employing negative feedback, where the phase shift of the fed back
signal causes this to be positive, even though the amount of the feedback may not
be enough to cause oscillation.
There are a number of advantages and disadvantages which are inherent in the use of
feedback in an amplifying system, and it is essential, in any design, that its designer
should be aware of the latent problems as well as the required benefits.
Benefits of Negative Feedback
If the feedback network (FB) is a phase inverting one, so that 13 is negative, then the
gain equation will be
A ' = AO/(1 + ~A o)
(5)
In this case the closed loop gain will decrease as the amount of feedback is increased,
up to the point at which ~A ~ is so much greater than 1 that the equation approximates
to
A'=A~
~ or A ' = l / ] 3
(6)
This relationship is of particular interest where A ~ is very high, because in these cases
the gain with feedback, A', is almost completely determined by the attenuation ratio,
13, of the feedback network, so that if]] is 1/10 then the gain A" will be 10, and if]] is
1/100, then the stage gain will be 100, and so on. This is a valuable feature in the case
of valve amplifiers, or other circuits where the performance of the components may be
expected to deteriorate with time, in that it will sustain the gain and other qualities of
the circuit in spite of component deterioration.
Some further valuable features can be predicted, particularly in the limit case where
A" tends towards 1/[3, for example that, if the feedback network has an attenuation
which is independent of frequency, then the action of the feedback will be such as to
make the amplifier gain also independent of frequency. Also, if the feedback network
is distortion free, then, if the gain (A ~ is high enough the output will also be distortion
free. A logical deduction from this is that, for those cases where A ~ is less than infinite
in value, the distortion (D ~) of the output signal will be reduced in direct proportion as
the value of [3A~ increases, giving the relationship
D ' : D~
+ [3A~
(7)
where D ~ is the distortion without feedback. However, this relationship makes the
assumption, not always found in practice, that the open loop gain (A ~ is still
sufficiently high, and negative, at the frequencies of the various higher harmonic
components which go to make up the total distortion. A similar prediction can be made
about the reduction of 'hum' and noise introduced by the amplifier block, provided
that the feedback network is itself hum and noise free. This feature is particularly
74
Valve and Transistor Audio Amplifiers
.
.
.
.
valuable in valve operated audio amplifiers, since the poor smoothing of the HT+
supply lines, and the likely induction of hum, from extemal AC fields, into the
windings of the output transformer means that the output signal from a simple
amplifier is typically not as free from hum as would be desired.
Another advantage of NFB is that it can improve the accuracy of rendition of transient
signals, in that any difference between the input signal, Ei, and that which is fed back,
~lEo, will generate an error voltage which will be amplified by A ~ and subtracted from
the signal output. All in all, an amplifier block employing negative feedback, where
this is applied correctly in a properly designed system, will be substantially superior
to that of a similar system without NFB. The fact that claims are sometimes made that
the performance of an amplifier without NFB is superior to that of one using this
technique seems to be due principally to the lack of design skills of those in respect of
whose designs this claim is made.
Stability Problems
It has already been noted that phase shifts within the amplifier block, or the feedback
network employed, can lessen the stability of the circuit to which NFB is applied.
Various methods have been employed to demonstrate this effect, but one of the
simplest, and most easy to understand, is that of the Bode plot shown in Figure 5.2. In
this, the closed loop gain and the phase angle of the fed back signal are plotted,
simultaneously, against frequency in the range of interest.
Gain (dB)
120-[_ _
100 80604020 _ ~ P h a s e
0
-20
"
1 ----e I
10
100
I
lk
~
1
~
l O o k - ~
t +90 Frequency
0"~.90~=
=
-180
Figure 5,2
The Bode plot
Stability in the system requires that the gain of the system should not exceed unity
(0dB) at any frequency at which the phase of the fed back signal reaches or exceeds
-180 ~ This requirement arises because the initial phase of a negative feedback signal
will be-180 ~ so that an added phase shift of 180 ~ will change the phase of the signal
fed back to -360 ~ (= 0~ and if the gain equals or exceeds unity the system
will oscillate.
Negative Feedback
75
Two methods are used to describe the stability of an amplifier using NFB; the gain
margin, which refers to the extent to which the loop gain is less than unity by the time
the phase shift has become-180 ~ and the phase margin, which refers to the extent to
which the phase angle of the fed back signal is less than -180 ~ by the time the loop
gain has decreased to unity. In general the negative signs are omitted from the
description, so a 20dB gain margin would imply that the loop gain would be - 20dB
(0.1) at the frequency at which the loop phase shift w a s - 1 8 0 ~ Alternatively, a 30 ~
phase margin would imply that the loop phase shift had only reached-150 ~ at the
frequency at which the loop gain was unity.
The problem in audio is that the phase shift within an amplifier will be influenced by
the characteristics of the load applied to it, as well as by the signal frequency, and
probably also the amplitude of the signal. Unfortunately, all of these things are
continually varying in any real-life audio system, so any system without an adequate
margin of stability may be flickering into and out of the regions of near or actual
instability for the whole of the time, and this will lead to the modification of the signal
waveshape, due to instantaneous changes of gain, or, at the worst, to the generation of
bursts of oscillation, superimposed on the wanted signal. It is difficult to generate
suitable input test signals to reveal this problem, and it is also hard to simulate in the
laboratory appropriate awkward loads to allow examination of these effects.
Understandably, badly designed amplifiers will give unpleasant results, and these
results may worsen when NFB is applied, but the answer is not to use reduced amounts
of NFB or no NFB at all, but to design them so that they have greater loop stability
margins, especially when used with reactive loads. It is inconvenient that this is a
region in which the overall stability in operation of an amplifier is often in conflict
with the attempts of the designer to achieve the lowest practical level of total harmonic
distortion, when measured with a steady state signal on a purely resistive load. For this
purpose, a large amount of NFB is often used, and the design of the circuit is then
tailored to sustain the loop gain at a high level up to the frequency at which it must fall
rapidly to reach 0dB before the loop phase approaches -180 ~ Circuits designed like
this, though stable, often have very small gain and phase margins,
Series and Shunt NFB
It is convenient in this analysis to use the conventional operational amplifier symbol,
shown as 'A' in Figure 5.3, which denotes a circuit block with a very high internal
gain, and two input points, labelled- and +, to indicate either inverting or noninverting operation of the amplifier. It is assumed in this convention that the amplifier
block will have a very low output impedance, and that both of the input points will
have a very high input impedance. It is also assumed that the amplifier acts only in a
differential mode, in which it will amplify only those signal voltages which exist
between its two inputs, and will ignore any potential difference which may exist,
jointly, between both of the inputs and the common 0V rail.
In the circuit of Figure 5.3a, the input signal is connected to the non-inverting input,
while the feedback signal is taken to the inverting input through the resistor network
76
Valve and Transistor Audio Amplifiers
R1 and R2. From the calculations shown above, the gain will be 1/13, or
A" = (R1 + R2)/R2
(8)
and, if the gain block has a very high impedance at its two inputs, the input impedance
of the circuit of Figure 5.3a will be that of the resistor Rin. This method of connection
is described as series feedback, because the feedback voltage is applied at the
amplifier input in series with the input voltage, since only the difference between the
two input voltages will be amplified.
Input C
=
RIN
....
, ,
ov C
. . . .
(a)
0 Output
i. I
I R2
.
.
.
.
.
.
0
ov
R1
Input C
.R/N=
~
ir~
,
-
ov C -
-
-
.
.
.
.
(b)
0 Output
O
ov
Figure 5.3
Feedback systems
There is a limit, in theory, to the performance which can be obtained from such a
feedback connected amplifier block, because there will be some failure, however
small, in the rejection of the voltage present on both input terminals - the so-called
common mode voltage. In the better transistor op amps, the extent of the common
mode rejection may be very good, particularly at the lower end of their operating
frequency range, but in a simple valve operated differential amplifier, such as the longtailed pair layout shown in Figure 3.1 d, the cancellation of common mode signals may
not be very good, and distortion due, for example, to the non-linearity of the Ia/Vg
characteristics of the two valves, will not be completely cancelled.
In the circuit layout of Figure 5.3b, both the input and the feedback voltages are
applied to the inverting input of the amplifier block, in parallel, or shunt, and this mode
of operation is known as shunt feedback. The gain of the stage is
A" = R]/Rin
(9)
Negative Feedback
77
and the input impedance is that due to Rin, because the inverting input will appear to
be what is termed a virtual earth, a concept which will be explained later. Since both
inputs are connected to the same point there is no common mode error.
-
_
i
l
v,
Vin
0 Vout
---
_
,. [
,,
FB
,,I .
,
-Oov
Figure 5.4
Series feedback system
The Effect of NFB on Input and Output Impedances
The aspect of the series NFB system can be explored with reference to Figure 5.4. In
this, the amplifier block has a gain of A ~ [3 is the proportion of the signal which is fed
back by the network, FB, V 1 is the actual voltage which appears across the input of the
gain block, and R a is the input resistance of the gain block. It is apparent that
Vou t -- AOgl
but
Win = ~Vou t + V 1
therefore
Vin = ~ A ~
+ Vl
= Vi(A~ + 1)
The actual input resistance (Rin) of the system is:
Rin = Vin/iin , and iin = V1/R a
so, since Vin = V I ( A ~ I] + 1), then, in the case of a series feedback system,
Rin =
VI(A~ + l)
V1/R a
=
(1 + A~
(10)
which is to say that the input impedance is effectively increased in proportion to the
amount of feedback employed.
78
Valve and Transistor Audio Amplifiers
,,,
In the case of the shunt feedback layout shown in Figure 5.5, if we define the circuit
input current as i 1, the amplifier input current as i2, and the current through the
feedback resistor, R2, as i 3, then:
i 1 = i2 + i 3
(11)
and, if the current amplification of the amplifier is A ~ then
(12)
i 3 = A ~ i 2 or i 2 = i3/A~
Combining these two equations we get
i 1 = i3/AO + i 3 = i3(1 + 1/A ~
from which
i 1 = i3(A ~ + 1)/A ~
so, by inversion,
i 3 = ilA~
~ + 1)
(13)
If Vin = ilR 1 and Vou t =-i3R2, because the gain block is a phase inverting amplifier,
then we can determine the circuit gain with feedback (A') which is
A
,
_
_
Vout _ R2
gin
A~
(14)
R 1 A~+ 1
which, if A ~ is high enough, will approximate to"
A'=
_ R2/R 1
The impedance at point 'X' of an amplifier employing shunt connected NFB (which I
have called Ra) can be determined, approximately, by the following argument.
Let us postulate that the gain block is an ideal amplifier in which neither input draws
any current, so that i 1 = i 3. However, because it is an inverting amplifier, the voltage
across R2 will be (A ~ + 1)Vin, and i 1 = {(A ~ + 1)Vin}/R2
SO,
Vin
Vin
Rin . . . . . . . . . . . . . . . .
iin
{(A ~ + 1)Vin }/R 2
which simplifies to
Rin = R2/(A~ + 1)
(15)
This can be very low indeed if A ~ is high enough - much lower in practice than the
input impedance (Ra) of any likely amplifier b l o c k - so that neglecting the amplifier
Negative Feedback
79
input current, i2, will not significantly affect the calculated value for Rin. The very low
input impedance at the inverting input of any amplifier using shunt feedback has given
rise to the term virtual earth to describe its characteristics, and allows the circuit
arrangement shown in Figure 5.6 to be used as a summing amplifier or signal mixer
with very little interaction between the various inputs.
_
9
l
1
R1
_
,
l
a
R2
i
t
vvv
A
,
(x)
I1
-0
Vout
Vin
0 ov
Figure 5.5
Shunt feedback layout
To summarise: the series feedback system offers a very high input impedance, which
will increase as the open loop gain of the system is increased. By contrast with this,
the shunt feedback layout has a very low input impedance, which will decrease as the
open loop gain of the amplifier is increased. In normal shunt FB voltage amplifier
circuits, there will be an input resistor, such as R1 in Figure 5.5, and this will be
effectively the input resistance of the circuit.
A further important difference between shunt and series connected NFB is that if the
feedback limb, between output and inverting input, has zero impedance then, in a
shunt feedback system, the gain of the stage will be zero, whereas, in a series feedback
RA
R2
1Ok
0
Inputs
O-
RB
- W V ~-1Ok
RC
"#W~ 1Ok
i
I 0
Output
RD
1Ok
Figure 5.6
ov C
Virtual earth mixer system
, Oov
80
Valve and Transistor Audio Amplifiers
system, the gain will be unity. This difference is important in those circuit layouts,
considered below, in which the feedback network contains capacitors, whose
impedance will decrease, and will ultimately tend to zero as the signal frequency is
increased.
Figure 5.7
Arrangement to allow calculation of output impedance
Influence of NFB on Output Impedance
In both shunt and series connected NFB circuits the application of NFB will lower the
output impedance of the gain block, as measured at the point from which the NFB is
taken. The magnitude of this effect can be calculated by considering the case of an
amplifying block with a gain A ~ and an effective output resistance, without feedback,
of R o, and has a feedback signal returned to its inverting input via the network, FB,
which has an attenuation of [3.
If a signal voltage, V, is applied to its output, this will produce a voltage [3V at the
inverting input of the amplifier, and this will, in turn, generate a voltage of-A~
at
point 'X'. The voltage across R 0 (V0) is given by
V 0 = V - Vout
:
{V-(-A~
- V(I+A~
and the current through R 0 (i0) will be
i0
: V0/Ro
: V(1 + A~
so the output impedance with feedback, R"0, which is V/i 0, will be
v
R'~ =
V(1 + A~
Ro
(1 + A~
(16)
and this effect of lowering the output impedance is the same for both feedback
systems.
81
N e g a t i v e Feedback
Effect of NFB on the Harmonic Components of the Signal
A frequently overlooked consideration, in the application of NFB to improve the
linearity of non-linear amplifier circuits, is that, in small amounts, NFB can both
worsen the total harmonic distortion and modify the distribution of the harmonic
components within the distorted signal. The reason for this is simple - that if, for
example, an amplifier block introduces second and third harmonic distortion, these
harmonics will be present in the feedback signal to the amplifier, and will then be
distorted further by the amplifier to produce new fourth, sixth and ninth harmonic
components, not present in the original distorted output. So, while the second and third
harmonic components present in the feedback will tend, following amplification, to
cancel the original distortion components, the higher order distortions will look like a
new input signal, which will pass through the system, to be amplified and distorted in
their turn.
A practical example of this problem was shown graphically by P. J. Baxandall
(Wireless WorM, December 1978, pp. 53-56), whose results are reproduced in the
graph shown as Figure 5.8. These figures were determined by measurement on a
simple FET amplifier stage, for various levels of applied NFB. For a system using
more than 15dB of NFB, the reduction in distortion is as would have been predicted
from the simple mathematical model, but for small amounts of NFB, up to, say, 10dB
(equivalent to a reduction in gain of 3.3"), the distortion could actually worsen due to
the increase in the magnitude of higher order components. This problem is more likely
to arise in valve operated than in transistor based amplifier systems, partly because it
is difficult to get high stage gains from single stage valve circuits, and partly because
of the difficulty in applying NFB around multiple stage amplifiers - especially those
which employ transformer coupling.
100
a~ 10-~
.o
2nd
t-
L__
0
3rd
1--
._
"0
"E 0.1--
4th
u
0
E
L
-60
dB
-100
0.001
Figure 5.8
i- -40
-80
-r 0.01
O.O001-~-40dB
_L-20
20dB
Amount of negative feedback
Effect of NFB on harmonic distortion
~--120
Ode
82
Valve and Transistor Audio Amplifiers
Practical Valve NFB Systems
Negative feedback circuit layouts are widely used as frequency response shaping
systems, using resistors and capacitors in networks chosen for their impedance vs.
frequency characteristics. Such a feedback path, whose input is derived from the
anode, can be returned either to the valve grid or to its cathode. By far the most popular
arrangement for this type of system is that shown schematically in Figure 5.9. In this
type of layout a single valve, usually a pentode, is operated under conditions chosen
to give a high stage gain, and with shunt connected NFB taken through some
frequency dependent network, (Z) from the anode to the valve control grid.
+V,
Output
input 0 . ~ | |
,-?
AIAA
tl
OV
R2
C2
R4
R6
C3
9 OOV,
Figure 5,9
Simple NFB arrangement
This arrangement would give an approximate stage gain of Z/R1. In early valve
preamplifier circuitry both the components forming the feedback network Z and the
input network, R1,C1, would be controlled by a multi-position wafer switch to allow
a range of different gain/frequency options to be selected to suit radio, or gramophone
pick-up, or tape recorder, or microphone inputs.
Although the alternative series NFB connection is possible, this usually means that the
feedback signal must be introduced into the cathode of the amplifier valve, which is a
very low impedance point, and this presents problems.
A very successful series feedback circuit arrangement, nicknamed the ring of three for
obvious reasons, and much favoured in physics labs for use as a high gain, linear, wide
bandwidth gain module is shown in outline form in Figure 5.10. With suitable
components this could give a gain of up to several hundred, with very little waveform
distortion and with gain/bandwidths flat from, say, 20Hz up to 20MHz or more. The
approximate gain of this circuit, which will be constant over a wide frequency band,
will be (R7 + R8)/R8.
Negative Feedback
i
R4
-
- -~0
+V,
C2
V3
OV
R2
83
Output
Input
'RS
C3
ovo
Figure
1
-
-.
~
T [
_
_
~
Oov,
5.10
The 'ring of three' feedback circuit
The Baxandall Negative Feedback Tone Control System
Contemporary fashion decrees that the user of Hi-Fi equipment should not attempt to
modify the frequency response of his or her equipment to increase or diminish the
relative loudness of any part of the audio frequency spectrum with respect to the
remainder, and that, instead, the best available fiat frequency response equipment
should be purchased, and the tonal rendering given by this should then be left alone.
However, in more practical, and perhaps more realistic, times it was accepted that
imperfections could indeed exist, and most of the better audio systems provided some
form of 'tone control' to allow the user to compensate for any tonal shortcomings in
the bass or treble output of his or her audio equipment.
Of all the circuits suggested for this purpose, by far the most popular - and adopted by
audio designers on a world-wide basis - was the method proposed by P.J. Baxandall
(Wireless WorM, October 1952, pp. 402-405) of which I have shown the basic layout
in Figure 5.11. I regret that, among the purists, even this excellent circuit is thought
likely to impair the quality of the final sound - even when the controls are adjusted so
that the circuit gives a fiat frequency response - and component arrangements are
favoured in which any desired adjustment to the frequency response is made only by
the use of passive components (such as resistors or capacitors) in the belief that these
cannot introduce any unwanted worsening of the sound quality.
Unfortunately, this argument is fallacious in that, for any passive tone control to work,
it must introduce an overall attenuation in its fiat frequency response mode which is at
least equal to the desired boost in gain at the desired frequency (treble or bass) relative
to its gain in the mid-range. So, in order for the overall gain of the system, including
84
Valve
and
Transistor
Audio
C3
RVl
-'-II
0.01uF
4
Amplifiers
CA
vD'V'E)
vlvv
' ~
11
I
. . . . .
O.OluF L
[47k
2k2
2k2
Cl
~'
47k
-
~ 4k7
- O Output
8k2
~u
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8k2
cs
I
/
IJ
li
/
*
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O'047uF I
c6
Jl
9
-
I
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RV2 (BASS)
100k
ovC
Figure
.......
-
+V
R2
.
0.47u F
0
-
F
-
-
-
9 _
-
-
-
~
~0
0v
5.11
Baxandall
NFB
Tone
Control
the control circuit, to be the same as it had been before the circuit was inserted, an
additional (distortion introducing) gain stage must now be used to make up for the loss
of gain due to the passive circuitry. Unfortunately, this amplifier stage is likely to
introduce more distortion than that of the equivalent stage within the feedback system,
because any distortion introduced by this will be reduced by the internal NFB within
the circuit. This argument applies equally to such circuit as filters or fixed frequency
response adjustment networks (i.e. for gramophone PU output equalisation) as it does
to tone controls.
To understand the operation of the Baxandall circuit it is convenient to postulate that
the two tone controls are set at their mid positions (in Baxandall's original circuit, the
potentiometer RV2 was centre-tapped, and this tap was connected to the 0V line, but
in normal practice, this refinement is omitted, since the circuit works quite well
without it). In this case, the input limb of the circuit (to V1 grid, which acts as a virtual
earth because the circuit employs shunt connected NFB) is R6,C3 and half RV1 in
parallel with R7,C5 and half RV2. The feedback limb is, similarly, R8,C4 and half
RV 1 in parallel with R9,C8 and half RV2. Clearly, at all audio frequencies, these two
limbs are equal in impedance and the circuit therefore has a gain of unity.
If the slider of RV 1 is moved to the left, then, at (treble) frequencies at which C3 and
C4 have a low impedance, the impedance of the left hand (input) limb of this network
will be less than that of the fight hand (feedback) one, and the circuit gain will be
increased up to a limit value of
Treble gain (or attenuation) - (2200 + 47 000)/2200 = 22.3
which is just over 26dB. Conversely, if the slider of RV1 is moved to the right, the
gain at treble frequencies will be decreased to an ultimate attenuation of-26dB.
Negative Feedback
85
A similar argument can be used to determine the limit increase in gain, or the limit
value of attenuation at those very low audio frequencies at which the impedance of C5
and C6 is much higher than the resistance of RV2. In this case, the bass gain will be
Bass gain (or attenuation) = 108 200/8200 = 13.2
which is just over 22dB. Intermediate settings of RV1 and RV2 will provide
intermediate levels of lift and cut of either treble or bass, and these adjustments can be
made simultaneously, and with very little cross influence. Also, the amplifier valve
will have an open loop gain of about 50, and this will reduce the closed loop distortion
(at mid-frequency range) to about 0.01% at 6V rms output.
By comparison with this, a similar gain stage following a passive tone control layout,
with NFB used to adjust its gain to, say, 25, will have a distortion (at mid-frequency
range), of about 0.2% at 6V rms output. The drawback in both cases is that any treble
boost, even in a distortion-free circuit, will enhance the amount of harmonics of the
signal as it increases the treble gain, relative to that of the waveform fundamental
frequency. On the other hand, treble attenuation will reduce the THD. An analogous
and very obvious phenomenon, within the experience of any user of vinyl gramophone
record discs, is that any use of bass boost will exaggerate the audible, low-frequency
rumble noise caused by imperfections in the turntable beatings.
Frequency Response Equalisation
In the early years of the manufacture of 78rpm emery-loaded shellac gramophone
record discs a variety of modifications were made to the frequency response of the
recorded signal in the hope that the inverse frequency response, used on replay to
recover a fiat frequency output from the recorded signal, would lessen the surface
noise of the disc, and increase the practicable recording levels. By the time the vinyl
(LP) gramophone record was introduced, in 1951, universal agreement had been
obtained on the type of record and replay characteristics. The agreed standard, for both
45rpm and 33rpm discs, is shown in Figure 5.12, and is generally known as the RIAA
response curve because it was proposed by a consultative committee from the Radio
Industries Association of America.
A typical gramophone record replay circuit, of which the characteristics have been
tailored to give an RIAA type replay frequency response, is shown in Figure 5.13. This
circuit gives a stage gain of 10 at 1kHz, and since the input signal is likely to be small,
a cascode layout is chosen to give the lowest noise practicable from a shunt feedback
circuit.
As seen from Figure 5.12 it is required to generate a gain/frequency response curve
which rises, below l kHz, at which frequency the relative gain is 0dB (unity) to +3dB
at 500.5Hz, and +17dB at 50.05Hz. Above lkHz the relative gain is required to
decrease from-3dB at 2121.5Hz to -20dB at 21.2 l kHz. This type of response curve
is produced by the two parallel RC groupings shown as R2C2 and R3C3 in the circuit
drawing, since the overall gain will be determined by the relative impedance ratios of
86
Valve and Transistor Audio Amplifiers
the two limbs R2C2 and R3C3 relative to R1C1. The method by which the required
component values can be calculated is given in my book The Art of Linear Electronics
(Linsley Hood, J. (1994), pp. 172-174, Butterworth-Heinemann), as is also a rather
fuller account (in Chapter 7) of the use of NFB in audio systems.
Relative
20-
gain (dB)
+ 17dB at 50.05Hz
f
10-
dB at 500.5Hz
Frequency
O-
10
!
I
50
100
-
'
~
lk
~
~
f
~
-3dB at 21;1.5Hz
-10-
i"
1Ok
'
I
20k
-2OdBat 21.215kHz
-20-
Figure
5.12
RIAA equalised frequency response
Filter Circuits Based on NFB
D.T.N. Williamson, in part of his celebrated series of articles describing a very high
quality audio amplifier and preamplifier (Wireless World, October/November 1949),
described two circuits in which he used a combination of a frequency selective
feedback arrangement and a parallel-T notch filter to provide a steep cut high pass
rumble filter and an equally steep cut, but switched frequency low pass filter which
would serve to reduce the amount of audible high frequency hiss resulting from the
emery powder loading of the shellac body of the contemporary 78rpm records.
Negative Feedback
87
+V
Output
560pF
180pF
1/2ECC88
PU Input
C1
R1
C4
0.1uF
47k
0.1uF
0V
C6
47uF
0V
Figure 5.13
Gramophone (RIAA) pick-up input stage
The basic layouts of these two filter types are shown in Figures 5.14a and 5.14b, and
the frequency response of the final circuitry incorporating these is shown in Figure
5.15. The method of operation of both of these filter circuits is the same: if NFB is
applied around a circuit in which there are two RC low pass networks operating at a
similar roll-off frequency, the action of the NFB will be to flatten the frequency
response up to some turnover frequency determined by the RC networks, beyond
which the output will fall at an ultimate rate of-12dB/octave.
In the case of the low pass circuit shown in Figure 5.14a, the two RC networks are
formed by R4,C1 and R7,C5. By introducing a parallel-T notch filter (R2,R3,C4 and
C2,C3,R5) into the signal path, and then taking the NFB point from the output of this,
the overall flatness of the frequency response is maintained, but the slope of the HF
roll-off beyond the turnover frequency is increased by the notch circuit to about
-40dB/octave.
The action of the high pass filter circuit of Figure 5.14b is essentially similar, except
that only the first of the two CR networks (C 1,R10 and C4,R7) is within the feedback
loop, and the T network is bridged to reduce the ultimate attenuation rate to about
-30dB/octave, which Williamson judged to be sufficient to remove rumble noises due
to turntable beating imperfections.
The detailed design of these circuits is given in a reprint of his original articles
(Williamson, D.T.N., Wireless World, August 1949-May 1952. Reprinted in 1990 by
Audio Amateur Publications, Inc., Peterborough, NH, USA).
An alternative example of valve operated preamplifier design, the Brimar SP55 stereo
preamp, is shown in Figure 5.16. This dates from the middle 1950s and is quoted in
88
Valve and Transistor Audio Amplifiers
+V
Input
C1
'
II
~
R2
....
R3
vvv
V1
vvv
,P
C2
Output
q
II
',R7
'IR 4
:
R5
==C4
m
._.
9.
...
1
J,
~.
Figure 5.14a
Low pass filter
0
._,
Input
C
+V
C4
-
II
'" ' JlO
R
'
_lL,, _ ,T.. . -]
ou,p,
T. "L 1"' l
OV
C
Figure 5.14b
High pass filter
:~_,
0
f
P//
]-~~
v
-~o~
10
20
100
ik
Frequency
Figure 5.15
Filter characteristics
1Ok
(Hz)
20k
lOOk
R11
t
INPUT
o
,- "~s & ~
,,,
o~
To~
R7
C6
.iiii~1M0~,
C3
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1
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RV1
C8
R15
R23
.1~176
~ C2
500k Treble
C1 100K
0.1uF4~R17
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91M5iC5 ":~....
,~R12~qlr-i
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100k _~/212AX7 o,,i~
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Sl
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- 4 H , ,,-,,,
~
Slb
OV
Input selector
Figure 5.16
The Brimar SP55 preamp
+ 250VDC
i
%R16
RV2
1~ I~ss .
Tonecontrols
19
~n~F
~)I
~ RV3
~R'11~
t ~100K out,,u,
45~ 1M0 N,,,.,.,,,&c.,,l I
3k3 5Ou
--
~'~ I
:.,.,~3k3 I
2,~
"
.
.
........
V3
j/212AX7
T_Lc16
50nF
4 b r ~ v ,~
1
;
.
.
.
! T":Trl
.
.
Rumblefilter
0 0V
90
Valve and Transistor Audio Amplifiers
the Brimar Valve and Teletube Manual Volume 9. This is a typical, and wellengineered, example of circuit design at a period when the situation with regard to
gramophone record replay facilities had stabilised with the more or less universal
adoption of 33 '/3 rpm LPs and 45 rpm EPs in place of the easily broken 78rpm shellac
discs. Because hiss on record replay was no longer a problem, steep-cut treble filters
of the type Williamson proposed were no longer much needed, and were seldom
offered. Also, with the new vinyl discs came an agreed (RIAA standard) replay
frequency compensation characteristic and an effective choice between high
performance and expensive electromagnetic pick-up cartridges, and inexpensive and
popular piezo-electric systems, usually based on a piezo-electric ceramic element.
For both of these cartridge types a standard load of 100k was adopted (contemporary
usage would favour a load resistance of 47k) which was supposed to constrain the
ceramic cartridge into an electromagnetic style of performance, for which, with both
types of input, the frequency response would be corrected by the network R9,C4,C5
in the anode-grid feedback path of V1. For all the other inputs provided, V 1 would act
as a fiat frequency response stage whose gain was determined by the input and
feedback networks, with the exception of the microphone input where no input stage
NFB was employed.
This allowed V1 to have a stage gain of about 200, and gave input sensitivities, for
maximum output:
Mag. p.u, 8mV, Ceramic p.u, 60mV, Radio/Tape recorder, 100mV and
microphone, 2.5mV.
The resistance network connected around V2 implements a Baxandall type feedback
tone control circuit, for which the quoted performance is:
Treble lift/cut, + 13dB at 10kHz, ref. l kHz.
Bass lift/cut, + 16dB at 50Hz, ref. l kHz.
The final stage provides an input channel balance facility coupled with a permanently
connected rumble filter, with a fiat frequency response down to the cut-off point
beyond which the gain would fall rapidly, with an ultimate attenuation rate of
-18dB/octave. This filter characteristic was generated by NFB through the two
feedback loops, C 14,R22 and C 16,R20,C 13, connected between anode and grid of V3.
A roll-off frequency of 28Hz was adopted in this design since it had by now become
generally accepted that there was little below 30Hz which was either part of the
musical content of the recording or capable of reproduction by anything but the very
best of LS systems.
The design figure for signal output, with tone and balance controls set at mid-position,
and the volume control set at its maximum output position, was 500mV, at which the
distortion figure would be less than 0.05%. The quoted signal/noise ratio for the disc
inputs was better than -55dB, and for the radio and tape inputs, better than -65dB.
Negative Feedback
91
Although modern design practice would allow some improvement on these
signal/noise figures, it is in this respect that the biggest difference exists between valve
and solid state technology, for which a disc signal/noise ratio of, say,-85dB would be
normal, and a flat frequency response auxiliary input position (such as provided for a
CD replay input) should offer signal/noise ratios of the order of-100dB.
CHAPTER 6
VALVE OPERATEDAUDIO POWER AMPLIFIERS
There was only a limited range of circuit options open to the audio amplifier designer
before the widespread availability and adoption of solid state semiconductor
components, and for this reason the variety of commercially successful designs
evolved for valve operated audio power amplifiers was also fairly limited. In this
chapter I have mainly concerned myself with those circuits produced by UK
manufacturers, or those designs offered both by the valve manufacturers' laboratories
and by independent designers for home construction.
By the early 1950s the expected standard of performance of a high quality audio
amplifier was that it should have an output power of at least 10 watts, that it should
have a full power bandwidth of at least 20Hz-20kHz, and that its harmonic distortion,
at 1kHz and full output power, would be better than 0.2%. In order to achieve this sort
of performance, it was necessary, in practice, that overall (output to input) NFB should
be used. It was not normally feasible to do this with the simple transformer coupled
layout shown in Figure 4.4 because the inevitable phase shifts, introduced within the
feedback loop by the two coupling transformers, would make the amplifier oscillate
continuously, except where the designer had chosen to use very s m a l l - and probably
unhelpful- feedback levels.
In the case of the designs using only one transformer (that for coupling the output
valves to the loudspeaker) it was possible to design the circuit to allow useful amounts
of NFB to be employed- a value for 13A~ of 20-26dB was a typical figure - provided
that the output transformer was well designed, and this meant that, for the designer,
the extent of the choices available to him were in the type of input amplifier stage
(triode, pentode, cascode or long-tailed pair), the type of phase splitter (split load,
floating paraphase, long-tailed pair or simple inverter), the type of output
valves,(pentode or beam-tetrode, straight or triode connected), whether an additional
amplifier stage (usually a push-pull pair of triodes) between the phase splitter and the
output stage was judged to be necessary and, finally, in the case of pentode or beamtetrode output valves, whether the output transformer had primary tapping points to
which the screened grids could be connected- the so-called ultra-linear or distributed
load type of connection.
In spite of the seemingly large number of different design layouts these choices
allowed, in practice, when one looked at the designs offered at the time, they did not
appear to differ greatly from one another, and I will examine the more notable
examples of these later in this chapter. Three designs, by McIntosh of the USA, Quad
94
Valve and Transistor Audio Amplifiers
of the UK, and Sonab of Sweden, did, however, strike off on directions of their own,
and are worthy of particular consideration.
The Mclntosh Amplifier
In the years preceding and following the 1939-1945 war, the McIntosh company of
the USA became notable, among the radio and radiogramophone equipment which
filled the British radio shops, for producing massive, immaculately engineered
equipment offering a superb performance at a high price, for those who wanted and
could afford the best. (The company is still in business, and its philosophy appears to
be unchanged). At a time when it was thought that amplifier output powers in the
range 7-15 watts, and THD figures in the range 0.5-1%, would be entirely adequate
for domestic audio purposes, McIntosh offered the circuit shown in Figure 6.1, which
had an output power of 50 watts, and a THD figure of 0.2% from 50Hz to 10kHz
(McIntosh, F.H., and Gow, G.J., Audio Engineering, December 1949).
I
+Ht2
+ HT
61.6G
NFB
Input
TR2
O u t p u t to LS
NFB
Figure 6.1
The Mclntosh system
There were three basic design problems to be solved in this application. The first of
these was that obtaining such high output power levels, even with fixed bias, entails
driving the output valves in class AB 1. This is a condition where the level of negative
grid bias is chosen so that one or other of the output valves will be driven into cut-off
at some part of the operating cycle. In normal circumstances this will lead to crossover distortion, due, in part, to the imperfect coupling between the two halves of the
output transformer primary, and to shitts in the input bias level due to the high output
impedance of the driver stage.
Valve Operated Audio Power Amplifiers
.....
95
Mclntosh solved both of these problems by using a trifilar wound output transformer,
in which there was very tight inductive coupling between the windings, and the
cathodes as well as the anodes of the output valves were coupled to the load. The final
problem, that of achieving satisfactory loop stability with two coupling transformers
within the loop, was solved in part by taking great care in the design of the coupling
transformers to achieve a high winding inductance at the same time as preserving low
levels of leakage reactance - the driver transformer is also trifilar wound (i.e. the
windings are made by laying the three wires of the separate windings alongside each
other and at the same time). The other method used to preserve NFB loop stability was
to derive the NFB lines from the cathode of each output valve to that of each input
valve rather than from the output to the loudspeaker. The latter arrangement is
preferable on technical grounds.
An additional contribution to the close coupling of the two halves of the output stage
is provided by connecting the output valve screening grids (G2) to the anodes of the
valves on the opposite side of the transformer.
The Williamson Amplifier
The period which saw the introduction of the Mclntosh amplifier was one of very great
activity on the part of audio amplifier designers, and a number of novel circuit designs
were published, of which some were offered specifically for the use of the home
constructor. Among these, by far the most celebrated, was the Williamson amplifier,
of which the circuit design is shown in Figure 6.2. This was designed by an
enthusiastic audiophile who was working for the GEC receiving valve development
laboratories at Hammersmith, London. His circuit, first described in a GEC internal
memo in 1944, was subsequently published in A p r i ~ a y 1947 (Williamson, D.T.N.,
Wireless WorM, April 1947, pp. 118-120; idem. May 1947, pp. 161-163).
The springboard for Williamson's design was the prospect of the renewed availability
of the GEC KT66 output beam-tetrode for general use. (This valve design had been
introduced in 1937, but, in the UK, its use in domestic applications during the
1939-1945 period of hostilities had been restricted due to the overriding priority given
to military requirements.) The KT66 beam-tetrode valve was particularly suited to
audio output use, in that its output waveform distortion, unlike that of an output
pentode in which the distortion was mainly third harmonic, was predominantly second
harmonic and, if the output valves were matched, this type of distortion could be
substantially reduced by the use of the output valves in push-pull. Moreover, by
connecting its G2 to the anode, the KT66 could be made to behave in a manner very
closely analogous to that of an output power triode, without any of the problems
associated with high power triode construction.
In his introduction to the Wireless Worm articles, Williamson examined the various
circuit options available to the audio amplifier designer, and concluded that for the
requirements of low harmonic distortion, negligible phase shift within the chosen
frequency band (10Hz-20kHz), good transient response and low output resistance to
be met, it was essential that the loudspeaker output transformer should be included
1.2 10H
. .LI
. . . 30H
..
, ~
--- .,,
._
,198
2~
,R7
'22k
I
(3BV)
___
I
R19
l ......
Rt3
47k
c2.rv~
VI
1/2~N7
,RI5
q00k
R21
1S(:R:'
q12
Rll
-i
'lO0k
c-
i
~9- -
!,,,
_
Figure 6.2
The Williamson amplifier (voltages in brackets are AC signal levels)
Ill
- _0.-[111
Valve Operated Audio Power Amplifiers
97
within the negative feedback loop. This, in turn, required that the transformer primary
incremental inductance should be greater than 100H, and the primary:primary and
primary:secondary leakage reactances should be less than 33mH. These figures were
calculated from the premise that in order to ensure loop stability the phase shift
introduced by this component should not exceed 90 ~ at 3.3Hz and 60kHz: a
requirement which was easier to meet if the anode impedance of the output valves was
not too high, so that the required primary:secondary turns ratio for the output
transformer would also not be too large. For a triode connected KT66, R a would be
2500~ and the optimum total primary to secondary turns ratio for a 15~ LS load
would be 25:1.
A triode valve, used as a low power voltage amplifier, has a very low level of nonlinearity distortion, especially if the output signal voltage is small in relation to the
total possible output voltage swing. In Williamson's circuit the output voltage swing
at the anode of the input valve, V 1, was only 2.7V peak, which is very small in relation
to the possible 100V swing at this point.
In order to minimise the LF phase shift within the amplifier, the anode of V 1 was
directly connected to the split-load phase splitter, V2, and since the DC voltage on V 1
anode was about + 100V, the voltage on V2 cathode would be about 105V and on its
anode would be about +200V. Since V2 is effectively a cathode follower, it will have
a signal gain very close to unity and negligible waveform distortion. Moreover, since
the same current flows through both R7 and R8, it must follow that if the load
impedances are identical, the signal voltages across both of these load resistors would
be identical.
Because of the relatively low gain from the input of V1 to the output of the phase
splitter, it was necessary to have a further gain stage between this and the output
valves, and here, again, a push-pull pair of triodes was used. Since the cancellation of
second harmonic distortion requires that the output valves are matched - both in
characteristics and in operating conditions- Williamson included preset controls
(RV1 and RV2 in Figure 6.2) to allow matching of both grid bias and signal drive
levels.
Since the final circuit structure now gave an adequate gain without the use of
(electrolytic) cathode bypass capacitors, these were omitted and this provided a small
amount of local negative feedback in each gain stage. This helped to improve the
gain/bandwidth product and the gain and phase linearity of these stages, and also
eliminated a component whose performance was sometimes less than desirable, as
well as being prone to long-term deterioration.
Provided that the specification of the output transformer, as given by Williamson, was
met, the performance of this amplifier was excellent, and was far ahead of any of its
competitors. The articles describing its circuit were reprinted on a world-wide scale,
and its performance served as a model to which other designers could aspire. Since it
was not easy to improve on this design in terms of performance, most of the
competitive designs sought to offer a higher output power than the 15 watts which the
98
Valve and Transistor Audio Amplifiers
Williamson gave, though, in the UK, many of the commercial designers judged that
12 watts would be adequate for all normal domestic purposes.
Distributed Load Systems
The dilemma which faced audio amplifier designers in their choice of output valves
was that of balancing output stage efficiency (and available output power) against
harmonic distortion. It was a matter of general observation among the audio
enthusiasts of this period that the type of sound quality given by a triode output stage,
which mainly generated second harmonic distortion, was preferable to the rather hard
or shrill reproduction which was typical of the output pentode, which mainly produced
third and fitch harmonics. Output beam-tetrodes, of the KT type, which tended more
to a triode distortion characteristic, sounded better than pentodes, but triodes were still
the enthusiasts' preference. (With modem beam-tetrodes or pentodes it was possible
to get something fairly closely approaching triode characteristics by connecting G2 to
the anode.) The problem was in output efficiency, which was only 27% for a push-pull
pair of triodes, as compared with nearly 40% for a pair of pentodes using the same HT
voltage and anode current.
Understandably, circuit designers began to explore ways in which an output beamtetrode (or pentode) valve could be made to give a higher output power while still
retaining the low distortion characteristic of the triode stage, and it was shown by
Hailer and Keroes (Audio Engineering, pp. 15-17, November 1951) that this could be
achieved by connecting G2 to a tap on the output transformer primary winding, as
shown in Figure 6.3. The effect of this method of transformer connection, in terms of
relative output power, output impedance and harmonic distortion, is shown
graphically in Figure 6.4. Hailer and Keroes termed this the ultralinear (U-L)
connection, and this raised the hackles of the linguistic purists, one of w h o m - I forget
w h o - likened it to the thirteenth chime of a crazy clock, which served only to cast
doubt on all that had gone before. Nevertheless the name stuck.
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Figure 6.3
The ultralinear connection
Valve Operated Audio Power Amplifiers
99
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50%.
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Figure 6.4
Distributed load effects
The practical effect of the U-L connection was to offer the designer a graded choice
between the virtues and drawbacks of triode or pentode outputs. In terms of distortion
and output (anode) impedance- low values of which made the design of high quality
output transformers easier- there was a continual improvement as one moved from
pentode to 100% triode, but for maximum output power for a given level of distortion,
the 25% tapping was the best. The final point of concern to the designer was that the
stage gain in the pentode connected mode was substantially higher than the triode, and
this would facilitate the design of the preceding stages of the amplifier. The best
compromise value for this was the 40% tap, and this was the ratio favoured for the
high quality end of the market.
Much attention was drawn to this technique by the publication by Hailer and Keroes
of a circuit which offered ultralinear operation as a means of improving the
performance of the Williamson amplifier (Audio Engineering, pp. 26-27, June 1952).
This prompted an indignant response from Williamson and Walker (the latter being
the head of the Acoustical Manufacturing Company, the design of whose Quad audio
amplifier used a similar, but allegedly superior, philosophy and had been on sale since
1945). Their response (Amplifiers and Superlatives, Williamson, D.T.N., and Walker,
P.J., Audio Engineering, pp. 75-81, April 1954) gives one of the more comprehensive
analyses of this topic, and compares U-L operation with cathode coupled systems of
the kind used by both Quad and Mclntosh.
The Quad System
This method of operation of the output valve is shown schematically in Figure 6.5.
This arrangement offered a number of practical advantages, among which was the fact
that the local negative feedback introduced into the cathode circuit, by a winding
which was about 10% of that in the anode, reduced the output stage distortion from
100
Valve and Transistor Audio Amplifiers
2-3%, a typical value for a triode connected output valve, to some 0 . 7 % - a figure
which would be reduced still further by overall NFB. It also offered an output stage
efficiency which was of the same order as that of an output pentode. Moreover, the
local NFB reduced the phase shift due to the output transformer and made the
specification for this component less stringent. The full circuit of the Quad 12 watt
power amplifier is shown in Figure 6.6. Since this design was offered commercially in
1945, it has the distinction of being one of the first true Hi-Fi power amplifiers, with
the NFB loop including the output transformer as well as the whole signal amplifying
chain.
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F i g u r e 6.5
The Quad circuit
There are a number of interesting (and surprising) features in the Quad II design,
which I have redrawn in Figure 6.6 so that these features are easier to see.
Superficially the circuit uses a single pentode amplifying stage followed by a simple
phase inverter of the kind shown in Figure 4.5, in which the ratio of R7 to R8 (R4:RVI
in Figure 4.5) is chosen to give the correct output drive voltage at V2 anode. However,
there are complications in that the NFB signal, from a tap on the output transformer
secondary, is applied to both of the input valve cathode circuits, and while this will
appear as negative feedback at V 1, it will be positive feedback when applied to V2.
PFB has the effect of increasing both the stage gain and the distortion of the stage to
which it is applied, and its use, within the NFB loop, to increase the loop gain and
thereby to reduce the overall distortion level has been employed many times.
However, in this instance, the increase in the gain of V2 makes it necessary to reduce
the input signal to V2 grid to a much lower level than would normally have been used.
The actual values of R7 and R8 would imply a stage gain of 253 for V2, which is
rather more than twice that which would have been expected.
Another unusual factor is the cross-coupling, by C 1, of the G2 grids of V 1 and V2,
instead of the more normal separate decoupling of both grids to the ground (0V) or
Valve Operated Audio PowerAmplifiers 1 0 1
cathode line. This, and the shared cathode bias resistor (R4), helps to restore the
equality of the two drive signals applied to the output valves.
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The Baxandall Amplifier
There was clearly a degree of rivalry between amplifier designers at this time, since,
shortly after Williamson had described his 15 watt design, in 1947, Baxandall
proposed in 1948 a circuit design for an amplifier offering a comparable standard of
quality at a lower c o s t - but also over a more limited frequency range (30Hz-15kHz)
and with a lower (10 watt) power output - (Baxandall, P.J., Wireless World, pp. 2-6,
January 1948). There were two innovative features shown in Baxandall's design, of
which the first was the decision to use pentode (or tetrode) valves throughout,
including a push-pull beam-tetrode output stage. Baxandall argued that the extra gain
obtained this way, for the same number of gain stages, would allow more NFB to be
applied, without instability, and this would reduce the hum and noise in the system and
would also allow a substantial reduction in the overall THD.
The second innovation proposed in this circuit was to derive the overall NFB signal
from a separate secondary winding on the output transformer. The absence of any
significant output load on this winding greatly reduced the internal phase shifts within
this part of the transformer and facilitated the use of quite a high level of NFB - up to
36dB, adjustable by RV3 - without the need for the elaborate and costly design
102
Valve and Transistor Audio Amplifiers
described by Williamson. Unfortunately this idea had been patented by C.G. Mayo, of
the BBC Engineering Research Dept., and any commercial manufacturer who sold
transformers to Baxandall's design would have to pay royalties to the BBC.
I have shown the circuit of the Baxandall 10W amplifier in Figure 6.7. In terms of its
circuit structure, it uses a pair of SP61 high slope pentode valves (V1 and V2) as a
floating paraphase phase splitter directly driving an output pair of 6L6s, V3 and V4.
These valves were the RCA (USA) equivalent of the Marconi-Osram KT66s and had
a somewhat lower permissible anode voltage rating and anode dissipation.
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The Baxandall a m p l i f i e r
Interestingly, Baxandall proposed the use of a square wave test signal as a means of
uncovering wide-band performance defects, and used this method, with a 50Hz input
signal, to show the very considerable improvement which NFB made to the overall LF
performance- a possible weak feature in any valve system. It is a pity, in retrospect,
that the performance of this amplifier was not tested also with a high frequency square
w a v e - say 1 0 k H z - while driving a reactive load, in that this would have allowed a
more direct comparison to be made between this amplifier and contemporary solid
state audio designs, which are often tested in this way.*
Baxandall had taken particular care in tailoring the loop phase shifts within this
design, and had chosen the values of the coupling capacitors (C2,C5) and those used
for screen decoupling (C3,C6) to constrain the amplifier LF phase lead to an
* My own measurements, in 1966, on two valve amplifiers, one o f commercial origin and one
o f my own design, showed that the handling o f a 10kHz square wave was relatively poor, even
on a resistive load. Unfortunately, although the memory remains, the oscilloscope photographs
from these tests are no longer available.
103
Valve Operated Audio Power Amplifiers
adequately low level. A similar function, at HF, was performed by the step network
(C1,R3) across the anode load resistor (R4) of V1, and the Zobel networks (R21,C9
and R22,C10) across the two transformer half primaries. The Zobel networks also
protected the output transformer (and output valves) against damaging high voltage
swings if the amplifier was over-driven, or operated into a very high impedance (or
open-circuit) load.
Unfortunately, although the more attractive features of this design were copied,
usually in a simplified form, by many amateur constructors, Baxandall's design did
not succeed in displacing the Williamson circuit as the leader in public esteem, or in
overall sound quality. The weak link, both in terms of the cost and complexity of the
output transformer, and in the amplifier performance as a whole, was, of course, the
fact that the NFB was drawn from a different winding from that which fed the
loudspeaker. This led to the possibility that if the loudspeaker load current was
distorted due to the non-uniformities of the moving coil magnetic s y s t e m - which
would usually be the case - the output signal from the amplifier would also be
distorted- because of the effective high output impedance of the power amplifier.
Baxandall noted this fact in two places in his article describing the design, but blamed
the fault on imperfections in the loudspeaker- as if there would ever be a perfect LS
unit.
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However, as I noted, this didn't prevent the general layout of his circuit being copied
by various amateur designers. An example of this is given in a circuit of my own,
shown in Figure 6.8. This was designed around a relatively inexpensive 20W. output
1 0 4
Valve and Transistor Audio Amplifiers
transformer made by the Wharfedale Wireless Works, which had rather poorer figures
for primary winding inductance and leakage reactance than Williamson's, but which
would nevertheless allow the circuit shown to work quite stably with about 15dB of
NFB if local feedback, around the output valves, (R16,R19) restricted their stage gain
to about 12". I made several of these amplifiers at the time (1951-1952), one of which
was made for a local music loving cabinet maker in exchange for a most magnificent
oak radiogram cabinet which he then built to my design.
Baxandall's 5 Watt Design
In March 1957, Baxandall returned to the field of audio amplifier design with a
simplified, relatively low power (5W) design, because he felt that there was a niche
for an inexpensive design intended for use with relatively efficient loudspeaker
systems (Baxandall, P.J., Wireless WorM, pp. 108-113, March 1957). In this design,
of which I have shown the circuit in Figure 6.9, Baxandall retained both the use of
output pentodes and a fairly high level of NFB to keep the residual hum level low, and
the total harmonic distortion (THD) below the 0.1% target level at 400Hz and 5 watts
output, though he now used the split load phase-splitter system (V2) proposed by
Williamson.
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The use of high levels of NFB with an output transformer of relatively simple
construction brought with it the probability of HF instability - which, if it occurred
while handling a speech or music signal, would lead to a greater degree of impairment
Valve OperatedAudio PowerAmplif!ers 1 0 5
in the audible quality than any possible improvement due to reduced steady-state
THD. To provide the very necessary correction to the poor HF loop stability,
Baxandall added a further output Zobel network (R22,C10) across the whole
secondary of the transformer, in addition to the networks (C8,R20, C10,R21) retained
across the output transformer half primaries. Further circuit refinements for the
purposes of HF NFB loop compensation were the lag-lead networks (C2,R8 + C3,R9)
interposed in the signal line between V 1 and V2.
The Radford Valve Amplifiers
These designs were marketed by Radford* Audio Ltd, of Long Ashton, Bristol, over
a period of nearly twenty years, and enjoyed a reputation among music lovers which
was second to none, mainly because of the very high quality of the output transformer
and the general attention to detail in components and design. They were normally
available in output power ratings from 7 to 30 watts, though units up to 100 watts
could be provided to special order. The circuit I have shown in Figure 6.10 is that of
their MA15 15 watt mono version, but the other designs were very similar in form.
In terms of its circuit structure, the output stage uses a push-pull pair of output power
pentodes, operating in class A, and coupled to an output transformer which is used in
a 40% U-L mode. The output valves are individually cathode biased and are driven by
an ECC83 double-triode long-tailed pair phase splitter. Negative feedback is taken
from the whole of the output transformer secondary winding to the cathode circuit of
V1 via a phase correcting network (R28,C17), of which the component values are
switched to suit the output load impedance chosen. (This is also switch selected, but I
have omitted the details of this in the interests of clarity.)
Local NFB is applied between anode and grid of V1, and additional loop phase
correcting networks are included across V1 anode load (R7,C3) and in the grid drive
to the output valves (R16,C8 and R17,C9). Apart from the primary inductance of the
transformer the only LF phase error inducing components are C 10,R 18 and C 11 ,R 19,
and this ensures loop LF stability.
*Arthur Radford, the owner and founder o f the firm, was, in his own view, a transformer
designer who 'dabbled a bit in electronics '. However, he had not come to terms with 'solid
state' electronics, and although his firm manufactured and sold transistor operated audio
power amplifiers, they were not o f his design, and the amplifier which he preferred, and used
in his own home, was the STA25 - a 25 watt stereophonic amplifier which was one o f the last
o f his own creations. I had the great privilege o f acquaintanceship with Mr Radford, and was
invited to his home on one occasion in the early 1970s so that we could do a comparative
listening trial between his own domestic STA25 and my recently designed 75 watt (Hi-Fi News)
transistor operated unit. I had expected that the two power amplifiers, which, f o r the purposes
o f the trial, were both driven from Radford's own valve preamp, would sound the same, and, so
far as I could tell, this was indeed the case. However, I was flattered to find that Mr Radford
thought that my design was superior in sound quality to his, and he then invited me, as a
consultant, to do some electronic circuit design work for him, which suggested that his
judgement was not just a compliment intended to please a guest: not that I thought that such an
act would have been in his nature.)
106
Valve and Transistor Audio Amplifiers
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The Leak TL/12
This circuit, which I have shown in Figure 6.11, was conservatively rated at 15 watts,
and used a circuit which was very similar to that of the Radford, the main differences
being in the capacitative - rather than direct - coupling between V1 anode and V2
grid, and that the output valves were triode (rather than U-L) connected. As in the
Radford an HF loop phase correction network (R23/C12) was included in the NFB
line, and, once again, the values of these components were switched to suit the output
transformer secondary ratio chosen. The only other loop compensation component
was the lnF capacitor across R6 in the cathode circuit of V1.
The Mullard 5/10 and 5/20 Designs
The Mullard 5/10, so described because it had an output power of 10 watts and used
five valves, including the rectifier, and its companion design with 20 watts output,
formed the third of this closely similar group of designs, and, because Mullard Ltd
was a wholly owned subsidiary of the Dutch Philips Electronics NV organisation,
these circuits could be taken as representing a broad consensus of European audio
amplifier design. Unlike the situation in the UK and the USA, beam-tetrode valves
were not used, output stages being based solely on pentode valve types.
The design I have shown in Figure 6.12 is the Mullard 5/10, a design which was
enormously popular among amateur constructors, and for which output transformers
Valve Operated Audio Power Amplifiers
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and mains transformers were available from a wide range of suppliers. The main
differences between the 10 and 20 watt units were that the higher powered version
used a higher HT line voltage, used EL34s rather than the smaller, pentode connected,
EL84s as the output valves, and that the output was U-L connected. Mullard noted that
the best performance - presumably in respect of output power - was with the screen
tapping points being taken at 20% from the centre, though they did give technical
details for the use of the 43% tapping connections, which was an operating mode
favoured by the average constructor and transformer supplier.
The quoted harmonic distortion for both these designs, at their rated output powers,
was in the range 0.3-0.4%, which, though entirely adequate for most amateur use, did
not compete, in the minds of the audio enthusiasts, with the less than 0.1% THD at full
output power which was claimed for the Williamson, the Radford or the Leak designs.
Full details of these and other Mullard audio designs were reprinted by Mullard in
1959 in a booklet Mullard Circuitsfor Audio Amplifiers and this was reprinted in 1993
by Audio Amateur Publications, of Peterborough, NH, USA.
The GEC Audio Amplifiers
A major milestone in audio amplifier design was the Williamson amplifier, of 1947,
and the British General Electric Company, as the owners of the Marconi/Osram valve
manufacturing plant were D.T.N. Williamson's employers. The celebrity which his
work had generated, even after Williamson had left the GEC and had joined Ferranti,
108
Valve and Transistor Audio Amplifiers
provided an incentive to their valve development laboratories to develop a range of
audio amplifier designs, of which the most significant, from my point of view, was a
30 watt U-L design shown in Figure 6.13, which was basically a somewhat modified
and simplified version of Williamson's 15 watt circuit.
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109
Valve Operated Audio Power Amplifiers
This amplifier could be built with KT66s, but gave a better performance, <0.2% THD
at 30 watts, if the (then) newly introduced KT88s were used as the output valves. With
either KT88s of the recently introduced KT77s (a redesign of the KT66) the power of
this amplifier could be increased to 60 watts by employing fixed bias rather than
cathode bias. This is done by simply removing the cathode bias resistors of V5 and V6
and returning their cathodes to the 0V rail, and then applying a negative grid voltage
of some 48 volts in value to the earthy ends of the grid resistors R14 and R15. In the
revised 60 watt circuit the negative grid bias for each output valve was obtained
separately from small preset potentiometers connected across a simple 55V negative
DC supply.
The GEC 912+ Amplifier
The period from 1950 to 1965 was one of great activity on the part of amateur audio
enthusiasts who tried out a wide variety of experimental ideas. The GEC company
reflected the mood of the moment and offered a metal cone loudspeaker driver unit,
and an economical and easy to build 12-14 watt audio power amplifier- the GEC
912+. The + indicated that it included a simple preamplifier with a gramophone record
replay frequency response equalisation stage and a passive tone control circuit. The
actual output power depended on the design and efficiency of the output transformer.
In the circuit which I have shown in Figure 6.14, I have deleted the input, single valve,
preamplifier and tone control circuitry, which would provide the normal input to V 1,
in the interests of clarity. I have also deleted the presence switch, in the NFB line, with
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110
Valve and Transistor Audio Amplifiers
which an RC series network could be switched across either R11 or R12, depending
on the effect sought.
In the view of the amateur constructor this amplifier was in direct competition with the
highly successful Mullard 5/10 design, with the GEC unit offering a somewhat greater
power, at a rather worse full output power (typically 0.8% THD) distortion figure. In
terms of circuit design, the GEC amplifier used a single triode input amplifier (V 1),
followed by a split load phase splitter (V2) which provided the drive for the output
(U-L connected) pentodes. The total loop feedback was not high, because of the
limited gain of the amplifying stages. This was the reason for the relatively poorer
THD figure for this design, but the greater loop stability then allowed 'presence
control' type tinkering with the frequency response.
Other GEC Audio Designs
It is noteworthy that GEC had produced a number of application reports during the
1950s which covered a wide range of valve amplifier designs in this field, from 3W to
1100W. Most of these designs were gathered together in a booklet, An approach to
audio frequency amplifier design, originally published in 1957, and reprinted in 1994
by Audio Amateur Publications, of Peterborough, NH, USA. In addition to these
design booklets GEC's valve development laboratories continued to produce
application reports relating to valve audio amplifier design up to 1979 - by which time
almost all the audio design effort had followed the engineering developments in
semiconductor technology into the field of the solid state.
It is somewhat to be regretted that amongst all this effort little novel circuitry emerged
from the GEC's audio engineers, except in the very high power designs, and these
were aimed at public address use, rather than high quality domestic audio applications,
and mainly were not of a type which would allow the use of overall loop NFB to attain
low levels of harmonic and intermodulation distortion, and mainly didn't employ
overall NFB.
The Brimar 25P1 Design
This is, in many ways, similar to the Leak TL 12 or the Radford STA 25 designs, but
has some interesting design features, and achieves a very good performance with a
relatively simple circuit layout. The quoted performance is 25 watts output at less than
0.1% THD, at lkHz, a bandwidth of 25Hz-20kHz, +0.2dB, an input sensitivity of
480mV for 25W output, and a hum and noise figure of >85dB referred to 20 watts
output. Other figures which are very good for a valve amplifier design are: Rise time
6~tS, Phase shift, <20 ~ at 25kHz, and Zou t <0.4f2 from 50Hz to 10kHz.
Structurally, the circuit consists of a high gain pentode input stage, with an HF phase
compensation network, C2,R24, across its anode load resistor. This drives a pair of
triodes, V2,V3, which act as a floating paraphase phase splitter, and which in turn
drive an output push-pull pair of beam-tetrodes, V4,V5. (The output valves specified
were STC type 5B255Ms, but these were closely similar to the contemporary KT66s,
A
- i
22k
C5
16uF
T
R12
27k
+ HT 440V
ill, ~
AIIL .d~.
/ v v v ~ ...._ _ -0
L1 1OH ~
/C
1
1
16uF
_..
T
OV
OV
C1
V4
op
C7
OOk
0.1uF
C6
m
1Ok
0.25uF
V2
... 112 1
Input
680R ~i-jC9
~150uF
~;;;;;3C10
,R5
;470
1Ok
OV
TR1
N
[~ LS
KT66
OV
R23 4K7
The Brimar 25P1 amp
15 ohms
680 R
,Rll
:e2o
,R6
:IOOR
6.15
V--T L Ls
,R15
6k8
R1
470k
Figure
TR1
Partridge P5352
r
_Jtc! 2__/
iilOOp F
.
.
.
.
.
.
.
.
.
112
Valve and Transistor Audio Amplifiers
which I have shown.) An innovative feature is the use of a resistor, R12, which is
common to both V2 and V3 anode circuits to derive the paraphase input signal to V3.
This leads to rather less loss of gain than the rather more common grid-derived
paraphase input signal. A further subsidiary NFB loop, which contributes a further
-10dB to the total, and assists in achieving a good loop stability, is drawn from V2
anode circuit by way of R9. The circuit shown is for a 15~ output load, and the values
of R23 and C12 are those appropriate for this load figure.
I consider this circuit to be an excellent example of valve audio design practice, along
with the Brimar SP55 preamplifier quoted in Chapter 5, which could well serve as
models to those designers wishing to retum to valve operated audio systems
The Sonab Design
Sadly, from the point of view of the historian, there have been relatively few genuinely
original valve circuit designs in the audio amplifier field, but the OA6 Mk 1 design
offered by Sonab, of Sweden, during the period 1966-1970, is a remarkable example
of circuit innovation. I have shown the basic layout of this design in Figure 6.16,
simplified by omitting the various NFB loops as well as the power supplies to the
second grids of the output pentodes. A somewhat unusual feature of the design is that
it requires a conducting path between its LS output terminals, and it will not work,
electrically, without such a low resistance output load. I have assumed that this will
always be the case for the purposes of the circuit analysis.
R18
1M5
,Rll
'220k
) R19
~'1M5
V 4 / r
EL86
/
C15
nn
16uF
Output to LS
V3
1/2 ECC83
V2~
1/2 ECC83
Vl
L1/2 ECC83
Input
0.68u F
Figure 6.16
The Sonab system
TR1
lk5
33n
R15
lk5
R25
300R
Valve OperatedAudio PowerAmplifiers 1 1 3
The output stage is connected as a White type cathode follower (see Jones, M., Valve
Amplifiers, Newnes, 1995, p. 91). In this layout, the output cathode follower, V4, feeds
an active load, V5, which is driven in phase opposition to V4. This phase inversion of
the signal is provided by an additional triode amplifier stage, V3, inserted between V2
and V4. The cathode potential of V4 is arranged to sit at about half the input HT line
voltage by taking its grid to a voltage divider network, R18/R19, which is connected
between V+ and the 0V line via the LS load. Because of the low impedance of this
output stage- approximately half that of V4 acting as a cathode follower on its o w n it is possible, with this layout, to drive a high impedance LS unit (say, 50-100 ohms),
or a pair of headphones, directly, without the need for an impedance matching output
transformer.
In the complete circuit of the OA6/1 design, shown in Figure 6.16, several further
innovative features are added. Firstly, the input valve, V 1, is operated in a virtual earth
configuration, with the input and NFB signals being summed at its grid. This shunt
type NFB is theoretically superior to the more common series type NFB, where the
feedback signal is returned to the input valve cathode, because it is free from any
possible common-mode errors. Secondly, both voltage mode NFB (taken across the
LS load, via NFB 1) and current mode NFB (derived, via NFB2, from the current
flowing through the LS load by a current transformer, TR1) signals are employed. In
addition, a third NFB path is taken directly from the amplifier output to the cathode
of V2.
Since Sonab felt that the availability of suitable high impedance LS units might be
limited, they provided, as the other half of the OA6 design, a mirror image circuit in
which only voltage feedback was used, and in order to provide a low Z output the
amplifier output was coupled to the load via a conventional output transformer. Since
the voltage NFB signal in the low Z half of this amplifier was taken from the primary
of the output transformer, this would leave the design open to the inherent fault
Baxandall found, in his design of Figure 6.7, that the inevitable loudspeaker load nonlinearities would distort the waveform at the amplifier o u t p u t - regardless of the
absence of distortion in the circuit of the amplifier itself.
By modern standards, the resistance values employed in this design, such as in the
input and NFB paths to V1, are very high, and would lead to a relatively high
background thermal noise level, as well as some DC level instability, due to valve
aging processes, as a consequence of high value grid leak resistances.
Increasing Available Output Power
As has been seen, the use of the U-L output transformer connection provides a simple
means for increasing the output power obtainable from a given design, but, with
suitable protection arrangements to prevent inadvertent operation of the system
without output grid bias, fixed bias operation can also substantially increase the
available output. For example, in the case of U-L connected KT77, at 430V HT, an
output power of 34 watts is quoted at 2.5% THD, while a similar fixed bias operation
will allow an output of 48 watts at 1% THD. For the KT88, the comparable figures, at
460V HT, are 50 watts and 80 watts respectively.
+ HT3
0
41-- NFB1
iuF
.
+ HT1
1Ok
, R18
33k
T
'1MS
|
C14
lk5
,R19
C15
16uF
J
Input
60pF
220k:
C1
R21
220k
33n
V5
41-NFB3
V1
\112 ECC83
1/2 ECC83
R26
+ HT2
lk5
R1 5
lk5
R25
300R
1MO
R3
lk5
ovo
Figure 6.17
The Sonab 0Q6/1 amp
R29
C17
27k
0.1uF
WV
R16
24k
R17
41-- N F B 2
OV
Valve OperatedAudio PowerAmplifiers 1 1 5
Conclusions
With hindsight it is easy to see the pitfalls into which some of the pioneering valve
amplifier designers fell. The need, for example, for the amplifier LS output to be taken
from the winding, and the whole of that winding, from which the loop NFB signal was
drawn, was not generally appreciated until the late 1950s, this allowed the situation in
which the amplifier could be virtually distortion flee, until the LS load was connected.
By the time of the Radford and Mullard designs it was accepted that the NFB signal
must be taken from the LS output connections, and if the transformer
primary:secondary ratio was changed to suit different LS loads then the attenuator
network (cf. R28 and C17 in Figure 6.10) in the feedback path must also be changed
to suit this new condition.
In a valve amplifier, the output transformer is a source of waveform distortion, so, for
an output THD of better than 0.1% over the bulk of the audible frequency spectrum,
overall loop NFB, including the output transformer, must be employed. The amount
of this which it was needful to employ would depend mainly on the quality of the
output transformer, and on the type of output valve used. For a well made LS
transformer, and triode connected output valves, this degree of linearity could be met
with some 20dB of NFB. For U-L connected beam-tetrodes, this performance would
require some 26dB of NFB, while pentode or beam-tetrode output valves, an NFB
ratio of nearer 30dB was needed. Beam tetrodes were, in general, slightly more linear
than pentodes.
In order to have an adequate gain margin to allow sensible amounts of NFB to be
employed- without the need for very high input signal voltages - either a driver stage
must be used between the phase splitter and the output valves, or a phase splitter with
both a high gain and a large permissible output voltage swing, such as a floating
paraphase or a long-tailed pair, is required.
Unless the required gain can be obtained with relatively few gain stages, the use of
NFB will lead to problems with HF and LF instability. Poor loop stability at the LF
end of the spectrum will probably cause motor-boating which will be audible. HF
instability may cause continuous oscillation at frequencies which are outside the
heating range, which will spoil the sound quality and may damage the amplifier or the
LS units. An insidious problem with poor loop stability margins is that the instability
may be sporadic, triggered by certain combinations of signal frequency, output signal
level and load reactance. This is most easily checked by the use of an oscilloscope, a
variable frequency square wave input and a range of non-resistive loads. A number of
HF/LF phase compensation networks have been shown in Figures 6.7, 6.9 and 6.10.
CHAPTER 7
SOLID STATE VOLTAGE AMPLIFIERS
The solid state device technologies which are available to the amplifier designer fall,
broadly, into three categories: bipolar junction transistors (BJTs), and junction diodes;
junction field effect transistors (FETs); and insulated gate F E T s - usually referred to
as MOSFETs (metal oxide silicon FETs), because of their method of construction.
These devices are available both in P t y p e - operating from a negative supply l i n e and N t y p e - operating from a positive line. BJTs and MOSFETs are also available in
small-signal and larger power versions, while FETs and MOSFETs are manufactured
in both enhancement-mode and depletion-mode forms. Predictably, this allows the
contemporary circuit designer very considerable scope for circuit innovation, by
comparison with electronic engineers of the past, for whom there was only a very
limited range of vacuum tube devices.
In addition, there is a very wide range of integrated circuits (ICs) which are complete
functional modules in some (usually quite small) individual packages. These are
designed both for general purpose use, such as operational amplifiers, and for more
specific applications, such as voltage regulator devices, current mirrors, current
sources, phase sensitive rectifiers, and an enormous variety of designs for digital
applications, which mostly lie outside the scope of this book.
In the case of discrete devices, I think it is unnecessary for the purposes of audio
amplifier design to understand the physical mechanisms by which the devices work
provided that their would-be user has a reasonable grasp of their operating
characteristics and limitations, and, above all, a knowledge of just what is available.
Junction Transistors
These are nearly always three-layer devices, fabricated by the multiple and
simultaneous vapour phase diffusion and etching of small and intricate patterns on a
large, thin slice of very high purity single crystal silicon. A few devices are still made
in germanium, mainly for replacement purposes, and some VHF components are made
in gallium arsenide, but these will not, in general, lie within the scope of this book.
The fabrication techniques may be based on the use of a completely undoped
(intrinsic) slice of silicon, into which carefully controlled quantities of impurities are
diffused through an appropriate mask pattern from both sides of the slice. These will
be described in the manufacturers' literature as double-diffused or triple-diffused and
so on.
118
Valve and Transistor Audio Amplifiers
In a later technique, evolved by the Fairchild Instrument Corporation, all the diffusions
were made from one side of the slice. These devices were called planar and had,
normally, a better HF response and more precisely controlled characteristics than, for
example, equivalent double-diffused devices. In a further, more recent, technique, also
due to Fairchild, the silicon slice will have been made to grow a surface layer of
uniformly doped silicon on the exposed side (which will usually form the base region
of a transistor) and a single diffusion was then made into this doped layer, to form the
emitter junction. This technique was called epitaxial and led to transistors with
superior characteristics, especially at HF. Since this is the cheapest BJT fabrication
process it will normally be used wherever it is practicable, and if no process is
specified it may reasonably be supposed to be a planar-epitaxial type.
Ic
.~lb
v
Figure 7.1
BJT non-linearity
hfe
Frequency
v
Figure 7.2
Decrease in hfe with frequency
119
Solid State Voltage Amplifiers
In contrast to a thermionic valve, which is a voltage controlled device, the BJT is a
current operated one. So while a change in the base voltage will result in a change in
the collector current this has a very non-linear relationship to the applied base voltage.
In comparison to this, the collector current changes with the input current to the base
in a relatively linear manner. Unfortunately, this linear relationship between I c and Ib
tends to deteriorate at higher base current levels, as shown in Figure 7.1. This
relationship between base and collector currents is called the current gain, and for AC
operation is given the term hfe, and its non-linearity is an obvious source of distortion
when the device is used as an amplifier. Alternatively, one could regard this lack of
linearity as a change Of hFE (this term is used to define the DC or LF characteristics of
the device) as the base current is changed. A further problem of a similar kind is the
change in hfe as a function of signal frequency, as shown in Figure 7.2.
However, as a current amplifier (which generally implies operation from a high
impedance signal source) the behaviour of a BJT is vastly more linear than when used
as a voltage amplifying stage, for which the input voltage/output current relationships
are shown for an NPN silicon transistor as line 'a' in Figure 7.3. (I have included, as
line 'b', for reference, the comparable characteristics for a germanium junction
transistor, though this would normally be a PNP device with a negative base voltage
and a negative collector voltage supply line.) By comparison with, say, a triode valve,
whose anode current/grid voltage relationships are also shown as line 'c' in Figure 7.3,
the BJT is a grossly non-linear amplifying device, even if some input (positive in the
case of an NPN device) DC bias voltage has been chosen so that the transistor operates
on a part of the curve away from the non-conducting initial region.
la/Ic
I -
5
4
4
-Vg (Volts)
f~
3
~
2
I
1
e
I
I
I
i
0.5
I
[
+ Vb (volts}
"~
I
!
~"
1.0
Figure 7.3
Comparative characteristics of valve, germanium and silicon based BJTs
Control of Operating Bias
There are three basic ways of providing a DC quiescent voltage bias to a BJT, which
I have shown in Figure 7.4. In the first of these methods, shown in Figure 7.4a, an
arrangement which is fortunately seldom used, the method adopted is simply to
120
Valve and Transistor Audio Amplifiers
connect an input resistor, R1, between the base of the transistor and some suitable
voltage source. This voltage can then be adjusted so that the collector current of the
transistor is of the fight order to place the collector potential near its desired operating
voltage. The snag with this scheme is that transistors vary quite a lot from one to
another of nominally the same type, so this would require to be set anew for each
individual device. Also, if the operating temperature changes, the current gain of the
device (which is temperature sensitive) will alter- and, with it, the collector current
of Q 1 and its working potential. The arrangement shown in Figure 7.4b is somewhat
preferable in that a high current gain transistor, or one working at a higher
temperature, will pass more current, and this will lower the collector voltage of Q 1,
+Vcc
R2
Output
Input
.C.1
]
i i ~ " Q1
+Vcc
]
R2
OV
(a)
Output
Input
+Vcc
R3
Output
Input
ql
C1
C2
0V
OV
Figure 7.4
Biasing circuits
(c)
C1
- ~,
(b)
OV
Solid State Voltage Amplifiers
1 21
which will, in turn, reduce the bias current flowing through R1. However, this also
provides NFB and will limit the stage gain to a value somewhat less than R1/Zin.
The method almost invariably used in competently designed circuitry is that shown in
Figure 7.4c, or some equivalent layout. In this, a potential divider (R1,R2) having an
output impedance which is low in relation to the base impedance of Q1, is used to
provide a fixed DC base potential. Since the emitter will, by emitter-follower action,
sit at a potential, depending on emitter current, which is about 0.6V below that of the
base, the value of R4 will then determine the emitter and collector currents, and the
operating conditions so provided will hold good for almost any broadly similar device
used in this position. Since the emitter resistor would cause a significant reduction in
stage gain, as seen in the equivalent analysis of valve cathode bias systems, it is
customary to bypass this resistor with a capacitor, C2, which is chosen to have an
impedance which is low in relation to R4 and R3.
Stage Gain
The stage gain of a BJT, used as a simple amplifier, can be determined from the
relationship:
Vout
hfeRL
Vin
R s +r i
where R s is the source resistance, R L is the collector load resistor, hfe is the smallsignal (AC) current gain, and r i is the internal emitter-base resistance of the transistor.
An alternative, and somewhat simpler, approach is similar to that which one would use
for a pentode valve gain stage, in which
gout/gin-- gmRL
where the gm of a typical modern planar epitaxial silicon transistor will be in the range
25-40mS per mA of collector current. Because the gm of the junction transistor is so
high, high stage gains can be obtained with a relatively low value of load resistor. For
example, a small-signal transistor with a supply voltage of 15V, a 4k7 collector load
resistor and a collector current of 2mA will have a low frequency stage gain, for a
relatively low source resistance, of some 300". If some way can be found for
increasing the load impedance, without also increasing the voltage drop across the
load, very high gains indeed can be achieved - up to 2500 with a junction FET acting
as a high impedance constant current load (see Linsley Hood, J., Wireless WorM,
September 1971, pp. 437--441).
A predictable, but interesting aspect of stage gain is that the higher the gain which can
be obtained from a circuit module, the lower the distortion in this which will be due to
the input device. This is so because if increasingly small segments are taken from any
curve they will progressively approach more closely to a straight line in their form.
This allows a very low THD figure, much less than 0.01% at 2V rms output, over the
122
Valve and Transistor Audio Amplifiers
frequency range 10Hz-20kHz, to be obtained from the simple NPN/PNP feedback
pair shown in Figure 7.5, which would have an open loop gain of several thousand.
The distortion contributed by Q2 will be relatively low because of the high effective
source resistance seen by Q2 base. A similar low level of distortion is given by the
amplifier layout (bipolar transistor with constant current load) described above,
because of the very high stage gain of the amplifying transistor and the consequent
utilisation of only a very small portion of its Ic/Vb curve.
+Vcc
C3
Q2
Input
C1
Output
RS
R4
R7
~-~, C2
OV
ov
F i g u r e 7.5
NPN/PNP feedback pair
Basic Junction Transistor Circuit Configurations
As in the case of the thermionic valve, there are a number of layouts, in addition to the
simple single transistor amplifier shown in Figure 7.4, or the two stage amplifier of
Figure 7.5, which can be used to provide a voltage gain or to perform an impedance
transformation function. There is, for example, the grounded base layout of Figure 7.6,
which has a very low input impedance, a high output impedance, and a very good HF
response. This circuit is far from being only of academic interest in the audio field, in
that it can provide, for example, a very effective low input impedance amplifier circuit
for a moving coil pick-up cartridge. I showed a circuit of this type, dating from about
1980, in an earlier book (Audio Electronics, Newnes, 1995, p. 133).
The cascode layout, described in Chapter 4, is also very widely used as a voltage
amplifier stage, using a circuit arrangement of the kind shown in Figure 7.7a. As in the
case of the valve amplifier stage, this circuit gives very good input/output isolation and
an excellent HF performance due to its freedom from capacitative feedback from
output to input. It can also be rearranged, as shown in Figure 7.7b, so that the input
stage acts as an emitter-follower, which gives a very high input impedance.
Solid State Voltage Amplifiers
123
+Vcc
R5
Output
Q2
C1
Inputo--~k
R6
R2
OV
OV
Figure 7.6
Grounded base stage
The long-tailed pair layout, shown in its simplest form in Figure 7.8a, gives a very
good input/output isolation, and also, since it is of its nature a push-pull layout, it gives
a measure of reduction in even-order harmonic distortion. Its principle advantage, and
the reason why this layout is normally used, is that it allows, if the tail resistor (R 1) is
returned to a - v e supply rail, both of the input signal ports to be referenced to the 0V
line- a feature which is enormously valuable in DC amplifying systems. The designer
may sometimes seek to improve the performance of the circuit block by the use of a
high impedance (active) tail, in place of a simple resistor, as shown in Figure 7.8b.
This will lessen the likelihood of unwanted signal breakthrough from the-ve supply
+Vcc
~
+Vcc
ql
Output
Input
Input,
Q2
Output
'R5
0V
OV
(a) Basic NPN/NPN cascode
Figure 7.7
Cascode layouts
OVq
OV
(b) Complementary NPN/PNP cascode
124
Valve and Transistor Audio Amplifiers
rail, as well as ensuring a greater degree of dynamic balance, and signal transfer,
between the two halves.
Although like all solid state amplifying systems it is free from the bugbears of hum
and noise intrusion from the heater supply of a valve amplifying stage- likely in any
valve amplifier where there is a high impedance between cathode and g r o u n d - it is
less good from the point of view of thermal noise than a similar single stage amplifier,
partly because there is an additional device in the signal line, and partly because the
gain of a long-tailed pair layout will only be half that of a comparable single device
gain stage. This arises because if a voltage increment is applied to the base of Q 1, then
Q 1 emitter will only rise half of that amount due to the constraint from Q2, which will
also see, but in opposite phase and halved in size, the same voltage increment. This
allows, as in the case of the valve phase splitter, a very close similarity, but in opposite
phase, of the output currents at Q 1 and Q2 collectors.
+Vcc
+Vcc
Output
Input
V ref 1
Output
Input
V ref
V ref 2
R1
0 ov
(a)
C
-- -
"
....
0
0v
(b)
Figure 7.8
Long-tailed pair layouts
Emitter-follower Systems
These are the solid state equivalent of the valve cathode follower layout, though
offering superior performance and greater versatility. In the simple circuit shown in
Figure 7.9 (the case shown is for an NPN transistor, but a virtually identical circuit,
but with negative supply rails, could be made with a similar PNP transistor) the emitter
will sit at a quiescent potential about 0.6V more negative than that of the base, and this
will follow, quite accurately, any signal voltage excursions applied to the base. (There
are some caveats in respect of capacitative loads, and I will explore these potential
problems under the heading of slew rate limiting.) The output impedance of this circuit
is low, because this is approximately equal to 1/gm, and the gm of a typical smallsignal, silicon BJT is of the order of 35mA/V (35mS) per mA of emitter current. So,
if Q 1 is operated at 5mA, the expected output impedance, at low frequencies, will be
1/(5"35) kilohms, or 5.7 ohms; a value which is sufficiently smaller than any likely
Solid State Voltage Amplifiers
1 2 5
-------0 +Vcc
Input o ~ Q
1
....
0 Output
RI
OVO-
-
00V
Figure 7.9
Emitter-follower
value for R1 that the presence of this resistor will not greatly affect the output
impedance of the circuit.
The output impedance of a simple emitter-follower can be still further reduced by the
circuit elaboration shown in Figure 7.10, known as a compound emitter-follower. In
this, the output impedance is lowered in proportion to the effective current gain of Q2,
in that, by analogy with the output impedance of an operational amplifier with overall
NFB, any change in the potential of Q1 emitter, brought about by an externally
impressed voltage, will result in an opposing change in the collector current of Q2.
This layout gives a comparable result to that of the Darlington pair, of two transistors,
in cascade, connected as emitter-followers, shown in Figure 7.11, except that the
arrangement of Figure 7.10 will only have an input/output DC offset equivalent to a
-0 +Vcc
R1
Input o ~ ~ Q 1
]
v
- -o Output
R2
OVO- ,,
Figure 7.10
Compound emitter-follower
o 0v
126
Valve and Transistor Audio Amplifiers
single emitter-base forward voltage drop, whereas the layout of Figure 7.11 will have
two, giving a combined quiescent voltage offset of the order of 1.3-1.5V.
Nevertheless, in commercial terms, the popularity of power transistors, internally
connected as a Darlington pair, mainly for use in the output stages of audio amplifiers,
is great enough for a range of single chip Darlington devices to be offered by the
semiconductor manufacturers.
+Vcc
In
q2
Output
R2
0vi
0v
Figure 7.11
Darlington pair
Thermal Dissipation Limits
Unlike a thermionic valve, the active area of a BJT is very small, in the range 0.5mm=
for a small signal device to 4mm= or more for a power transistor. Since the physical
area of the component is so small - this is a quite deliberate choice on the part of the
manufacturer because it reduces the individual component cost by allowing a very
large number of components to be fabricated on a single monocrystalline silicon slice
- the slice thickness must also be kept as small as possible - values of 0.15-0.5mm
are typical - in order to assist the conduction of any heat evolved by the transistor
action away from the collector junction to the metallic header on which the device is
mounted.
Whereas in a valve, in which the internal electrode structure is quite massive, and heat
is lost by a combination of radiation and convection, the problem of overheating is
usually the unwanted release of gases trapped in its internal metalwork, in a bipolar
junction transistor the problem is the phenomenon known as thermal runaway. This
can happen because the potential barrier of a P-N junction (that voltage which must be
exceeded before current will flow in the forward direction) is temperature dependent,
and decreases with temperature. Since there will be unavoidable non-uniformities in
the doping levels across the junction this will lead to non-uniform current flow through
the junction sandwich, with the greatest flow taking place through the hottest region.
If the ability of the device to conduct heat away from the junction is inadequate to
Solid State Voltage Amplifiers
1 2 7
prevent the junction temperature rising above permissible levels, this process can
become cumulative. This will result in the total current flow through the device being
funnelled though some very small area of the junction, and this may permanently
damage the transistor. This malfunction is termed secondary breakdown, and the
operating limits imposed by the need to avoid this failure mechanism are shown in
Figure 7.12. Field effect devices do not suffer from this type of failure.
Ic
Amperes
Current l i m i t
,,
10
-1-
~tion
limitabrryeakdown
limit
0.1-
~
0.01-
1.0
Devicevoltagelimit
,
10
Collector
! ,
voltage
100
~(V)
Figure 7.12
Bipolar breakdown limits
Junction Field Effect Transistors (JFETs)
These are, almost invariably, depletion mode devices- which means that there will be
some drain current at a zero applied gate-source potential. This current will decrease
in a fairly linear manner as the reverse gate-source potential is increased, giving an
operating characteristic which is, in the case of an N-channel JFET, very similar to that
of a triode valve, as shown in curve 'c' of Figure 7.3. Like a thermionic valve, the
operation of the device is limited to the range between drain (or anode) current cut-off
and gate (or grid) current. In the case of the JFET, this is because the gate-channel
junction is effectively a silicon junction diode - normally operated under reverse bias
conditions. If the gate source voltage exceeds some 0.6V in the forward direction, it
will conduct, which will prevent gate voltage control of the channel current.
P-channel JFETs are also made, though in a more limited range of types, and these
have what is virtually a mirror image of the characteristics of their N-channel
equivalents, though in this case the gate-source forward conduction voltage will be of
the order of-0.6V, and drain current cut-off will occur in the gate voltage range of +3
to +8V. Although Sony did introduce a range of junction FETs for power applications,
these are no longer available, and typical contemporary JFETs cover the voltage range
(maximum) from 15 to 50V, mainly limited by the gate-drain reverse breakdown
potential, and with permitted dissipations in the range 250-400mW. Typical values of
gm (usually called gfs in the case of JFETs) fall in the range of 2-6mS.
I 2 8
Valve and Transistor Audio Amplifiers
JFETs mainly have good high frequency characteristics, particularly the N-channel
types, of which there are some designs capable of use up to 500MHz. Modern types
can also offer very low noise characteristics, though their very high input impedance
will lead to high values of thermal noise if their input circuitry is also of high
impedance; however, this is within the control of the circuit designer. The internal
noise resistance of a JFET, R(n), is related to the gfs of the device by the equation:
R(n) ohms = 0.67/gfs
and the value of gfs can be made higher by parallelling a number of channels within
the chip. The Hitachi 2SK389 dual matched-pair JFET achieves a gfs value of 20mS
by this technique, with an equivalent channel thermal noise resistance of 33 ohms
(Linsley Hood, J., Low noise systems, Electronics Today International, July 1992, pp.
42-46).
Although JFETs will work in most of the circuit layouts shown for junction transistors,
the most significant difference in the circuit structure is due to their different biasing
needs. In the case of a depletion mode device it is possible to use a simple source bias
arrangement, similar to the cathode bias used with an indirectly heated valve, of the
kind shown in Figure 7.13. As before, the source resistor, R3, will need to be bypassed
with a capacitor, C2, if the loss of stage gain due to local NFB is to be avoided. As
with a pentode valve, which the junction FET greatly resembles in its operational
characteristics, the simplest way of calculating stage gain is by the relationship:
A
= gfs RL
The device manufacturers will frequently modify the structure of the JFET to linearise
its Vg/I d characteristics, but, in an ideal device, these will have a square-law
relationship, as defined by"
gfs = Id/Vg = Idss[1--(Vgs/Vgc)]=/Vg
I
Cl
Input 0~---~!
O+Vcc
R2
0 Output
L~Q1 -
IF
R1
ov o
Figure 7.13
Simple JFETbiasing system
C2
--
v
0~4
Solid State Voltage Amplifiers
1 2 9
For a typical JFET operating at 2mA drain current, the gfs value will be of the order of
1-4mS, which would give a stage gain of up to 40 if R2, in Figure 7.13, is 10kD. This
is very much lower than would be given by a bipolar junction transistor, and is the
main reason why they are not often used as voltage amplifying devices in audio
systems, unless their very high input impedance (typical values are of the order of
10'2D) or their high, and largely constant, drain impedance characteristics are
advantageous.
i1.
0 + Vcc
R3
0 Output
R2
Input
OV
Figure
I
_
_
oov
7.14
Bipolar/FET cascode
. 0 +Vcc
e~Output
Input
OV
Figure
OV
7.15
FET/FET cascode
The real value of the JFET emerges in its use with other devices, such as the
bipolar/FET cascode shown in Figure 7.14 or the FET/FET cascode layout of Figure
7.15. In the first of these, the use of the JFET in the cascode connection confers the
130
Valve and Transistor Audio Amplifiers
very high output impedance of the JFET, and the high degree of output/input isolation
characteristic of the cascode layout, coupled with the high stage gain of the BJT. The
source potential of the JFET (Q2) will be determined by the reverse bias appearing
across the source/gate junction, and could typically be of the order of 2-5V, and this
will define the collector potential applied to Q 1. A further common application of this
type of layout is that in which the cascode FET (Q2 in Figure 7.15) is replaced by a
high voltage BJT. The purpose of this arrangement is to allow a JFET amplifier stage
to operate at a much higher rail voltage than would be allowable to the FET on its own,
and this layout is often found as the input stage of high quality audio amps.
A feature which is very characteristic of the JFET is that for drain potentials above
about 3V the drain current, for a given gate voltage, is almost independent of the drain
voltage, as shown in Figure 7.16. Bipolar junction transistors have a high
Id (Ic) mA
4
BJT
-0.SV
3
-1 .nv
2
-1 .SV
1
0T
-2.nv
' "'2LSV Vd (Vc)
,=
o
1'o
2'0
I
30
Figure 7.16
Drain current characteristics of junction FET
A
+V
Q1
RV1
v
Figure 7.17
Current source layout
,
Solid State Voltage Amplifiers
131
characteristic collector impedance, but their Ic/Vc curve for a fixed base voltage, also
shown, for comparison, in Figure 7.16, is not as fiat as that of the JFET. The very high
dynamic impedance of the JFET resulting from this very fiat Id/Vd relationship
encourages the use of these devices as constant current sources, of the kind shown in
Figure 7.17. In this form the JFET can be treated as a true two-terminal device, from
which the output current can be adjusted, with a suitable JFET, over the range of
several milliamperes down to a few microamperes, by means of RV 1.
Insulated Gate FETs (MOSFETs)
These devices, usually called MOSFETs, are by far the most widely available, and
most widely used, of all the field effect transistors. They normally have a rather worse
noise figure than an equivalent JFET, but, on the credit side, they have rather more
closely controlled operating characteristics. The range of types available covers the
very small-signal, low working voltage components used for VHF amplification in
TVs and FM tuners (for which applications a depletion-mode dual-gate device has
been introduced which has very similar characteristics to those of an RF pentode
valve) to high power, high working voltage devices for use in the output stages of
audio amplifiers, as well as many other high power and industrial applications. They
are made both in depletion- and enhancement-mode forms (the former having gate
characteristics similar to that of the JFET, while the latter description refers to the style
of device in which there is normally no drain current in the absence of any forward
gate bias), in N-channel and P-channel versions, and, at the present time, in voltage
and dissipation ratings of up to 1000V and 600W respectively.
All MOSFETs operate in the same manner, in which a conducting electrode (the gate)
situated in proximity to an undoped layer of very high purity single-crystal silicon (the
channel), but separated from it by a very thin insulating layer, is caused to induce an
electrostatic charge in the channel, which will take the form of a layer of mobile
electrons or holes. In small-signal devices this channel is formed on the surface of the
chip between two relatively heavily doped regions, which will act, respectively, as the
source and the drain of the FET, while the conducting electrode will act as the current
controlling gate.
Although modem photolithographic techniques are capable of generating exceedingly
precise diffusion patterns, the length of the channel formed by surface masking
techniques in a lateral MOSFET will be too long to allow a low channel 'on'
resistance. For high current applications the semiconductor manufacturers have
therefore evolved a range of vertical MOSFETs. In these, very short channel lengths
are achieved by sequential diffusion processes from the surface, which are then
followed by etching a V or U shaped trough inwards from the surface so that the active
channel is formed across the exposed edge of a thin diffused region. Since this channel
is short in length its resistance will be low, and since the manufacturers generally
adopt device structures which allow a multiplicity of channels to be connected
electrically in parallel, channel 'on' resistances as low as 0.008f2 have been achieved.
132
Valve and Transistor Audio Amplifiers
Like a JFET, the MOSFET would, left to itself, have a square-law relationship
between gate voltage and drain current. However, in practice this is affected by the
device geometry, and many modem devices have a quite linear Id/Vg characteristic, as
shown in Figure 7.18 for an IRF520 power MOSFET.
Id (amperes)
IRF520
10
.
,+Vg (volts)
0
"'1
!
l
I
I'-
5
I '
1
I-
---~
10
Figure 7.18
Power MOSFET
The basic problem with the MOSFET is that of gate/channel over-voltage breakdown
- in which the thin insulating layer, of silicon oxide or silicon nitride, between the gate
electrode and the channel breaks down. If this happens the gate voltage will no longer
control the drain current, and the device is defunct. Because it is theoretically possible
for an inadvertent electrostatic charge, such as might arise in respect to the ground if
a user were to wear nylon or polyester fabric clothing and well-insulated shoes, it is
common practice in the case of small-signal MOSFETs for protective diodes to be
formed on the chip at the time of manufacture. These could be either Zener diodes or
simple junction diodes connected between the gate and the source or the source and
drain as shown in Figure 7.19.
Figure 7.19
Diode gate protection
Solid
State Voltage Amplifiers
1 3 3
In power MOSFETs these protective devices are seldom incorporated into the chip.
There are two reasons for this: firstly that the effective gate/channel area is so large
that the associated capacitance is high, and this would then require a relatively large
inadvertently applied static charge to generate a destructive gate/channel voltage
(typically >40V), and secondly that such protective diodes could, if they were
triggered into conduction, cause the MOSFET to act as a four-layer thyristor, and
become an effective electrical short-circuit. However, there are usually no
performance penalties which will be incurred by connecting some external protective
Zener diode in the circuit to prevent the gate/source or gate/drain voltage exceeding
some safe value, and this is a common feature in the output stages of audio power
amplifiers using MOSFETs.
IcAmperes
Current limit
10
n
0.1-
0.01
limit
Device
.
1.0
.
.
.
.
10
Collector voltage
I
voltage limit
~
100
!
Figure 7.20
Power MOSFETSOARlimits
Apart from the possibility of gate breakdown, which, in power MOSFETs, always
occurs at less than the maker's quoted voltage except at zero drain current, MOSFETs
are quite robust devices and the safe operating area rating (SOAR) curve of these
devices, shown for a typical MOSFET in Figure 7.20, is free from the threat of
secondary breakdown whose limits are shown, for a power BJT, in Figure 7.12. The
reason for this freedom from localised thermal breakdown in the MOSFET is that the
mobility of the electrons (or holes) in the channel decreases as the temperature
increases, and this gives all FETs a positive temperature coefficient of channel
resistance.
Although it is possible to propose a mathematical relationship between gate voltage
and drain current, as was done in the case of the JFET, with MOSFETs the
manufacturers tend to manipulate the diffusion pattern and construction of the device
to linearise its operation, which leads to the type of performance (quoted for an actual
device) shown in Figure 7.21. However, as a general rule, the gfs of a MOSFET will
increase with drain current, and a forward transconductance (slope) of 10S/A is quoted
for an IRF 140 at an I D value of 15 amperes.
134
Valve and Transistor Audio Amplifiers
Id (Amperes)
I
IRFF11025 ~ C
4
+TV
3-
+6"V
2
+5V
Ilif
1~~/
0
....
+_4v _
Vd (Amperes)
0
10
20
30
40
50
Figure 7.21
MOSFET characteristics
Power BJTs vs. Power MOSFETs as A m p l i f i e r O u t p u t Devices
Some rivalry appears to have arisen between audio amplifier designers over the
relative merits of bipolar junction power transistors, as compared with power
MOSFETs. Predictably, this is a mixture of advantages and drawbacks. Because of the
much more elaborate construction of the MOSFET, in which a multiplicity of parallel
connected conducting channels is fabricated to reduce the conducting 'on' resistance,
the chip size is larger and the device is several times more expensive both to produce
and to buy. The excellent HF characteristics of the MOSFET, especially the N-channel
V and U MOS types, can lead to unexpected forms of VHF instability- which can, in
the hands of an unwary amplifier designer, lead to the rapid destruction of the output
devices. On the other hand, this excellent HF performance, when properly handled,
makes it much easier to design power amplifiers with good gain and phase margins in
the feedback loop, where overall NFB is employed. By contrast, the relatively sluggish
and complex characteristics of the junction power transistor can lead to difficulties in
the design of feedback amplifiers with good stability margins.
Also, as has been noted, the power MOSFET is intrinsically free from the problem of
secondary breakdown, and an amplifier based on these does not need the protective
circuitry which is essential in amplifiers with BJT output devices if failure is to be
avoided when they are used at high power levels with very low impedance or reactive
loads. The problem here is that the protective circuitry may cut in during high
frequency signal level peaks during the normal use of the amplifier and this can lead
to audible clipping. (Incidentally, the proponents of thermionic valve based audio
amplifiers have claimed that the superior audible qualities of these, by comparison
with transistor based designs, are due to the absence of any overload protection
circuitry which could cause premature clipping, and to their generally more graceful
behaviour under sporadic overload conditions).
Solid State Voltage Amplifiers
135
A further benefit enjoyed by the MOSFET is that it is a majority carrier device, which
means that it is free from the hole-storage effects which can impair the performance
of power junction transistors and make them sluggish in their turn-off characteristics
at high collector current levels. However, on the debit side, the slope of the Vg/Id curve
of the MOSFET is less steep than that of the Vb/I c curve of the BJT and this means
that the output impedance of power MOSFETs used as source followers is higher than
that of an equivalent power BJT used as an emitter follower. Other things being equal,
a greater amount of overall negative feedback (i.e. a higher loop gain) must therefore
be used to achieve the same low amplifier output impedance with a power MOSFET
design than would be needed with a power BJT one. If a pair of push-pull output
source/emitter followers is to be used in a class AB output stage, more forward bias
will be needed with the MOSFET than with the BJT to achieve the optimum level of
quiescent operational current, and the discontinuity in the push-pull transfer
characteristic will be larger in size, though likely to introduce, in the amplifier output
signal, lower rather than higher order crossover harmonics.
U and D MOSFETs
I have, so far, lumped all power MOSFETs together in considering their performance.
However, there are, in practice, two different and distinct categories of these, based on
their constructional form, and these are illustrated in Figure 7.22. In the V or U MOS
devices - these are just different names for what is essentially the same system,
depending on the profile of the etched s l o t - the current flow, when the gate layer has
been made sufficiently positive (in the case of an N-channel device) to induce a mobile
electron layer, will be essentially vertical in direction, whereas in the D-MOS or
T-MOS construction the current flow is T shaped from the source metallisation pads
across the exposed face of the very lightly doped P region to the vertical N-/N+ drain
sink. Because it is easier to manufacture a very thin diffused layer (= short channel) in
the vertical sense than to control the lateral diffusion width, in the case of a T-MOS
device, by surface masking, the U-MOS devices are usually much faster in response
than the T-MOS versions, but the T-MOS equivalents are more rugged and more
readily available in complementary (N-channel/P-channel) forms.
All power MOSFETs have a high input capacitance, typically in the range
500-2500pF, and since devices with a lower conducting resistance (Rds/on) will have
achieved this quality because of the connection of a large number of channels in
parallel, each of which will contribute its own element of capacitance, it is
understandable that these low channel resistance types will have a larger input
capacitance. Also, in general, P-channel devices will have a somewhat larger input
capacitance than an N-channel one. The drain/gate capacitance- a factor which is very
important if the MOSFET is used as a voltage amplifier- is usually in the range
50-250pF. The turn-on and turn-off times are about the same (in the range 30-100nS)
for both N-channel and P-channel types, mainly determined by the ease of applying or
removing a charge from the gate electrode. If gate-stopper resistors are used - helpful
in avoiding UHF parasitic oscillation, and avoiding latch-up in audio amplifier output
source-followers - these will form a simple low-pass filter in conjunction with the
device input capacitance, and will slow down the operation of the MOSFET.
136
Valve and Transistor Audio Amplifiers
GATE
SOURCE
Si 0 2
N+
N-
NN+
DRAIN
V or U MOS
SOURCE
Si 0 2
GATE
I.
N+
DRAIN
T or D MOS
Figure 7.22
MOSFET design styles
Although circuit designers tend to be rather lazy about using the proper symbols for
the components in the designs they have drawn, enhancement-mode and depletionmode MOSFETs should be differentiated in their symbol layout, as shown in Figure
7.23. As a personal idiosyncracy I also prefer to invert the symbol for P-channel fieldeffect devices, as shown, to make this polarity distinction more obvious.
N-ch JFET
P-ch JFET
n-ch depletion
MOSFET
N-channel enhancement
MOSFET
Figure 7.23
MOSFET symbols
P-ch depletion
MOSFET
P-channel enhancement
MOSFET
Solid State Voltage Amplifiers
1 3 7
Useful Circuit Components
By comparison with the situation which existed at the time when most of the
pioneering work was done on valve operated audio amplifiers, the design of solid-state
amplifier systems is greatly facilitated by the existence of a number of circuit artifices,
contrived with solid state components, which perform useful functions in the design.
I have shown a selection of the more common ones below.
Constant current (CC) sources
A simple two-terminal CC source is shown in Figure 7.17, and devices of this kind are
made as single ICs with specified output currents. By comparison with the discrete
JFET/resistor version, the IC will usually have a higher dynamic impedance and a
rather higher maximum working voltage. In power amplifier circuits it is more
common to use discrete component CC sources based on BJTs, since these are
generally less expensive than JFETs and will be available higher working voltages.
The most obvious of these layouts is that shown in Figure 7.24a, in which the
transistor, Q 1, is fed with a fixed base voltage- in this case derived from a Zener or
avalanche diode, though any suitable voltage source will s e r v e - and the current
through Q 1 is constrained by the value chosen for R 1, in that if it grows too large the
base-emitter voltage of Q 1 will diminish, and Q 1 output current will fall. Designers
seeking economy of components will frequently operate several current source
transistors and their associated emitter resistors (as Q1/R1) from the same reference
voltage source.
i
V ref
ou~
o
i out
V ref
i out
R2
i ref
Q2
..... Oov
(a)
~
(b)
ov
--
-
'-00V
(c)
Figure 7 . 2 4
Constant current sources
In the somewhat preferable layout shown in Figure 7.24b, a second transistor, Q2, is
used to monitor the voltage developed across R1, due to the current through Q 1, and
when this exceeds the base emitter turn-on potential (about 0.6V) Q2 will conduct and
will steal the base current to Q1 provided from Vref through R2. In the very simple
layout shown in Figure 7.24c, advantage is taken of the fact that the forward potential
138
Valve and Transistor Audio Amplifiers
of a P-N junction diode, for any given junction temperature, will depend on the current
flow through it. This means that, if the base emitter area and doping characteristics of
Q 1 are the same as those for the P-N junction in D 1 (which would, obviously, be easy
to arrange in the manufacture of ICs) then the current (iout) through Q 1 will be caused
to mimic that flowing through R1, which I have labelled iref. This particular action is
called a current mirror, and I have shown several further versions of these in Figure
7.25.
i ref
i out
i out
+V
i ref
Symbols
(d)
Constant
current s o u r c e
R2
-
..
O0v
(a)
_
(b)
j
0 0v
' ..(c)
O0v
(e)
Current
mirror
Figure 7.25
Current mirror circuits
Current mirror (CM) layouts
This allows, for example, the output currents from a long-tailed pair to be combined,
which increases the gain from this circuit. In the version shown in Figure 7.25a, two
matched transistors are connected with their bases in parallel, so that the current flow
through Q2 will generate a base-emitter voltage drop which will be precisely that
which is needed to cause Q 1 to pass the same current. If any doubt exists about the
similarity of the characteristics of the two transistors, as might reasonably be the case
for randomly chosen devices, equality of the two currents can be assisted by the
inclusion of equal value emitter resistors (R1,R2) as shown in Figure 7.25b. For the
perfectionist, an improved three-transistor current mirror layout is shown in Figure
7.25c. Commonly used circuit symbols for these devices are shown in Figures 7.25d
and 7.25e.
Circuit Oddments
Several circuit modules have found their way into amplifier circuit design, and I have
shown some of the more common of these in Figure 7.26. Both the DC bootstrap,
shown as 7.26a, and the JFET active load, shown as 7.26b, act to increase the dynamic
impedance of R1, though the DC bootstrap- which can, of course, be constructed
using complementary devices- has the advantage of offering a low output impedance.
The amplified diode, shown as Figure 7.26c, is a device which is much used as a
means of generating the forward bias required for the transistors used in a push-pull
Solid State Voltage Amplifiers 1 3 9
+V
Input
+V
l
Output
~
Output
R2
Oov
(a) DC bootstrap
(b) JFET active load
+V
=
=ll=
(~
PN
'
Output
PNP
P
p
OV
(c) Amplified diode
(d) Offset cancelling
emitter-follower
Figure 7.26
Circuit oddments
Input~
Output
a
ov C
Input
Figure 7.27
Cause of slew rate limiting
Input ~
. . L C load
T_o0v
_/
Output from a
,
~Output
load
o
~ o
Output from b
14 0
Valve and Transistor Audio Amplifiers
pair of output emitter followers, particularly if it is arranged so that Q 1 can sense the
junction temperature of the output transistors. It can also be used, over a range of
relatively low voltages, as an adjustable voltage source, to complement the fixed
voltage references provided by Zener and avalanche diodes, band-gap references (IC
stabilisers designed to provide extremely stable low voltage sources) and the wide
range of voltage stabiliser ICs. Finally, when some form of impedance transformation
is required, without the Vbe offset of an emitter follower, this can be contrived as
shown by putting two complementary emitter followers in series. This layout will also
provide a measure of temperature compensation.
Slew Rate Limiting
This is a potential problem which can occur in any voltage amplifier or other signal
handling stage in which an element of load capacitance (which could simply be circuit
stray capacitance) is associated with a drive circuit whose output current has a finite
limit. The effect of this is shown in Figure 7.27. If an input step waveform is applied
to the network 'a', then the output signal will have a waveform of the kind shown at
'a', and the slope of the curve will reflect the potential difference which exists, an any
given moment, between the input and the output. Any other signal which is present at
the same time will pass through this network, from input to output, and only the high
frequency components will be attenuated.
On the other hand, if the drive current is limited, the output waveform from circuit
7.27b will be as shown at 'b' and the slope of the output ramp will be determined only
by the current limit imposed by the source and the value of the load capacitance. This
means that any other signal component which is present, at the time the circuit is
driven into slew rate limiting, will be lost. This effect is noticeable, if it occurs, in any
high quality audio system, and gives rise to a somewhat blurred sound- a defect which
can be lessened or removed if the causes (such as too low a level of operating current
for some amplifying stage) are remedied. It is prudent, therefore, for the amplifier
designer to establish the possible voltage slew rates for the various stages in any new
design, and then to ensure that the amplifier does not receive any input signal which
requires rates of change greater than the level which can be handled. A simple input
integrating network of the kind shown in Figure 7.27a will often suffice.
CHAPTER 8
EARLY SOLID STATE AUDIO AMPLIFIER DESIGNS
Towards the end of the 1950s, transistors ceased to be merely an interesting laboratory
curiosity, and emerged into the component catalogues as valid and usable devices, for
a number of low power and low voltage applications. However, few people seriously
considered them as possible competitors for valves in higher power applications, or
possible options for use in high quality audio systems. There were a number of good
reasons for this, of which the major one was that transistors, at that time, mostly meant
germanium PNP diffused junction devices, and these were very temperature sensitive,
both in respect of the output (collector) current for a given forward bias, and in respect
of the electrical characteristics of the device itself. This latter problem arose because,
if the diffused junction regions within the transistor got sufficiently hot, say > 180~
the diffusion processes, by which they were manufactured, would continue, and the
internal junction boundaries would shift.
At this time, a germanium transistor would be made by taking a small, thin wafer of
single crystal germanium containing trace quantities of arsenic or antimony, as an
N-type impurity, and then heating the wafer so that two small pellets of i n d i u m previously spot welded facing each other on opposite sides of the w a f e r - would cause
P-type impurity diffusion zones to move in towards each other from opposite sides of
the wafer, as shown in Figure 8.1, to form a PNP device. Since the high frequency
response of the device, its current gain and its breakdown voltage would depend on the
thickness of the zone (the base junction) separating these two regions, if this changed
during use it would cause the circuit performance to change also. (In principle, an
NPN transistor could also be made by the same technique, and would, presumably,
suffer from the same snags. However, neither arsenic nor a n t i m o n y - the only
practicable N-type electron donor materials - were found to behave very satisfactorily
as alternatives to indium.)
Bp
,.E
Figure 8.1
Diffused junction transistor
142
Valve and Transistor Audio Amplifiers
Because of their small size and low supply voltage requirements, germanium PNP
devices offered, within the constraints imposed by their sensitivity to heat, a number
of advantages for use in things like heating aids and portable radios. For reasons of
economy in power consumption, and the need to avoid much heat dissipation in the
output transistors, a typical circuit for a low power audio amplifier- having, say, an
output power of 500mW - would use a circuit of the kind shown in Figure 8.2, in
which the output devices were operated in class AB, with a very low quiescent
collector current. Inevitably, this arrangement led to a fairly high level of crossover
distortion, and because the circuit used both driver and output transformers, it would
be difficult to employ a useful amount of NFB to reduce its distortion, but perhaps, as
the output stage for a portable radio, its performance would be thought to be adequate.
,r
~, -gv
ii2k2
R1
1Ok
I
t120R
IS.
.3
ohms
i
TR1
Signal in
C1
c5
~3
4u7F
250nl
t
OV
)C81
,~01
.R2
~8k2
p
t
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. . . .
9m C 6
C ~ lO00uF
9.~2
: :R3 , ,R7
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OU
~" R9
~
uF:I
OV
Figure 8.2
Transformer coupled amp
However, the circuit of Figure 8.2 is a very simple one, and much better designs are
possible, as, for example, in the 15 watt power amplifier shown in Figure 8.3, due to
Mullard Ltd (Reference Manual of Transistor Circuits, 2nd edition, 1961, pp.
178-180). In this, by rearranging the circuit somewhat, it has proved possible to
dispense with the output LS coupling transformer entirely, so that if the driver
transformer is reasonably well designed, it will be possible to employ a small measure
of overall NFB, from LS output to signal input, to reduce the distortion and make the
frequency response of the circuit more uniform. The quoted performance for this
design was: THD <4%, bandwidth 150Hz-7kHz, +l.5dB. The way that NFB is
applied to the power amplifier, as shown, makes the overall gain dependent on the
ratio of R7 to the external circuit impedance seen at Q 1 base.
Early Solid State Audio Amplifier Designs
143
-28V
Ro
ib~
15R
4.
k
SR
0C35
lO00uF
R8
0.5R
bR7
:2k2
~ TH1
Input
C6
2.2uF
P~lOOOuF
LS Output 4 ohm
OV
Figure 8.3
1 5 watt audio amplifier
The Lin Circuit
As has been seen in the case of the valve audio amplifiers described above, the output
transformer is a bulky and expensive component whose performance is likely to have
a crucial effect on the overall performance of the amplifier design. Since the output
impedance of a transistor, used as an emitter-follower, could well be less than an ohm,
it should be possible to implement the basic audio amplifier structure shown in Figure
4.1 (a variable gain voltage amplifier stage driving the loudspeaker via some means of
impedance conversion) by the use of a pair of push-pull emitter-followers, rather than
a transformer, as the output impedance-matching mechanism.
However, there was at this time an additional requirement- that these output emitterfollowers should be based on output transistors of the same polarity. This need arose
because although a few NPN transistors were available these were all small-signal
types, and were quite unsuitable for use as one half of an output emitter-follower. All
of these problems appeared to have been solved at a stroke by the ingenious circuit
layout proposed by Lin (Lin, H.C., Electronics, pp. 173-175, September 1956), and is
shown in Figure 8.4. In this, the output emitter-followers were what Lin termed a
quasi-complementary pair, in which the upper half (Q2,Q4) was a conventional
Darlington pair and the lower half (Q3,Q5) was a compound emitter-follower. This
1 4 4
Valveand Transistor Audio Amplifiers
allowed the output voltage, at C4, to follow the amplified signal voltage at the
collector of Q 1, but at a low enough impedance to drive a 16 ohm LS directly.
0
+Vcc
- .
R1
lk0
R4
.
A~IIL_
A.
i .
T50uF
n q, i
.
2k7
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R7
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PNP
R3
2k7
0V
o--
.
LSOutput
.
.
.
.
,
0V
o
Figure 8.4
Audio amplifier due to Lin
27k
The performance of Lin's design (THD <1% at 400Hz at 6W output, bandwidth of
30Hz-15kHz, + 1.5dB), while not yet as good as could be obtained from a run of the
mill valve audio amplifier, nevertheless offered a workable design option. As would
be expected, the stability of the voltages and currents in this design was not very good,
though Lin had employed DC NFB, via R2, in addition to the AC NFB through R9
and C5, to hold the collector voltage of Q1 to a suitable value. The forward bias
applied to the output quasi-complementary emitter-followers, which would need to be
about 0.2V, at 25~ was provided by the voltage drop across R5, caused by the
collector current of Q 1, and an attempt was made to compensate for changes in the
junction temperature of the output transistors by connecting a negative temperature
coefficient thermistor (TH1) across R5.
As can be seen from Figure 8.5, germanium junction transistors have a less abrupt
turn-on characteristic than silicon ones, and since the output devices will probably run
warm, their actual Vb/I c graph is more likely to be that of the dashed line curve than
the solid line, 20~ o n e - and this will act to lessen the magnitude of the potential
crossover-type discontinuities which lurk within any push-pull system.
However, the somewhat unpredictable performance of germanium transistors of that
time, together with their proneness to thermal runaway, discouraged audio amplifier
manufacturers from making commercial designs of this type. This had to wait for
another five years for the introduction of silicon transistors made by variants of the
Early Solid State Audio Amplifier Designs 1 4 5
Fairchild planar process. These became available initially in small signal versions,
and, because the manufacturing techniques favoured this, in NPN (+ve rail)
constructions. These allowed the design of high quality, low noise, low distortion,
small-signal gain stages which needed no setting-up adjustments, and which, in my
view at the time, were a great improvement, in terms of freedom from mains hum and
microphony, on their thermionic valve predecessors.
u~
//
L
E
r-
,// /
Germanium
//
//
//
//
/J
0
Figure 8.5
I
0.1
I
0.2
I
0.3
I
0.4
Base
I
0.5
I
0.6
!
0.7
I
0.8
voltage
Turn-on characteristics
The implementation of audio power amplifier designs with predictable and stable
performance characteristics demanded equally reliable and robust output transistors,
which meant, in practice, those made using silicon planar construction, and when these
became available, they were offered principally as NPN types, so it was using these in
Lin-type quasi-complementary circuit arrangements that the first high fidelity solidstate audio amplifiers were made. Unfortunately, this approach led to a type of
malfunction which was overlooked by the designers at the time, but which fairly soon
became the subject of hostile comment from the users of this new Hi-Fi equipment,
and this was the problem of output stage asymmetry.
Quasi-complementary Output Stage Asymmetry
This problem is illustrated in Figure 8.6. The input base voltage vs. collector current
relationship of a simple NPN/NPN Darlington pair based on silicon junction
transistors is shown in the upper fight-hand quadrant of the drawing, and the
equivalent characteristics of a silicon transistor compound emitter-follower are shown
in the lower left-hand quadrant. Not only are these curves different in slope - which
makes the push-pull transfer characteristics asymmetrical, even when operated at the
optimum quiescent bias, as shown in the diagram 'a' - but, since the output impedance
of an emitter-follower is approximately 1/gm, and the slope of the curve in the lower
146
Valve and Transistor Audio Amplifiers
LH quadrant is markedly steeper than that of the upper RH, the output impedance will
be different as well; a factor which will become apparent on low output impedance
loads, such as, for example, LS driver units at some parts of their frequency response
curve.
?.vb
Input
o~NPN
A
2i0 lj6 1L2 0j8 0i4
-Vb
]
Input
i
,
~
O +Vcc
O O m
0.4 0.8 1.2 1.6 2.0
!
r
--
O Out
JNPN
O-V
-Ic
Figure 8.6
Output pair asymmetry
/
10--!
0.01
I
0.01
30 w a t t
"
|
transistor
~ "i
'
I
0.1
1.0
Power output
/'/
"
~~~
10
30 ( O v e r l o a d )
(watts)
Figure 8.7
Comparative THD curves for transistor and valve amplifiers
Inevitably, this asymmetry in the transfer curves of the two halves of the output stage
leads to a degree of residual crossover distortion, which is worsened if the chosen
quiescent current setting (the choice of which will always be a matter of some
compromise, because what would be the best setting for one half of the output pair
would not be the best for the other) is not the optimum value. Since the circuit layout
Early Solid State Audio Amplifier Designs 1 4 7
of the amplifier is likely to be fairly simple, with a limited number of phase-shifting
elements, it is possible to use a large amount of NFB to reduce the measured full
output power total harmonic distortion (THD) level. Since it is inconvenient, on a
production assembly line, to have to adjust the output quiescent currents of each half
of a stereo amplifier, some designers - having noted that, in an amplifier using a high
degree of NFB, the actual quiescent current setting which was chosen made relatively
little difference to the measured value of the residual full output power THD - opted
either to use some fixed value which was rather less than the optimum, or to use no
forward bias at all. This could lead to the situation shown in Figure 8.7, in which the
THD could be quite low at the rated power output, but would worsen as the output
power level was reduced.
I have shown a typical solid state audio amplifier design of this period, the middle
1960s, in Figure 8.8. The circuit I have chosen is that of the Leak Stereo/Delta 70. This
is a typical example of design thinking of that time, and most of the contemporary HiFi shops would have similar designs from a wide range of audio amplifier
manufacturers on their shelves.
"~ +759'
R6
27k
R7
i "~"; i"lkO
i
"'
Q3
2N2102
RIO
lk8
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R11
220R
~ Set
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I
I
5
~2N3055
R15
1R0
C7
10uFl~
..=.
CI
20nF
,R8
R16
1R0
:12k
,D2
2000uF
R17,
1OR:
R1
L1
IOuH
2k2
2N4036
Input
q2
2N2102
10R
R12
220R
Output
0V
Figure 8.8
Leak Delta 70 amplifier
2k2
148
Valve and Transistor Audio Amplifiers
Listener Fatigue
It was fairly generally accepted that there was a difference in the sound quality given
by the new transistor amplifiers and that of the valve amplifiers that they sought to
supplant, and this quickly led to the emergence of two camps in the Hi-Fi field, those
who liked the new sound, and those who rejected it, and described the tonal quality as
hard, or thin or clinical. On the other side were those who argued that, since the output
power available was now greater, and the full output power bandwidth and distortion
figures were both better than those of earlier systems, what the listener was now
heating was the actuality of the music, and not some rounded-off version, all of whose
rough edges had been removed by the inadequacies of the amplifier output
transformer. The anti solid state protagonists retaliated by describing the new
technology as giving 'transistor sound' and complaining that it caused listener fatigue.
In reality, the new solid state amplifiers suffered from a number of shortcomings,
which were largely overlooked by the design engineers because they occurred in areas
which had not previously been regions of concern. The first of these, already
mentioned, was that of the dependence of the distortion level on the output power. In
valve designs operating in class A (that condition in which the amplifying devices are
conducting for the whole of the signal voltage excursion) it could be taken for granted
that the worst output THD figure would occur just below the onset of clipping, and this
would decrease, ultimately disappearing into the noise background, as the output
power decreased. However, as had been seen in Figure 8.7, a solid state amplifier
operating under zero forward bias (class B) conditions would have a distortion figure
which would worsen as the output power was reduced.
An additional factor is also shown in Figure 8.7, in respect of the output power
available before the onset of signal peak clipping. A valve amplifier which used only
a modest amount of NFB, say 15dB or less, would have a distortion figure which
would worsen only gradually as it was driven into overload, and if the listener was
prepared to accept a moderate level of peak clipping, the valve amplifier could
actually sound louder than the apparently higher powered transistor version. The
relatively soft clipping of the traditional valve amplifier, when driven into overload, is
one of the more highly valued characteristics of this system, in the view of some
contemporary users. This is mainly a feature of the low level of NFB which is used in
some such designs, coupled with their freedom from latch up, a frequent feature of
badly designed solid state amplifiers.
The use of large amounts of NFB to reduce the apparent distortion level of the
amplifier, especially under class B operation, leads to two further problems, of which
the first is that when the output transistors are cut off, the system gain is zero and
consequently the amount of NFB applied through the feedback loop is also zero. In
practice, this means that if the signal voltage swing takes it through the zero voltage
axis it will pass into a dead zone, beyond which the full amplifier gain will operate to
urge the voltage swing across to the opposite conduction region. However, in the dead
zone the amplifier is switched off, and any low level signals which are present in this
region will be lost - thus justifying the allegations of the thinness of tone of the
Early Solid State Audio Amplifier Designs
149
amplifier, a characteristic feature of zero-biased or under-biased output stage
operation.
In the circuit design of Figure 8.8, the maker' s recommended quiescent current setting
is 30mA, although, in practice, experience would suggest that the optimum current for
the upper Darlington pair will be of the order of 800100mA, and the lower compound
emitter-follower stage would be optimally biased at an Iq of some 40-50mA. Using a
higher bias current setting than the optimum would, as shown in diagram of Figure 8.9,
lead to a worsening of the THD above some low power level, determined by the actual
Iq setting, in exchange for a substantial improvement in the distortion at very low
output power levels as the output stages effectively returned to class A operation. Such
an amplifier performance might look worse on the specification sheet, but could be
more pleasant to listen to.
e-
.o_ 10I=
0
.~_
u
"E 1 . 0 -
over-biased
transistor pair
0
E
L
r
0.1-
f
0.010.01
\
.
. . . . a
T
w
O. 1
1.0
10
Power output (watts)
-
30 (Ovedoad)
F i g u r e 8,9
Effect on THD of increased value of quiescent current
The second incipient problem in solid state amplifier designs of this period was that of
inadequate stability margins in the feedback loop. Like most of the other problems,
this was worsened by the lack of symmetry of the output stage, in that not only were
the dynamic (amplitude and rate of change related) electrical characteristics of the
transistor itself frequency, temperature and current dependent, but they would also
vary depending on which of the two output emitter-follower groups was conducting.
To add to this complex mix of difficulties, the LS load which was coupled to the
amplifier had a reactance which was continuously variable, dependent on the
frequency and amplitude levels of the input signal. All of this presented a substantially
greater challenge to the loop stability of the amplifier than that offered by the
conventional resistive dummy l o a d - so that amplifiers which behaved quite stably on
the test bench might well pass through regions of instability under live conditions with
an LS load, which would lead to the occurrence of brief bursts of HF oscillation
buried, but not hidden from the ears of the listener, within the signal.
150
Valve and Transistor Audio Amplifiers
Design engineers working in this field in the mid-1960s were acutely aware of the
need for some improvement on the type of performance given by the standard quasicomplementary (Q/C) output pair, and a number of options were explored with this
aim in mind.
Alternative Circuit Choices
In the absence of PNP power transistors, or, when such devices had become available,
obtainable only in relatively low voltage and low power versions, there were two
possible options open to the designer - to improve the symmetry of the quasicomplementary pair circuit, so that NPN output power transistors could be used
exclusively without audible performance penalties, or to bias the output emitterfollowers so that they operated in class A, in which condition the crossover distortion
would be greatly reduced. The first of these routes was chosen by the Acoustical
Manufacturing Company in their Quad 303 power amplifier, in which they elaborated
the two-transistor quasi-complementary pair into a triplet, as shown in Figure 8.10a
(report, Wireless WorM, April 1968, p. 67). In this layout, an almost exact symmetry
of the Vin/Iou t c u r v e s was obtained, though there was still some small difference
between the two triples in respect of the optimum quiescent current, of which the mean
value was only, in any case, about 4mA. This low value of optimum !q led to the minor
drawback that it did not allow any significant margin of operation m class A, which
would act as a cushion if there were unexpected variations in the operating conditions
or device characteristics.
At this time the design of a high power, high quality audio amplifier presented an
interesting technical challenge, in the absence of any high voltage, high power PNP
transistors which could be used in conjunction with NPN power transistors in an
output stage having complementary symmetry. The general philosophy used by Quad
seemed to offer an answer to this problem, and I have shown in Figure 8.10b a layout
for a Q/C triplet which I tested for use in a high power amplifier. Under DC or LF
conditions, the two halves of this triple were virtually identical, and the optimum
quiescent current (--100mA) was also the same for both emitter-follower groups. Used
in the output stage of an experimental amplifier design, this output configuration gave
a distortion figure at low output power levels which was less than my then ability to
measure i t - at the time my test bench THD meter had a lower measurement limit of
about 0.05% over the range 100Hz-10kHz - and the amplifier did not appear to
exceed this threshold value over the output power range from 10mW to 30 watts.
Another method for improving the symmetry of the output stage was suggested by
Shaw (Shaw, I.M., Wireless World, June 1969, pp. 265-266), and shown in Figure
8.1 l a. Baxandall did an analysis of this layout (Baxandall, P.J., Wireless WorM,
September 1969, pp. 416--417) and proposed a rather more straightforward way of
achieving the same end, using the circuit layout shown in Figure 8.11 b. Unfortunately
he did not extend his analysis to show a fully worked out amplifier design based on
his analysis. I was attracted by the simplicity of this approach, in which the diode, D 1,
simulated the effect on the driver transistor of the base-emitter junction of the lower
output transistor. The effect of this diode in imitating the missing output transistor
+ 67V
+ Vcc
R5
I k0
q5
M PSU56 PN P
q3
9PNP
q7
2N3442 NPN
~5
!N3055
ql
NPN
R7
68R
100R
D4
R10 2
0R3
C2
C2
N FB etc.
To NFB etc.
,R8
:0R22
all 1 $9 20
R9
OR3
ET IQ
q2
R7
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q6
M PSU56 PN P
LS O u t p u t
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LS O u t p u t
PN
earlier stages
lk0
Signal f r o m
earlier stages
~!N3055
NPN
:68 R
0V
0V
(a) Quad output triples
Figure 8.10
Improved Q/C output stages
R6
100R
!N3Zl.42 N PN
0V
0V
(b) JLH output triples
+ 67V
qll
+V
R2
I kO
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Q1
I
q4
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2
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.
.
.
.
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9
; z~7o ~
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(a) Shaw's circuit
Figure
8.11
Improved Q/C output stages
0R22
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(b) B a x a n d a l l ' s
Oov
version
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ToLS
l
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= R5
4 ~OR5
LS output
Input
:0R22
-
3442
' O5~
(c) JLH 75W amp output stage
Early Solid State Audio Amplifier Designs
153
junction could be improved, especially at higher frequencies, by adding a capacitor,
C 1, across this diode to simulate the output transistor forward junction capacitance, as
shown in Figure 8.11c. I adopted this circuit for the output layout of a 75 watt
amplifier design, eventually published in Hi-Fi News (Linsley Hood, J.L., Hi-Fi News
and Record Review, November 1972, pp. 2120-2123) in what became a very popular
constructional project. It is easy for an author to think favourably of his own designs,
but my personal feeling, then and now, was that with this design, and others of similar
quality which were then being offered, junction transistor audio amplifiers had come
of age, and that their users need not feel that something had been lost for ever with the
passing of thermionic valve operated designs. I have, for the record, shown the circuit
of this 75 watt amplifier in Figure 8.12.
Although there are one or two innovations in this circuit, the design is fairly
straightforward, and consists of an input long-tailed pair stage, with a junction FET
used to provide a very high dynamic impedance constant current source tail to improve
the emitter signal transfer between Q1 and Q4. PNP transistors are used in this stage
so that Q5, the main voltage amplifying stage, could be a high voltage NPN device
with good HF response. The stage gain was increased by the use of a DC bootstrap
circuit (Q3,R3,R6) as the load for Q1. This also gives a low drive impedance for Q5,
which also helps to maintain the stage gain. The voltage drop which is developed
across an amplified diode, Q6, due to Q5 collector current, is used to provide the
forward bias (about 3V) needed to make the output transistor groups operate at the best
point on their combined push-pull transfer characteristics. Q6 is mounted on the output
transistor heat sink to provide a measure of thermal compensation for the quiescent
operating current, and helps to maintain this at the desired level (~-100mA).
A bootstrapped load resistor (C8,R13) is used to increase the dynamic impedance of
R13, the collector load of Q5. The operation of the Shaw/Baxandall technique used to
increase the symmetry of the output Q/C transistor layout has already been described
and illustrated in Figure 8.11. HF stability for all likely combinations of reactive
loudspeaker loads is ensured by the main, dominant lag capacitor, C9, connected
between Q5 collector and Q4 b a s e - in which position it does not contribute to slew
rate limiting or slewing induced distortion, an immunity which is assisted by the input
low pass network R2/C2. Since a large amount of NFB (approximately 46dB) is
employed to maintain a very low level of distortion over the whole available output
power range, the feedback loop characteristics are tailored by the HF step networks
R9/C6, R3/C3, R4/C4 and the output Zobel network C14/R31 so that the loop phase
characteristics are satisfactory. Typical performance figures for the design shown in
Figure 8.12 are: output power 75 watts into an 8 ohm load, bandwidth 15Hz-20kHz,
(upper end set by R2/C2), THD <0.01% at all power levels below the onset of
clipping, unconditionally stable into all combinations of load impedance or reactance.
Class A Operation
The other option which was open to the circuit designer, even in the absence of
satisfactory PNP power transistors, was to operate the amplifier in class A, a
4k7
_l_
R18
12k
ZD1
12V
R22
12
Q2
2N5457
Input 700mV
C1
t4.7OnF
150
220uF
,ql
BFR79
50u F
Q4 q
BFR79
120
'680
m
R29
0R22
R32
FU1
3A
R24
'120
7uF
qe
BC21
150
LS
(~ 0V
0V
,R21
'12k
OveHoad
Indication
330
Rll
WL
220
R4
Figure 8.12
75 w a t t a m p
+45V
C103
IIM5000uF
33q
I
Q5
BF257
R26
100R
I ~ C104
lllJll15000u F
-45V
Early Solid State Audio Amplifier Designs
155
possibility which, as shown in Figure 8.13, could be realised using only NPN polarity
output devices. Since this is not a push-pull layout, crossover distortion cannot occur,
but, since it is not a push-pull system, the output power available is limited, as in any
other single ended layout, by the choice of the output stage operating current, and this,
in turn, is limited by the permissible thermal dissipation of the output transistors. With
reasonably efficient loudspeaker units, the bulk of normal listening would take place
at output power levels which did not exceed a watt or two and the possible output
power from such a class A system would be entirely adequate.
I had designed and built this amplifier for my own use, like all of my audio circuit
designs up to that time, and I only offered it for publication because the use of output
transistors in class A had become, at that time, a matter of topical interest, principally
because a commercial amplifier using this principle, made by J.E. Sugden Ltd, had
attracted very favourable reviews in the Hi-Fi press, who applauded its freedom from
transistor sound.
The structure of the circuit shown in Figure 8.13 is very simple, with Q 1 acting as a
grounded emitter amplifier stage, with Q2 as an active collector load, driven in phase
opposition to Q 1 by Q3. The loop gain of the amplifier is increased by bootstrapping
the load resistor for Q3 by C1. Because the transition frequency of the output
transistors is of the order of 4MHz, whereas those of Q3 and Q4 are in the 400MHz
range, the circuit has an in-built dominant lag in its loop NFB characteristics. This
ensures that the loop gain has fallen below unity before the loop phase angle reaches
180 ~ No additional HF compensation networks are therefore necessary to ensure
complete loop stability, even with reactive loads.
R8
Vr
R1
Q2
IVU480
RVl
lk2
C1
220uF
Vx
R3
2K7
q4
Input
i
2N3906
0.5uF
cz
]711/2NIG131 ~
i22oR
2200uF
bR4
R2
8 k2
MJ4,BO
i
'
l kO
,
"
I LS o u t p u t
8-15R
0V
Adjust RV2 so that Vx = 1/2 Vc
Figure 8.13
JLH 10 w a t t class 'A' W W 4/69
156
Valve and Transistor Audio Amplifiers
Fully Complementary Designs
With the continuing development of epitaxial base and similar structures for silicon
transistors, PNP power transistors became more readily obtainable, although initially
in relatively limited voltage ratings, at prices which approached those of existing NPN
power devices. This provided an incentive to the circuit designers to provide amplifier
systems which took advantage of this new technology, and offered the possibility of
reducing low signal level crossover distortion to a level where it would no longer be
audibly detectable. Two of the circuit designs which made use of this new-found
freedom were due to Locanthi (Locanthi, B. N., dr. Audio Eng. Soc., July 1967, pp.
290--294) and Bailey (Bailey, A.R., Wireless WorM, May 1968, pp. 94-98). Of these,
the Bailey circuit offered a somewhat lower level of THD and I have shown the circuit
used in Figure 8.14.
Although NPN and PNP power transistors were nominally exact equivalents of one
another, in reality there were significant differences between these structures which
lessened the symmetry of the amplifier circuits built around them. Of these
differences, the most obvious was that the current carrying majority carriers were
electrons, in the case of the NPN devices, and holes in the case of the PNP ones, and
since electrons have greater mobility, performance differences show up at higher
frequencies. The second difference, due to the nature of the emitter/base diffusion
interface is that, although the makers quote identical safe operating area (SOAR)
curves, PNP power transistors are nevertheless more prone to failure in use than NPN
ones.
There are a number of interesting design features in the circuit of Figure 8.14, of which
the most important, in terms of its influence on subsequent designs, was the output
overload protection circuitry arranged around transistors Q5 and Q8. These are
arranged to monitor both the output current from Q7 and Q 10 (by means of the voltage
drop across R25 and R26) and also the voltage present across Q7 and Q10. If the
combination of voltage and output current is such to approach the secondary
breakdown region of the maximum working limits of the device (see, for example,
Figure 7.12), Q5 and Q8 will conduct, and limit the drive voltage applied to the bases
of Q6 and Q9.
Bailey had also configured the circuit so that it operated between a symmetrical pair
of voltage r a i l s - anticipating the circuit configuration used in the so-called direct
coupled layouts which subsequently became very popular (see, for example, Figure
8.12). However, he had chosen to retain a simple grounded base input amplifying
stage, of which the inevitable Vb_e DC offset of Q1 was lessened by a bias current
derived from the amplified diode circuit built around Q2, rather than the more
straightforward input long-tailed pair layout of Figure 7.8a. Since the circuit of Figure
8.14 gave a DC output voltage offset under no-signal conditions, a reversible (AC
working) electrolytic capacitor was required to isolate the loudspeaker from the
amplifier output. Such capacitors are not commonly used, and are therefore expensive.
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158
Valve and Transistor Audio Amplifiers
In the article in Wireless World in which he described this design, Bailey
demonstrated, by oscilloscope traces of the relative Vin/Iou t transfer characteristics, the
fundamental lack of symmetry of the existing and widely used simple quasicomplementary layout, and showed the superiority of the fully complementary design
in the way the harmonic distortion of the amplifier progressively decreased towards
zero as the output power was r e d u c e d - behaviour which was typical of valve
amplifiers operating in class A, but not found in most of the early class B (low or zero
operating quiescent current) transistor designs. Bailey also showed the performance of
his design when driven with a square-wave input signal and coupled to resistive or
reactive loads. A lack of overshoot in, or significant distortion of, a square wave or
similar type of signal, when the amplifier is caused to drive a reactive load, makes, I
believe, an important contribution to good overall sound quality in an audio amplifier.
Gain Stage Designs
The gain stages between the signal input point and the output devices are normally
operated in class A and are arranged to give as wide a bandwidth, as high a gain and
as low a phase shitt between input and output as is possible. To reduce the difficulty
in keeping the final amplifier stable, when overall NFB is applied, the gain block is
normally restricted to two amplifying stages, and to get as high a gain from these
stages as is practicable, the collector load for the second stage will be arranged to have
a high dynamic impedance. In the amplifier designs shown so far (Figures 8.4, 8.8,
8.12-8.14), this increase in the AC impedance for a given DC resistance has been
achieved by bootstrapping the load resistor (by coupling its supply-line end by a
capacitor connected to the output of the amplifier). In addition to increasing the AC
impedance of the load resistor this also has the practical effect of increasing the
possible output voltage swing which that stage can deliver. However, this technique is
essentially that of applying positive feedback around the output stage. When this is an
emitter-follower stage, or some similar arrangement, the gain will be less than unity
and the amplifier will not be unstable. On the other hand, positive feedback has the
effect of increasing both the stage gain and the distortion of the stage across which it
is applied. In the present context, if that stage suffers from crossover distortion, the ill
effects of this will be magnified by a drop in the dynamic impedance of the load
resistor and a reduction in the driver stage gain at the crossover point. Modem design
practice therefore tends to favour a high dynamic impedance load, such as a constant
current source or the output from a current mirror, as the means of optimising driver
stage gain. Typical arrangements of this kind are shown in Figures 8.15a-8.15c.
The layout of Figure 8.15a is a fairly typical long-tailed pair input stage, in which a
constant current source, of one of the types shown in Figures 7.17 or 7.24, has been
used as the tail in order to assure the integrity of the signal transfer between the
emitters of Q 1 and Q2. This approach is favoured in IC manufacture where it is easier,
and less expensive of chip area, to manufacture an active device than a resistorespecially one of high value. Similarly, the gain of Q3 could be increased by replacing
R2 with a further constant current source.
In the modification shown in Figure 8.15b, a current mirror, such as one or other of
the types shown in Figure 7.25, has been used to combine the outputs of the two
Early Solid State Audio Amplifier Designs
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transistors of the long-tailed pair, which will substantially increase the input stage
gain. Once again a constant current source could be used in place of R1, with a further
increase in stage gain. As modified in this way layouts of the kind shown in Figure
8.15b form the basic structure of the bulk of both operational amplifier circuits
(because it does not need any resistors) and of a large proportion of Hi-Fi audio
amplifier circuitry.
In all of these layouts the polarity of the devices could be reversed (i.e. by substituting
NPN for PNP devices, and vice versa) and other types of transistor, such as JFETs or
MOSFETs, could be used, at the choice of the designer. Similarly, the gain can be
further increased, especially at the higher end of the frequency band, by connecting a
cascode transistor, of one of the forms such as are shown in the layouts of Figures 7.7
or 7.15, between its collector and the collector load.
1 6 0
Valveand TransistorAudioAmplifiers
An interesting further development of this idea is shown in Figure 8.15c, in which the
current mirror, used to combine the outputs of the two antiphase signal streams, is
transferred to form the collector load of the second gain stage transistor, Q3. This idea
appears to have been originated by National Semiconductors, and has been used in
several of their IC designs, such as the LH0061. It has also been adopted by Hitachi
as the basis of an audio amplifier design (Hitachi Ltd, Power Mosfet Application
Manual (1981), pp. 110-115), having the desirable qualities of symmetry and a very
high gain from just two stages.
HF Compensation Techniques
This is the somewhat misleading term which is given to the adjustment of the amplifier
gain and phase characteristics, as a function of frequency, so that when overall loop
feedback is applied the amplifier remains stable - ideally with a wide margin in terms
of the gain or the phase angle which exists between the working condition of the
amplifier and the onset of instability. While a two gain-stage amplifier of the kind
shown in Figure 8.15 would most probably be stable if an NFB signal was returned
from the output to Q2 base, by way of some suitable network, if a push-pull output
emitter-follower pair, of the kind shown in Figures 8.12 or 8.14, were to be interposed
in the feedback path, the loop phase shift would approach 180 ~ at some upper or lower
frequency at which the loop gain was equal to, or exceeded, unity, and the amplifier
would oscillate.
With direct-coupled circuits the LF phase shift will not exceed a safe level, so the
problems of loop instability are confined to the HF end of the signal pass-band, and it
was (and is) customary to achieve the necessary HF loop stability by imposing a
single-pole, dominant lag characteristic on the gain/phase relationships of the system
by connecting a small capacitor (Cfb) between the collector and base of the second
amplifying transistor (Q3) in Figure 8.16a, since this arrangement gives the best
performance, in terms of THD, at the high frequency end of the pass-band. However,
this approach leads to the problem that it imposes a finite speed of response to any
rapidly changing input signal while Cfb charges or discharges through its associated
base or collector circuits - mainly being limited by the collector current of Q1. This
effect is illustrated in Figure 7.27.
If a composite input signal which includes some rapid rate of change of input voltage
level is applied to the input of the amplifier it is possible that the input device (Q 1)
will be driven into cut-off or saturation because no compensating feedback signal has
yet arrived at the base of Q2. If this happens, there will be a complete loss of signal
during this period because Q3 will be paralysed while the charge on Cfb is returning
towards its normal level. This problem was described by Otala in a published paper
(Otala, M.J., J. Audio Eng. Soc., 1972, No. 6, pp. 396-399), and he coined the term
Transient Intermodulation Distortion to describe the audible effects of this type of
malfunction. A simpler description, suggested by Jung (Jung, W.C., Hi-Fi News and
Record Review, November 1977, pp. 115-123), is slewing-induced distortion (or slew
rate limiting), and this effect can be seen clearly on an oscilloscope connected to the
output of an amplifier when a suitable input signal is applied.
Early Solid State Audio Amplifier Designs 1 6 1
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HF compensation methods
This type of problem is not an inevitable consequence of dominant-lag type HF
compensation since there are ways of avoiding it (Linsley Hood, J.L., Hi-Fi News and
Record Review, January 1978, pp. 81-83). Of these, the simplest is just to introduce
an RC low pass network at the input of the amplifier to limit the possible rate of
change of the input voltage - as R1/C 1 in Figure 8.16. A better approach is to include
the whole of the amplifier gain stages within the bandwidth limiting system, as used,
for example, by Bailey (C3 in Figure 8.14), and illustrated in Figure 8.16b. Then,
provided that the possible rate of change of the collector voltage of Q3 (which is
determined by its collector current and the circuit capacitances associated with its
collector) is faster than the rate of change permitted by R1/C1 (which is within the
control of the circuit designer) slew rate limiting will not occur.
Although the conventional scheme shown in Figure 8.16a is better from the point of
view of THD in the 10kHz-20kHz part of the pass-band, it is much inferior in respect
162
Valve and Transistor Audio Amplifiers
of the normal square wave into reactive load type of test, and it has always seemed
unwise to me to choose a design approach in which an inaudible improvement- from
0.1% to 0.02% THD at 2 0 k H z - has been bought at the cost of worsening the (almost
certainly audible) transient error from, say, 2% to 40%, as measured, for example, at
10kHz in the manner shown in Figure 8.17. In this, the amplifier, operating with a
simulated reactive load, is fed with a good quality square wave, and an amplitude and
time-delay corrected square waveform, derived from this, is subtracted from the
amplifier output. Ideally the residue should be zero, and the closer the amplifier output
approximates to its input waveform under reactive load conditions, the better it will
probably sound.
Amplifier under test
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Symmetry in Circuit Layout and Slewing Rates
A residual problem with any system in which there is stray, or other circuit
capacitances, is that the maximum possible slewing rates will not necessarily be the
same for a negative-going or a positive-going signal excursion. This is because the
outputs of circuits do not necessarily have the same ability to source or sink current,
so there must inevitably be differences in the rate in which any associated capacitances
can be charged or discharged. To take the case of the very simple amplifier layout
shown in Figure 8.15a, if there was some capacitance between its output and the 0V
line this capacitance could be discharged very rapidly if Q3 were tumed fully on, but
would, perhaps, only charge up again, towards the positive line, at a slower rate, which
would depend on the value of R2. This problem would be worsened if a constant
current source were used instead of a resistor, as in the, apparently much preferable,
circuit of Figure 8.16a. This prompted some designers, such as Bonjiomo (Audio,
February 1974, pp. 47-51) and Borbely (Audio Amateur, February 1984, pp. 13-24),
to propose circuits of the form shown in Figure 8.18, in which two amplifier blocks of
the kind shown in Figure 8.16a are coupled together as a mirror-image pair. The only
drawback with this type of layout is that there is normally rather greater difficulty in
achieving a stable quiescent current in the output transistors- a thing which is very
desirable in any class AB output stage for optimally low levels of crossover distortion.
Early Solid State Audio Amplifier Designs
163
Stability of Output Current
By the early 1970s audio amplifiers operating in class AB - by which I mean those in
which it is intended that a small residual current will pass through the output devices
under zero signal conditions - had mainly achieved very high standards of
performance, although the one remaining disadvantage which they all shared was that
the preferred setting of the quiescent current could be quite critical, and would need to
be set up on the test bench for each amplifier as the final step of its assembly.
Moreover, there was no guarantee that this Iq setting, when correctly adjusted (ideally
while monitoring the output with an oscilloscope and a distortion meter), would
remain at the chosen value, or even that this chosen value would still be the correct
one, during the aging of the circuit components, or as the ambient temperature of the
amplifier changed.
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Fully symmetrical gain stage
A number of design proposals were offered as a means for ensuring the stability of the
quiescent current, but, in general, these all suffered from disabling flaws in their
design, so that, in practical terms, the designers were left with the options of trying to
ensure quiescent current stability in the face of operating changes or to choose a design
approach in which the actual quiescent current value was not particularly critical. An
example of the latter approach was my 15-20 watt class AB design of 1970 (Linsley
Hood, J.L., Wireless WorM, July 1970, pp. 321-324).
Various circuit arrangements had been adopted to minimise the effect of changes in
the temperature of the output devices, but one bold approach, due to Blomley
(Blomley, P., Wireless WorM, February 1971, pp. 57-61 and March 1971, pp.
127-131), is shown, in a slightly simplified form, in Figure 8.19. In this the output
devices, a complementary pair of silicon planar transistors, are permanently biased
164
Valve and Transistor Audio Amplifiers
into conduction, and the input signal, after amplification, is chopped into two halves
by a pair of switching transistors (Q1 and Q2), and these halves are then passed to the
output transistor triples for reassembly into an enlarged and power-augmented version
of the original signal. The snag, of course, is that a correct forward bias must now be
chosen for the small-signal switching transistors, which merely moves the problem of
choosing and maintaining the operating current away from the output transistors and
back to the earlier switching stage.
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The Blomley amplifier
A typical example of bipolar transistor operated audio amplifier design of the mid1970s, incorporating many of the contemporary design features, is shown in Figure
8.20. This has an excellent performance both in terms of its THD (better than 0.01%
at 50 watts output over the frequency range 30Hz-15kHz) and square wave into
reactive load (no significant overshoot when coupled to an 8 ohm load in parallel with
capacitor values from lnF to 41aF). Q2 and Q3 form a constant current source for the
input long-tailed pair (Q l/Q4) with a preset potentiometer (RV1) connected between
its emitters to allow the DC offset at the IS output terminals to be reduced to a very
low level. Q5 and Q6 form a current mirror load for the input stage and Q10 acts to
protect Q7 from an excessive input drive signal.
Q8 and Q9 form a constant current load for the main gain stage transistor (Q7) which
drives the output devices on either side of the forward bias generating network
(RV2/C7) through hang-up prevention resistors (R1 l/R12). The output transistors are
Early Solid State Audio Amplifier Designs
1 6 5
connected as compound emitter-followers because this arrangement gives an output
which more nearly equals the input (because of its higher loop gain) and because it
offers a lower output impedance. This arrangement also has the advantage that the
base-emitter junctions (which will become hot in use) are not directly involved in
determining the best forward bias setting - this depends on Q 11 and Q 13 which have
a much lower output current level. Q9 senses the ambient temperature, and adjusts Q7
collector current and the voltage drop across RV2 as required.
Output overload protection could be by means of an output fuse, as shown, or an
output cut-out relay, or a current limited power supply. The use of Bailey-type output
protection circuitry had fallen from favour at that period because it was thought to
operate prematurely during high level signals, especially at higher audio frequencies
where the impedance of many commercial LS systems may fall to a low level, and be
seen as an apparent output short-circuit.
Although there were still areas in which improvements could be made, a comparison
between the performance given by the late 1950s design of Figure 8.3 and that of the
mid-1970s design shown in Figure 8.20 indicates the extent of the progress made.
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This Page Intentionally Left Blank
CHAPTER 9
CONTEMPORARY POWER AMPLIFIER DESIGNS
In the last chapter I looked at the evolution of audio amplifiers, based on bipolar
junction transistors, from early transformer coupled designs, having relatively
unsatisfactory electrical characteristics, to comparatively sophisticated circuit layouts
having a performance, both on paper and in the ears of any unprejudiced listener,
which was fully comparable to that of the best valve operated designs of former years.
This, I know, will be thought to be a contentious claim by many of those who base
their own technical assessments either on what they are told by those whose
experience and acoustic judgements they value - perhaps because they are well known
in Hi-Fi circles - or on the opinions they themselves have formed during listening
trials, a complex topic I propose to consider later in this chapter.
Remaining Design Problems
At the state of the art in the early 1970s, the design problems which remained- and
these were only small imperfections or minor difficulties which did not usually
inconvenience the u s e r - were:
9 the need, initially, and perhaps also from time to time, to adjust the quiescent
current of the output push-pull stage to obtain the best practicable performance;
9the need to ensure that this quiescent current value remained at its optimum setting
throughout the aging of circuit components or changes in junction temperatures of
the transistors;
9the need, especially in the direct coupled designs, where there was no DC blocking
capacitor between the amplifier output and the loudspeaker unit, to prevent
possibly damaging DC voltages appearing across the LS terminals in the event of
an improper input signal or a component failure;
9the need (since all sensibly designed amplifiers will employ negative feedback) to
make sure that the stability margins of the feedback loop were sufficient to allow
electrically awkward LS loads to be driven without significant waveform distortion
or amplifier instability;
9 the need to evolve circuitry and components which would allow progressively
higher output powers to be provided for use with increasingly inefficient LS
systems. In practice, changes in outlook and expectations have meant that while a
168
Valve and Transistor Audio Amplifiers
design having a 10 watt output would have been considered entirely adequate in
the late 1950s, and one of 30 watt capacity quite sufficient in the 1960s, one of
150-250W output would not be thought to be needlessly over-powered in the
1990s;
,, the need, because of a change in the majority usage of Hi-Fi equipment from
classical music to various kinds of high sound-level popular styles, for all the
electronically operated audio units to handle high level signal inputs without
overload, or when such signal overload did occur, that the circuitry involved
should handle this overload gracefully.
Power MOSFETs vs. Bipolar Junction Power Transistors (BJTs)
The major change in component availability since the late 1970s has been the growing
use of power MOSFETs. The construction and general characteristics of these devices
in T, D, V and U versions were explored in Chapter 7 (cf. Figure 7.22), and some of
their advantages and disadvantages were considered. Where they are used in audio
circuitry, such devices are normally employed as the amplifier output transistors,
almost always as a complementary, N-channel/P-channel, push-pull pair, since the
electrical characteristics of these two MOSFET types are much less dissimilar than
those of an NPN/PNP pair of junction transistors. However, in addition to the highand medium-power devices, small-signal versions, mainly of T-MOS devices, are also
manufactured, at voltage ratings up to 600V and current ratings up to 0.7 amperes, and
permitted dissipation ratings from 300mW to 1W, in the case of the TO-237
encapsulated N-channel transistors. Similar components, though only at present at
working voltages up to 450V (Vd_s), are obtainable in P-channel versions.
Id (amperes)
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ContemporaryPowerAmplifierDesigns 1 6 9
MOSFETs have a number of advantages and drawbacks in comparison with power
BJTs. Of these the first snag is that power MOSFETs are up to three times more
expensive to buy than comparable power BJTs, and the relative cost of small-signal
MOSFETs may be even higher. This extra expense is mainly a function of the chip
size, which determines the number of devices which can be fabricated, at the same
time, on a given size wafer of single crystal silicon, and, following from this, the
effects of the relative economies of scale in mass produced components. MOS devices
having a low conducting resistance (RDSon) because a large number of channels have
been connected in parallel, will obviously require a larger chip size, as will devices
having higher working voltages - which require wider spacings between the diffusion
zones. It is also claimed that MOSFETs are less linear than power BJTs when used as
output source-follower pairs. However, this is only marginally true, as can be seen
from the data given in Table 9.1.
Frequency Response of MOSFET Devices
The major feature of the MOSFET, which can be both an advantage and a drawback,
depending on the application, is that it is very much faster in responding to an input
voltage change than any BJT having comparable voltage and current ratings. One
consequence of this good HF response is that, in an incompetently designed MOSFET
circuit, the devices may burst into oscillation, at an extremely high frequency, as soon
as the supply rail voltage is applied. Since such inadvertent oscillation may occur at a
frequency which is well above the HF bandwidth of the oscilloscope which is being
used to monitor the circuit, the hapless designer may only become aware that he has a
problem when he finds that his output devices have become very hot and have
consequently expired, for reasons which may not be apparent. The simplest answer to
this particular problem is to connect a gate stopper resistor, of appropriate value (say,
150-1000~), in the MOSFET gate lead, as close as possible to the gate pin, and to
ensure that the layout of the gate, source and collector leads is not such as to encourage
inadvertent Colpitts, or Lecher-line-type, oscillation.
In a well-designed circuit, the MOSFET is likely to prove more robust in use than the
BJT since it does not suffer from thermal runaway, and its operating characteristics are
free from any secondary breakdown region, see Figure 7.20 vs. Figure 7.12. This saves
space and expense by avoiding the need for elaborate output transistor protection
circuitry.
A complementary pair of these is also, as noted above, much more truly symmetrical
particularly at high frequencies - than an NPN/PNP pair of junction transistors. A
further factor in favour of the MOSFET is that the operating current is carried by
majority carriers (electrons in an N-channel device and holes in a P-channel one), and
it is therefore free from the hole-storage defects which can contribute to the sluggish
turn-off characteristics of any BJT, following a condition in which it has been caused
to turn hard on, as might happen during a brief signal overload.
-
However, the major quality which endears MOSFETs to the audio amplifier designer
is its high effective ff, which can be in the 100MHz region. The speed of response of
170
Valve and Transistor A u d i o Amplifiers
any MOS device is mainly controlled by the time it takes the capacitance inherent in
the gate electrode to charge up to its target voltage through the finite resistance (or
current limit) imposed by the driver circuitry. (Since the channel current is controlled
by the gate voltage - in an enhancement mode MOSFET, which means all the normal
power devices - until this voltage appears on the gate, no channel current will flow.)
This means that the designer has to hand, in the form of the gate stopper resistor, a
simple tool for controlling the effective fv of the transistor. This is also true of the
power BJT, but in this case the normal problem is that the fT is not as high as one
would wish, rather than that it is too high.
Finally, since monocrystalline silicon has a negative temperature coefficient of
conductivity, MOSFETs can show a reduction in drain current for an increase in
temperature- at least at larger values of drain c u r r e n t - and this removes the
likelihood of thermal runaway under heavy load. It also implies, as seen in Figure 9.1,
that there will be some value of bias voltage/output current where the quiescent current
setting will be nearly independent of temperature; a fact which is convenient in pushpull output stage design. I have shown, for comparison, the influence of junction
temperature on the drain and collector currents of MOSFETs and BJTs in Figures 9.1
and 9.2, and I have also, for clarity, expanded the low current region of Figure 9.1 to
show on a larger scale the effect of the negative temperature coefficient of the
MOSFET drain current.
Ic (amperes)
5
100
4
OoC
3
2
1
+ Vb (volts)
=
0
F i g u r e 9.2
Power BJT characteristics
I
0.5
]
=
=
I"
I
1.0
ContemporaryPowerAmplifierDesigns 1 7 1
MOSFET Linearity
One of the criticisms levelled at MOSFETs, often made I suspect by those engineers
who have found experimental difficulties in obtaining stable operation, is that they are
much less linear than junction power transistors, when used as output push-pull pairs
of emitter- or source-followers. These claims have generally been based on the results
obtained from computer circuit simulations of the push-pull transfer characteristics
(cf. Self, D., Electronics Worm + Wireless WorM, May 1995, pp. 387-388), although
comparisons between actual audio amplifier circuits using MOSFETs or BJTs as the
output devices (cf. Toshiba, File Note X3504, March 1991) have shown the converse:
that amplifier circuits using MOSFET output devices have a lower distortion than
those using BJTs.
It is inevitable that any push-pull pair of devices whose input voltage/output current
relationships fall short of an ideal straight-line graph will introduce some distortion
into the output signal, whatever bias point is chosen, and there are big differences
between the BJT and the MOSFET in this respect, as shown in Figures 7.3 and 7.18.
These differences will influence the relative distortion introduced even by an
optimally biased output push-pull pair, and I have shown the comparative results from
tests using four such common circuits in Table 9.1.
Table 9.1 Distortion of output push-pull pair layouts
Configuration
(A) Darlington pair
(B) Compound E/F
(C) MOSFET s-follower
(D) BJT/MOSFET pair
Iq
THD (5k.Q load)
THD (10R load)
60mA
60mA
100mA
100mA
<0.02%
<0.02%
0.02%
0.015%
2.2%
1.15%
2.1%
1.0%
Note. These measurements were made at 20kHz, at a 5V rms output voltage and a 10f~
load. The circuits used are shown in Figure 9.3, and the 20kHz measurement
frequency was chosen to show up any differences between nominally complementary
devices due to dissimilarities in their HF response.
The comparative measurements shown above are of the output push-pull pairs on their
own, unencumbered by ancillary circuitry, and freed from any complicating effects
due to the HF compensation techniques which are necessary in any feedback (voltage)
amplifier circuit. The layout of Figure 9.3a is that of a conventional Darlington pair
output arrangement- very commonly used in solid state audio systems. That of Figure
9.3b is the rather better compound emitter follower system, in which R2 and R5 have
been included to make the setting of the required bias voltage rather less abrupt. Figure
9.3c employs a simple MOSFET complementary-pair source follower output stage-
172
Valve and Transistor Audio Amplifiers
;~ oo.
1 ~ Vbias
i
load
Iv
-Vdd
(a) Dadingtonpair
100R
Input
Output
V bias
R load
1OR
I0v
OV
04
-Vdd
(b) Compoundemitter-follower
Figure 9.3aJb
Output push-pull pairs
which is in essence the same as that of the Darlington pair circuit of Figure 9.3a except
that because the MOSFET has a relatively high input impedance, no impedance
transforming driver transistors, such as Q l/Q2 in Figure 9.3a, are needed.
(For the avoidance of possible spurious HF oscillation in the circuit of Figure 9.3c not
only must gate-stopper resistors (R1/R4) be used, but the MOSFET source electrodes
ContemporaryPowerAmplifierDesigns 1 7 3
~
"
od_
I
I
"=
"t *
~I
j'!
V bias
T
ql
O+Vcc
"
:0.~, ~.o,~.o
. : _jov
~o.
j~
-Vdd
(c) MOSFET source follower
A
+Vcc
4k7
~IE~'-~'~
, %n
Input
!~22
~bias
Output
,1OR
IR load
2
OV
IOOR
-I<'
OR
0 -Vdd
(d) BJT/MOSFETpair
Figure 9.3c/d
Output push-pullpairs
must be protected from capacitative loads. The small inductance (= l~tH) of the 0R22
wirewound source resistors serves this purpose adequately.)
The circuit of Figure 9.3d is that of the compound emitter follower in which the output
junction transistors have been replaced by complementary power MOSFETs. In this
case gate over-voltage protection may be provided simply by connecting 10V Zener
diodes across R1 and R6. I used this output stage arrangement as a recommended
improvement to a previously published audio amplifier design of my own, in which I
174
Valve and Transistor Audio Amplifiers
had originally used monolithic Darlington BJTs (Linsley Hood, J.L., Hi-Fi News and
Record Review, December 1980, pp. 83-85).
In view of the claims for high distortion levels in MOSFET output stages, the
surprising thing about the results of the trials detailed in Table 9.1 is how similar in
practice the distortion levels of the BJT and the MOSFET circuits were (a factor in
determining overall THD levels, as seen below). In both cases the compound
emitter-follower arrangements were superior in performance to the straight
Darlington pair, or the MOSFET source-follower circuit, by an approximate factor of
two. A second surprising feature, observed and used by Sandman (Sandman, A.M.,
Wireless World, September 1982, pp. 38-39), is that with a high impedance output
load (in the tests made, this would have been about 5k.Q) the residual distortion
introduced by a simple emitter- or source-follower is very low indeed and, I observed,
relatively unaffected by the actual Iq value chosen, over quite a wide range of settings.
To be able to obtain such a low level of crossover distortion and such an uncritical
setting for the Iq value would remove one of the major problems in solid state audio
power amplifiers, and this philosophy prompted Sandman to propose an audio power
amplifier design in which an ancillary output circuit was used to provide a very high
impedance load for the basic amplifier stage.
gm Values for BJTs and MOSFETs
In a bipolar junction transistor there is a predictable relationship between gm and
collector current, as shown in the equation. This is only affected to a minor extent by
the device geometry:
gm = Ic(q/kT)
where q is the charge on the electron (1.60 9 10-'9), k is Boltzmann's constant
(1.38 ~ 10-23), and T is the absolute temperature of the j u n c t i o n - giving a room
temperature value for gm of 38.9S/A. In the case of both the junction FET and the
MOSFET the drain current vs. gate voltage characteristics have theoretically a
square-law characteristic, defined, in the case of the MOSFET, by the relationship:
Id = (VGs- VT)=(PeCoZ)/2L
where VGS is the gate-source voltage, V T is the threshold (turn-on) voltage, ~te is the
electron mobility in the channel region, C Ois the capacitance, per unit area, of the gate
oxide layer, L is the channel length and Z is the channel width. This relationship can
be simplified to the approximation:
gm = 1/(RDSon)
which would give a gm in the range 5-20S for a typical 100V N-channel TMOS power
transistor. However, in both the JFET and the MOSFET the electrical characteristics
are greatly influenced by the internal structure of the transistor- an area in which
development work is continuing (c.f. the recently introduced Philips Trench-MOS
Contemporary Power Amplifier Designs
,,,
175
devices with their very low values of RDSon and their very high and linear gm) - and
most manufacturers use their control of this structure to manufacture components
which are somewhat more linear than the square-law behaviour predicted by the
formulae.
Quiescent Current Stability in Push-pull Output Systems
The problem of setting and maintaining, once set, the optimum forward bias for a class
AB or class B output stage* has been a problem with low dissipation solid state audio
amplifiers ever since such things were made. There are, basically, three solutions to
this problem:
9 To choose a circuit structure (such as, for example, a MOSFET complementary
source-follower output pair) in which the actual value of the quiescent current is
not particularly critical, so that some mid-point value can be chosen in the
expectation that the resulting performance will not be too far away from the
optimum; in practice, this will probably imply a fairly large value for Iq. (Typical
power MOSFETs show a region on their ID/V o curves, usually for drain currents
in the region 100-200mA, at which there is little change in Io over the temperature
range -50~ to + 150~
9 To use some circuit technique, such as those illustrated in Figure 9.4, that will
provide adequate compensation for changes in the junction temperatures of the
output devices (essential in the case of bipolar power transistors) - for which
proposed methods range from the simple Bailey amplified diode maintained in
thermal contact with the output heatsink to the elaborate custom-built IC used by
Pioneer to regulate the output device quiescent current shown in Figure 9.5.
The circuit shown in Figure 9.4a is a simple elaboration of Bailey's amplified diode
which I used in my 1972 75W amplifier (Linsley Hood, J.L., Hi-Fi News and Record
Review, November 1972, pp. 2120-2123), while that of Figure 9.4b makes use of the
Williams-type constant current source built around Q1/Q2 to control the current
flowing through R3, and hence the forward bias on the output devices. The layout of
Figure 9.4c is a further elaboration of the Bailey amplified diode arrangement, due to
Self (Self, D., Electronics World + Wireless WorM, October 1996, p. 755), in which
a further diode is used to increase its thermal sensitivity.
* These terms refer to the output stage biasing conditions, in that class AB operation implies
that there is some residual current flow under no-signal conditions, whereas the class B
designation implies that there is no such quiescent current, so that, on signal either one or other
of the two output halves will be cutoff alternatively, on each half cycle of the signal, as the
output signal voltage swings up or down. In practice, since this type of operation would lead to
audible discontinuities around the near-zero signal regions, the vast majority of transistor
operated audio power amplifiers, whose designers have sought the best compromise between
quality of sound and evolution of heat, will operate in class AB
176
Valve and Transistor Audio Amplifiers
L=~r~Bias to
R3 ~E = ~ C l ~
6
Signal output
(a)
+
devices
(b)
Vcc
~
_ =_
+Vcc
output
03
b R6
22R
R3
ql
;lkO
R.2A
.L
~D1
,~ 2; R
"1N41481
from driver
Output
V bias
Signal output
Figure 9.4
Output stage bias systems
(c)
(d)
Other Iq Control Techniques
A number of circuits have been proposed in which the emitter/collector currents of the
output transistors are monitored, and used to regulate the forward bias voltage so that
the desired quiescent current is held to some constant value. In general, this type of
control arrangement is only satisfactory with a system biased into class A operation,
such as that described by Nelson-Jones (Nelson-Jones, L., Wireless World, March
1970, pp. 98-103), and shown in Figure 9.4d. Even then, a large value capacitor (as
C2 in the diagram) is needed to remove the low frequency signal components from the
Iq control voltage.
Contemporary PowerAmplifier Designs 1 7 7
In all cases where BJTs are used as the output devices it is essential that the sensing
transistor (Q 1 in the diagrams of Figure 9.4) should be in as close thermal contact with
the output transistors as possible. Self even proposes gluing the sensing transistor to
the case of one or other of the output transistors (Self, D., Electronics Worm +
Wireless WorM, May 1996, p. 412) in an attempt to reduce the time lag between the
output transistor heating up during a high signal-level programme passage - which
will cause an increase in the output quiescent current of the B J T s - and this
temperature increase being sensed, or the corresponding time lag between the output
BJTs cooling down again, during a quiet passage in the p r o g r a m m e - which will
reduce their quiescent current setting - and the sensing transistor registering this
change in heat evolution. Of these two periods of discontinuity between the output
transistors heating or cooling and that fact being registered by the sensing transistor, it
is the second which is more important to the performance of the amplifier since the
nature of the crossover distortion- particularly in a BJT output stage - becomes less
acceptable at quiescent current settings below the optimum value.
9
ql
+Vcc
| _
J _ A ,
I
o
clvl3
...
Output
transistors
I I/
I
DEDICATED IC
R3
R load
OV
Output
transistors
-E in
-Vdd
Figure 9.5
Pioneer Iq control circuit
In the Pioneer M-90(BK) 250 watt amplifier, which has groups of eight parallel-pushpull connected BJTs in the output stage of each channel, a purpose-built IC, having the
circuit layout shown schematically in Figure 9.5, is used to sense the temperature of
the amplifier, the quiescent current of the output devices and the signal level, and to
178
Valve and Transistor Audio Amplifiers
anticipate and control variations in the quiescent current setting of the output devices.
A further system for quiescent current control in an output pair of current driven
MOSFETs was proposed by Van de Gevel (Van de Gevel, M., Electronics World +
Wireless World, February 1996, pp. 140-143), and is shown, in greatly simplified
form, in Figure 9.6. In this the source/drain current of the output MOSFETs (Q3/Q4)
is converted into voltages across R1 and R2, and these voltages are compared with
fixed reference voltages by Q5 and Q6. The sum of these voltages - converted into a
current by Q5 and Q 6 - is then used, via the two current mirrors, to modulate the
collector currents of Q 1 and Q2, and hence the bias voltages applied to the output
MOSFETs. The purpose of this arrangement is that the effect of input signals which
are applied differentially to the output devices is partially cancelled by subtraction,
and is not used to diminish the Iq level, whereas those changes which occur in both
devices (i.e. due to changes in static Iq) are added in the control system. It is claimed
that this also has the effect of ensuring that neither output transistor can be cut off at
any point of the input signal.
However, although the preceding circuit layouts partially solved the problems of
variable quiescent current and the need to set this, at least once, during manufacture,
or use, it still did not provide a design basis for an amplifier which required no setting
up. The first really satisfactory circuit arrangement which remedied this problem was
introduced in 1975 by the Acoustical Manufacturing Co. (Quad).
+Vcc 2
) +Vcc 1
Input
i0%
T ,o
-
Load
.7'
--
O-----OV
A1
o___v.
Figure 9.6
v
l~-Vdd
Van de Gevel's circuit
The Quad Current Dumping Amplifier
The circuit layout of this amplifier is shown, in its essentials, in Figure 9.7, and its
intention is to allow the output transistors (Q 1,Q2) to be operated at zero bias and zero
quiescent c u r r e n t - a condition which requires no initial setting-up adjustment. Its
operation can be considered to lie in either one or other of two distinct modes, that
when the output transistors are non-conducting, and that when the output transistors
are in their normal operating condition.
Contemporary Power A m p l i f i e r
Designs
179
l+Vcc R3
R2
9
,
-
!
A,W~-.-
o
Input
i~
x
~1. V out
: : RLOAD
oOV
1_ --
,
l-Vcc
Figure
9.7
Current Dumping circuit
Taking the first of these first, a low distortion small power amplifier, A1, is arranged
to drive the load through R3. Since there is in this operating mode no contribution
from the output transistors, the signal retumed to the input of A1, through the main
feedback loop (R1/Rin), will act to hold the output signal (Vout) to the level determined
by the feedback signals through (R1 + R4)/Rin, and R2/Rin, and, provided that the
required load currents are within the capability of A1, the overall system will operate
with very low distortion.
However, if the signal output level at Vy is increased to the extent that one or other of
the output transistors is driven into conduction, the voltage at V x, and hence the
current through R4 into the load, will be increased, thereby increasing the amount of
NFB through R1 to the input of A1, and reducing the gain of A1 to compensate for the
added gain of the output transistor which is now in circuit. If R4, R2 = R1, R3 then the
extent to which the gain increases or is reduced, as the output transistors pass into or
out of conduction, will be exactly compensated by changes in the amount of overall
NFB which is applied, and there will be no crossover distortion introduced by the
unbiased output devices.
So far, so good. The arrangement proposed by Quad provides a simple and readily
understood means of using unbiased output transistors without a major penalty in
respect of the system distortion. Unfortunately, the presence of R4 in the output signal
path to the load would lead to an undesirable waste of output power, so Quad replaced
R4 with a 3.3~tH inductor (Z4) and preserved the equivalence of R1, R3 = Z4, Z2 by
replacing R2 with a 120pF capacitor (Z2). Though this exchange preserves the
intrinsic philosophy of the design it made the actual operation of the amplifier
significantly more complex, which allowed those of the Hi-Fi press who resented
Quad's aloof approach to outside reviews a pseudo-technical reason for denigrating
the performance of this remarkable design.
180
Valve and Transistor Audio Amplifiers
The Sandman Class S System
As is shown above, in Table 9.1, all of the simple push-pull output layouts shown in
Figure 9.3 exhibit a conspicuous reduction in their levels of distortion when their
normally low impedance output load is replaced by a high impedance one: a reduction
in distortion from 1-2% down to less than 0.02% being typical. This effect was noted
by Sandman, who proposed the circuit shown in Figure 9.8 (Sandman, A.M., Wireless
World, September 1982, pp. 38-39). The way this circuit works is that a low power,
very linear amplifier, A 1, is arranged to drive the load through R4, while a high power,
low quality amplifier is used to drive the bridge network R3-R6 at point B, and,
through this, the output load. The voltage comparator, A2, monitors the extent of the
bridge unbalance, and drives the output transistors so that the current fed into the
resistor bridge at point B ensures that the difference in voltage level between points A
and C is very small. By making the signal voltage developed across R4 very small, the
high power devices effectively present the small power amplifier with an infinite
impedance load. Some crossover effects will, of course, arise during the period in
which the base voltage applied to Q1 and Q2 is swept across their non-conducting
regions. During this brief excursion A 1 will supply the load directly through R4. The
resistor values used in the bridge should be low enough to allow this.
R2
Input
-
i
+ Vcc
-
LOW power
amplifier
|
R load
OV
Figure 9.8
-Vd d
.......
OV
0
The Sandman circuit
The Technics Class AA System
This arrangement is shown in Figure 9.9, and is used in the Technics SE-A 1000 power
amplifier and all their current power amplifier designs. This circuit has obvious
similarities in its approach to the Sandman design, and clearly uses the same basic
philosophy in which an output push-pull power amplifier (they call this a class AA
stage) operates to provide a very high impedance load on the voltage amplifier. The
Sandman/Technics approach offers a unique and valuable way of reducing overall
amplifier distortion without requiring high loop gains in the power amplifier, or
thereby reducing the loop stability margins.
Contemporary Power Amplifier Desi,g.ns
R4a - ~aA~
R3
|
ocV-
I Low.p..ower
R10 I amplifier
/!--T1
l
r
220uF
. . . .
1 81
-
~OR06
R7
220RLS
tput
~22
~Q
OR comparaxor
2_
--0oV
-Vdd
Figure 9.9
The Technics circuit
Voltage Amplifying Stage Gain and Overall THD
In normal amplifying systems employing NFB the open loop gain (i.e. before NFB is
applied) will be made as high as feasible- so far as this is possible without introducing
an unacceptable level of loop phase error- so that there will be enough gain left over
to allow a useful amount of NFB to be applied. The relationship between distortion
and negative feedback is:
D" = D~
~
where D ~ is the distortion before NFB is applied, D" is the harmonic distortion after
the application of negative feedback, 13 is the feedback factor (by which I mean the
proportion of the output signal which is fed back to the input) and A ~ is the open loop
gain of the amplifier. This is very similar to the relationship between closed loop gain
and negative feedback:
A ' - A~
+ ~A ~
where A ~ and A" are the respective values of the stage gain before and after the
application of NFB. The implication of these two equations is that the distortion is
reduced in direct proportion to the reduction is stage gain. However, the relationship
is only an approximate one since the presumption is made, in the case of harmonic
distortion, for example, that the amplifier gain (and the feedback factor) remain
constant as a function of frequency from that of the fundamental to that of the nth
harmonic - an assumption which is unlikely to be true, and which is the major reason
why the distortion of an amplifier with NFB will worsen as the frequency is increased.
It also makes the assumption that the feedback remains negative throughout the
frequency range of interest, which may be only marginally correct.
However, if we assume for the purposes of this argument that the distortion is reduced
in the same proportion as the gain, and that the closed loop gain is the product of the
182
Valve and Transistor Audio Amplifiers
open loop gain and the feedback factor- which will be nearly true if the gain (A ~ is
very h i g h - then for an amplifier with an open loop gain of 1000", to which NFB has
been applied to reduce the gain to 10% if the initial distortion (D) was 2%, then the
distortion with NFB applied (D') will be 0.02%, and this is the basis of the design for
almost all the commercial audio amplifiers on sale in Hi-Fi shops, for which the two
performance factors of greatest interest to the (probably naive) purchaser are the total
output power and the total harmonic distortion. So far as the THD is concerned this is
mainly determined by the amount of NFB which can be applied, which, in turn,
depends on the open loop gain which can be obtained from the voltage amplifier stage.
This is a function of the circuit structure which is used, and I have shown how the gain
can be increased by refinements in the circuit layout in the various diagrams of Figure
9.10.
Basic Gain Stage Constructions
All of the gain stages I have shown are based on the long-tailed pair circuit
configuration- sometimes called a differential p a i r - because this allows the signal to
be operated upon between a symmetrical pair of supply lines, and permits the design
of amplifiers which do not need a large value output DC blocking capacitor- a
component disliked by the Hi-Fi afficionados because of its supposedly variable and
voltage dependent characteristics- and is therefore the approach used in all audio
amplifier designs having any pretensions to high audio quality. The test conditions for
all the circuits were: frequency lkHz, supply voltage +15V, 3V rms output.
In the layout of Figure 9.10a, 100% DC negative feedback is applied through R4 and
C2 to stabilise the output DC potential, which is held to 0V •
but, because C2
shunts the AC component of the feedback signal, allows virtually the full AC stage
gain (A ~ to be achieved. The current through both Q1 and Q2 is about 2mA. This
relatively high value is commonly chosen to increase the slew rate of Q3, between
whose collector and base a dominant lag HF stabilising capacitor of 50-100pF (not
shown) will normally be connected. Because of the need to ensure similar working
conditions for both of the two input transistors R2 must be chosen to provide a voltage
drop of about 0.6V at a current of 2mA, which requires an inconveniently low value
for this component from the point of view of stage gain, which, in the circuit shown,
is 4412".
Elaborating the circuit so that Q3 becomes a Darlington pair, Q3/Q4, as in Figure
9.1 lb, allows R3 to be increased to 680R, and this, predictably, very nearly doubles
the stage gain to 9091 ". Since R3 is still a low value, the benefit offered by the
relatively high input impedance of the Darlington pair cannot be utilised. If the need
to keep the slewing rate of Q3/Q4 as high as possible can be ignored - perhaps by the
use of a different method of HF loop stabilisation - the input transistor currents can be
decreased and R3 increased to 12k, which will increase the stage gain to 11,540".
A much more profitable approach would be to replace the load resistor of Q 1 with a
constant current source which would offer a dynamic impedance at least ten times
greater than a 680 ohm load resistor, giving a stage gain increase to some 100, 000 ~.
Contemporary Power Amplifier Designs
""
-
9
c
II C- - -1!
9
Input 0.47uFI
(0.68mV) ~i R1
/
r
1
"
1 8 3
0 +V
2k7
q2~
:IMO
I
_ OV
'(27mV)
R
lrC~3 14
~C.4
_
_
-V
A_
-0
(a) Gain = 4412
- 0 +v
i
1,%0
input0.47uF! ~
__5.=.1----~.---~"
~21 B C 2 ' ~
]
r
ilVlO
c2
0~470uF
I Output
-=>
(3v)
(20mY) E~C4141~
-v
o
(b) Gain = 9091
Figure 9.10a/b
Simple gain stages
Even better, and using no more components than a typical constant current source,
would be the use of a current mirror, as shown in Figure 9.10d, which combines the
outputs of Q1 and Q2 and will double the gain given by a simple constant current
source, to provide a measured stage gain of 187, 000". Ideally a matched pair of
transistors should be used for Q5 and Q6 (or, indeed one of the proprietary IC style
current-mirror devices, available in various input/output current ratios). Failing this, a
low value preset potentiometer should be inserted in the mirror transistors' emitter
184
Valve and Transistor Audio Amplifiers
leads, as shown in Figure 9.10e, to allow the currents through the two input transistors
to be balanced.
A practical snag with all of the circuits shown so far is that they offer very little
rejection of any intruding signal voltages present on the +ve supply rail, and this
.
.
.
.
-
O + V
k
2k7
v -
'~
~
(0.26mV) ~110
ov[
~~-~~
I
I
I~_ ~ O u F
(O~ft)put"
ovT_
v
(16mV)
~k
14
-V
(c) Gain = 1 1 , 5 4 0
,
-
C
IL
.,
Input o.47u
-
1
,
,
,,
O+v
I
(16uV)
1470uF
Output
(3V)
lC414
BC184
Am
84
(d) Gain = 187, 500
Figure 9.10c/d
Simple gain stages
Contemporary PowerAmplifier Designs 1 8 5
+V
q3
k~BC214
BC214
C'
Input O. ~
(11uV) ~
47k
Br
1
--i OV
2k7
R?
i
"~C?
T470uF
!i
ov_~
~
~
_
Q5
~J
BC184
I B.
-i
]
Output
(3V)
ov
VN0600M,
~C184
3k3
(e) Gain = 270, 000
+V
qe
BC214
Q7
BC214
R3
47k
R5
2k7
C'..
0.~
O----Input
(suv)
1MO
L
..
~07k
100u~~ d
0V.=
lC214
'
T470uF
~ OV
~
~
Q71
VN0600M
Q6BC184
3k3
(f) Gain = 600, 000
Figure 9.10e/f
Simple gain stages
t
Output
(3V)
0
186
Valve and Transistor Audio Amplifiers
would inevitably degrade the circuit performance. Substituting a two transistor
constant current source for R2 in the earlier designs would avoid this problem, and
possibly also allow a small further increment in stage gain. In the circuit diagram of
Figure 9.10e I have also explored the use of a small-signal T-MOSFET, which is very
linear in use as a class A gain stage, and also has a very high input impedance. This
increases the gain still further to 270, 000".
A final beneficial circuit refinement, shown in Figure 9.10f, is to insert a pair of
cascode connected transistors (Q3 and Q4) in the collector circuits of the input
transistors. This has the effect of clamping the collectors of Q 1 and Q2 to a fixed DC
voltage- approximately 4 . 5 V - so that there is no effective negative signal feedback
through the internal resistance and capacitance between collector and base of Q 1 and
Q2, and avoids any loss of gain in the input transistors due to this unwanted internal
feedback. With this refinement the circuit gain is increased yet further to 600, 000".
In all the circuits I have shown so far I have used a simple 2.7k resistive load for the
output, although it would clearly be possible to obtain a considerable increase in
unloaded stage gain if the output load resistor were to be replaced by, for example, a
high dynamic impedance constant current source. This choice of load is quite
deliberate, in that the circuits explored have all been of a kind which could be used in
the gain stages of an audio amplifier, where the output stage would be a push-pull pair
of Darlington connected emitter-followers or a similar pair of complementary
MOSFET source followers, for which, I felt, a 2k7 ohm resistor, with the possible
addition of a 2000pF shunt capacitor, would be a more representative type of gain
stage load. In any practical audio amplifier, the gain stage would be operated with a
high dynamic impedance load in order to increase the extent of supply-line signal
rejection, but this would not increase the open loop signal gain because of the loading
effects of the output devices.
Output-Input Signal Isolation
In most real-life amplifying devices there will be a signal feedback path from output
to input, and in a transistor the capacitative component of this will depend on the
junction area and the collector to base voltage. For a typical junction transistor of 1W
dissipation and 50V V c, this feedback capacitance (Cc_b) will be will be in the range
5-10pF, while for a comparable small-signal MOSFET, Cd_gwill be about 30-50pF,
and the Miller effect due to this feedback capacitance will reduce the gain at higher
signal frequencies. This loss of HF gain can be lessened by cascode connecting the
output device, as I have shown in Figure 9.11. This somewhat reduces the lkHz gain
to 330, 000, but allows a gain of 90, 000 at 20kHz, by comparison with the circuit of
Figure 9.10f, which has a gain of 600, 000 at lkHz, but only 30, 000 at 20kHz. This
is not an additional problem with most conventionally designed audio amplifiers, since
in these a capacitor in the range 50-100pF will be connected between the collector and
base of the device to cause a feedback stabilising dominant lag, but if some alternative
method of loop stabilisation is to be employed, an output cascode connected buffer
could be valuable.
Contemporary PowerAmplifier Designs 1 8 7
~BC214
R3
R5
~ 2k7
47k
Output
>.
(3v)
C1
Input 0.47uF
(0.33mV)
c31
R18
12k
uF
]
!
I
~
B~I
VNO3OOM
BC184
3k3
Gain = 330, 000 @ lkHz, 90, 000 @ 20kHz
Figure 9.11
Cascodeoutput isolation
Effect of NFB on Signal Distortion
If an open loop gain of 90, 000 is available, and the required gain, with feedback, is
30, then a factor of 3000 is available to reduce the distortion of the amplifier. If we
assume that the distortion of an output pair of emitter-followers worsens from 2% to
20% as the amplifier output approaches its overload point, then the (theoretical)
residual distortion, near overload, could be reduced from 20% to 0.006%. This is a
somewhat artificial assumption since there are a number of other factors which will
degrade the overall distortion figure, such as the unsuspected generation of signal
voltages between points which are notionally at the same potential, or the failure seen above in some valve amplifier designs - to derive the feedback signal from the
point at which the LS load is connected. However, I am left with the feeling that
designers who claim THD figures of the order of 0.0001%, or even 0.001%, are
talking about the predictions drawn from simplistic computer simulations rather than
the actual performance of real pieces of hardware. There are, of course, techniques,
such as those suggested by Sandman, by which the distortion of the output stage can
be reduced without relying entirely on the improvements brought about by NFB, and
these would result in a genuine improvement in results, but these are an exception to
the normal practice of commercial audio power amplifier design.
Symmetry in Gain Block Structure
A range of well-known solid-state gain stage layouts has been shown above and in
previous chapters, and it is clear that the contemporary audio amplifier circuit designer
- unlike his predecessors in the days of valve circuitry- has a very wide choice of
1 8 8
Valveand TransistorAudio Amplifiers
component arrangements at his disposal. So far as there is any general trend in
contemporary audio amplifier design it is towards the completely symmetrical type of
layout such as that due to Borbeley and shown in Figure 9.12 (Borbeley, E., Wireless
WorM, March 1983, pp. 69-75). The basic structure of each half of this circuit is
similar to that of Figure 9.10c with the added refinements of an output cascode buffer
between Q4 collector and the output load, and with a constant current source in place
of R2. Because the system is fully symmetrical, and employs active loads (Q 10/Q 11)
the lkHz stage gain, at 30, 000 ~, is twice that which would be given by the equivalent
single-ended layout of Figure 9.10c.
The basic aim of this circuit structure is to lessen or eliminate the inevitable effects of
slew rate limiting which will occur under limiting conditions in any single-ended
system. This purpose is assisted by the stabilisation of the feedback system by the use
of a pair of HF step networks (C2/R10, C1/R6) which do not begin to limit the HF gain
until 72kHz, which is well outside the 22kHz design bandwidth determined by R18
and C4. My own judgement is that this type of layout is much superior to the dominant
lag stabilised designs of the type shown in Figures 9.10c or 9.10d, which typify so
many of the middle-Fi offerings of the present day. This view is obviously shared by
a number of major Japanese audio equipment manufacturers who have adopted the
essential features of Borbeley's circuit design.
Negative Feedback and Sound Quality
Although it is possible, in principle, to design an amplifying module which exhibits
progressively smaller amounts of steady-state harmonic distortion, by the use of
negative feedback connected around a gain block of increasingly high gain, there is no
guarantee that the amplifier module so made will be satisfactory in use in any given
audio application. I will explain.
I feel, sometimes, that it is a pity that audio amplifying and sound reproducing
equipment succeeds or fails depending on the reactions it evokes in the ears of those
who listen to it. My misgivings arise because the characteristics of the human ear vary
from person to person, and from day to day, and since what is heard by the listener is
subject to his personal dislikes or preferences, the acoustic performance of the
equipment cannot be completely specified, with certainty, by any set of electrical or
engineering specifications. One can make guesses that certain features of the design
will be a good thing, or that certain other features which one has tried to avoid or
minimise would have been bad things, but one can never do better than feel confident
that what one proposes or has made will be favourably judged by one's peers.
It is because of this that those electronics engineers who have designed audio
amplifiers and other signal handling equipment, which has been judged by their users
to perform to their entire satisfaction, enjoy a higher seat in the electronic engineering
Pantheon - because of some supposed mastery of the black arts of audio - than other
equally competent engineers who had designed, for example, instruments to tell a
submarine captain how far he is beneath the surface of the sea, or who has designed,
say, a simple, efficient and reliable mechanism for opening and shutting the hatches
which enclose the landing gear on an aircraft.
Contemporary Power Amplifier Designs
1 8 9
+ 60V
R6
220
D2
3V
~
Input
R15
9 W,lr
10k
~uF
- _,L
r
330p
4t
"
I
,a bR1
~47R
"
:47R
l
..
= 11m22: ~ 2
C556
, 0 0pF
bl~.
J
-
BC5S
.....
22k
;:22k
bR17
i
~
0V
,0V
~
R10,I :
220R q p
~bcc2
aI 2mA
p R9
1~2k2
,t
_
9
,. .
Output
R16
~sAos2K3v
D1
bR11
~,
I
..
9
-60V
Figure 9.12
Borbeley symmetrical driver
Certain aspects of audio design can be specified with relative ease - such as the power
bandwidth, the uniformity of the frequency response over the audio band between,
say, 10Hz and 20kHz, the output power which can be delivered to certain specified
load impedances at certain levels of distortion, the size of the input signal required to
produce the specified output power, and the signal to noise ratio of the equipment or
the signal breakthrough from one channel to another, in a stereo or multi-channel
s y s t e m - however, all of these measurements relate to purely steady state
characteristics, such as could be made with an input signal derived from a low
distortion, constant amplitude, variable frequency sine wave oscillator, and bear only
a fleeting resemblance to the nature of the audio signals which are likely to be
presented to the equipment. These will consist of a multiplicity of simultaneous
signals, each having widely variable amplitudes and rates of change of amplitude, and
most of these waveforms will be non-repetitive quite unsymmetrical.
The task of designing a signal source which would simulate all of the relevant aspects
of an audio signal in demonstrating equipment shortcomings is a daunting one, but
quite a lot of information - particularly that relating to poor loop stability in amplifiers
using NFB - can be shown by the use of an input square-wave signal, as I have
illustrated in Figures 7.12a to 7.12g, in my book Audio Electronics (Butterworth-
190
Valve and Transistor Audio Amplifiers
Heinemann, 1995, pp. 280-284). Much more needs to be known and specified in
relation to any audio amplifier before it can be declared to be satisfactory, but, at the
very least, no statement of performance can be regarded as complete without the
illustration of the performance, as demonstrated by real-life hardware, under test on
reactive loads, photographed on the screen of a wide-bandwidth oscilloscope. This is
relevant because this is the area where the inadequacy of HF feedback loop
compensation methods will show up - both on the oscilloscope screen and in the ear
of the listener. As a supplement to the oscilloscope waveforms, it would be valuable
to specify the output square-wave settling time of the system, for various test
frequencies, amplitudes and load characteristics, defined as the time required, in
microseconds, for the output waveform to settle within +1% of the required output
level, following an input step-function-type change of input signal level, as shown in
Figure 9.13.
Input signal
I
I =
Settling time
Output waveform VOLTAGE
Settling time
TIME
Figure 9.13
Amplifier transient response and settling time
Power Amplifier Output Stages
Although for a time it seemed that the undoubted advantages offered by power
MOSFETs as output devices- greatly extended HF gain, freedom from secondary
breakdown (allowing simpler methods for output transistor protection), greater
intrinsic stability of the DC working point, greater ease of parallelling output devices
to allow increased output power, as shown, for example, in the output layout of the
Borbeley design, illustrated in Figure 9 . 1 4 - would ensure a growing number of
MOSFET based audio amplifier designs, the major mass-market manufacturers
continue to use bipolar power transistor layouts of the general form shown in the
Marantz PM-16 design of Figure 9.15. In this circuit, the forward bias for the required
output transistor quiescent current is generated by a simple amplified diode layout
built around a monolithic Darlington transistor pair (Q7/Q8), the output transistor
overload protection is provided by the simple Zener diode chain across the signal input
to Q 1 and Q6, and output LS protection is given by a relay cut-out whose contacts (not
shown) are situated in the LS output line.
ContemporaryPowerAmplifierDesigns 1 9 1
0 +vcc
010
M PSA55
4. ',0R22
hDOR22
Output
,R2
,0~
~:~..
220
Q9 ETC JJ'=-
Figure 9.14
I
L~-'4~~eI-~Ct-Vdd
220
Borbeley output layout
The most notable feature of this power amplifier design (whose output is rated from
90 to 150W per channel, depending on the output load) is the elaboration of the normal
output complementary Darlington pair into a symmetrical three-transistor cascade
(Q1/Q2/Q3 and Q6/Q5/Q4). The cross-coupling of the driver transistor emitter
circuits, via R6 and R3/C 1, is now quite conventional practice for the driver stages of
power junction transistors since the reverse biasing of the output transistor base
circuits helps drain the stored holes from the base-emitter region and thereby speeds
up the tum-off time of the transistor following any period in which it has been driven
momentarily into saturation.
A virtually identical output configuration- following a Borbeley-style symmetrical
driver stage- is used by Rotel in their RHB10 200-330 watt power amplifier, though
in this, the output transistors Q3 and Q4 in Figure 9.15 are replaced by four parallel
connected devices as each half of the output emitter-follower pair, current sharing
being ensured by the inclusion of a separate 0.22D emitter resistor and a 10D base
resistor for each transistor. Once again, the input bias needed to establish the output
quiescent current is achieved by a simple amplified diode, and output LS protection is
afforded by a relay cut-out whose normally closed contacts are located in the LS
circuit.
Output DC Offset Control
Almost all contemporary audio amplifiers are based on the split-supply rail
philosophy, in which the input-output signal path operates about a zero mid-point
voltage, and the output of the amplifier is connected by a DC path to the LS output
terminals. This type of circuit layout demands that there shall be no significant (say,
not greater than • 10mV) DC offset present across the output terminals, and this is now
achieved, almost universally, by the use of an op amp in the input signal path as shown
192
Valve and Transistor Audio Amplifiers
-
0
+Vcc#3
(+s3v)
cc1
7mA
O +Vcc#2
(s4v)
+Vcc#l
(ss.Tv)
q2
660
R3-L'(;1
Output
RV1
q4
qs
100
I
(-ss.Tv)
-V dd# 1
q6
(-s4v)
ETC
~) - V d d ~ 2
Figure 9.15
Marantz PM-16 output layout
in Figure 9.16, since most off-the-shelf op amps will have an intrinsic DC offset within
a few mV, even without any adjustment. Unfortunately, this type of offset removal,
though effective and inexpensive, brings with it the problem of switch-on plop, since
it is certain that the voltage required at the inverting input of A 1, for zero DC offset at
the power amplifier output, will not be zero, and until the capacitor C 1 has charged to
the required voltage through R4 there will be an unwanted amplifier output voltage.
The longer the time constant of R4 and C 1, the longer this error voltage across the LS
load will persist, and the time constant cannot be shortened without impairing the
amplifier LF gain.
I~:) ut =
A1
Power amplifier
J~
O
Output
t o LS
OV
Figure 9.16
DC output offset control
-
- -
Iv
,,
ContemporaryPowerAmplifierDesigns 1 9 3
If there are relay operated switching contacts in the LS line, as shown in Figure 9.16,
used to protect the LS load from any circuit failure which would generate a damaging
DC offset at the LS output terminals, then this relay circuit can be used to isolate the
LS from the amplifier output until there is zero residual DC present at this point
following switch-on. The effectiveness and simplicity of this method has resulted in
its widespread adoption among commercial systems. However, these switch contacts
may have to handle high output currents in use, so very good quality switching
contacts are essential if signal degradation is to be avoided.
Hybrid Systems
A number of hybrid systems have been proposed, such as that by Mortensen, using
valve driver stages and MOSFET outputs (Mortensen, H.J., Audio Amateur, January
1989, pp. 14-20) or Macaulay, using transistor driver stages and pentode valve outputs
(Macaulay, J., Electronics + Wireless WorM, October 1995, pp. 856-859), intended
either to take advantage of the robustness of transformer-coupled valves in respect of
overload, or the freedom from the need to use an output transformer offered by low
output impedance solid state designs, but these ideas seem to have had negligible
impact on mainstream circuit design which, apart from what I consider to be the
hunting grounds of wealthy eccentrics, has remained unshakably solid state.
Listening Trials
The purpose of Hi-Fi magazines is to sell Hi-Fi magazines, and, in the process, to
make money for their publishers, to give employment to their staff and to provide
entertainment and instruction for their readers. In this respect they are no different
from any other periodical, with the possible exception that their contributors must
deal, to a significant extent, in informed opinion rather than in simple statements of
fact, and this aspect of their editorial content comes very much to the fore in their
judgements of audio amplifiers, since the basic specifications of so many of these are
so similar that a judgement by the potential purchaser, solely on the basis of specified
performance, would be very difficult. Because of this, there has been a growth in the
numbers of reviewers who have made a speciality of the assessment of the quality of
the sound produced by any given audio power amplifier (or, to a lesser extent,
preamplifier) when it is used to reproduce any particular piece of music.
Inevitably, personal preferences on the part of the reviewer, in respect of the styling or
convenience of the equipment, or the general helpfulness or generosity of the
manufacturers, must, with the best will in the world, spill over into the area of the
reviewer's technical assessment of the performance of the unit, and the reviewer's
judgement of its tonal quality. This may not matter very much to the average reader,
who is probably unlikely to want to buy that particular product anyway, and may quite
enjoy reading slanging matches between august names, but it is a matter for
considerable concern for the manufacturers who may feel that their products are being
unfairly denigrated by a coterie of prejudiced or hostile reviewers, or by engineers
who feel that the wrong lines of technical development are being extolled. For this
reason, both the manufacturers (to defend their products from unfair attack) and the
'1 ~ 4
Valveand TransistorAudio Amplifiers
magazines and their reviewers (to establish in the eyes of their readership their ability
to make valid subjective judgements) have, from time to time, set up comparative
performance trials in which panels of listeners have been invited to record their
preferences for one or other amplifier or other piece of equipment, which should,
ideally, be randomly selected by a third p a r t y - or, indeed, by some electronic
sampling mechanism without any human interest in the proceedings or their outcome.
An early comparative trial along these lines was commissioned by Quad, and reported
by Moir (Moir, J., Wireless WorM, July 1978, pp. 55-58). In this trial, individual
members of a panel of six skilled jurors, mainly drawn from those having an
engineering involvement in sound reproduction, were asked to express their individual
preferences for one or other of a pair of units presented anonymously in randomly
ordered sequential tests. These tests could be between different amplifiers or (as a
check on the panel's ability to distinguish similarities) between the same unit twice
repeated. The units compared were Quad II valve amplifiers, Quad 303 transistor
amplifiers, and the Quad 404 Current Dumping power amplifier- a unit which does
not seem to have been fairly treated by the young cognoscenti of the Hi-Fi press. When
the results of this carefully prepared and executed trial, which involved some 373
individual tests, were subjected to statistical analysis the results indicated that, in the
judgement of the panel, there was no obvious and significant preference for one unit
rather than another, and that the members of this highly skilled listening panel were
generally unable to distinguish one unit from another.
The results of the Quad trial were so clearly at variance with the long-term experience
and expectations of the Hi-Fi press or their contributing reviewers that a repeat trial
was organised by Martin Colloms and Hi-Fi News (HFN/RR November 1978, pp.
110-117), and reported on by Colloms and Adrian Hope. The purpose of this repeat
trial, with a initial judging panel of seven members, largely drawn from the Hi-Fi
press, was to try to eliminate the possible factors which might have blurred the
distinctions between different systems, a confusion of outcome which the critics felt
must have occurred in the trials sponsored by Quad. This second trial employed a
Quad 405, a Naim NAP 250 and a TVA Export (high power valve operated) amplifier
- and so embraced a wide range of technologies as well as a wide spectrum of popular
esteem. In the event, the results were remarkably similar, in that, with two exceptions,
the (again highly experienced) panel was unable to differentiate between the units
under test.
Since great care and self-control had been exercised by the members of this panel to
avoid influencing each other's judgements it was surprising to discover that there was
obvious evidence of cross-influence in the results. However, ignoring this discovery,
which did not, in this case, affect the results, the outcome was largely the same as that
from the Quad trial, that the panelists were, as a whole, unable to decide which unit
they were listening to, or whether any one unit was superior to another. Once again, it
was felt by Colloms that the outcome was, in some way, distorted, perhaps by the very
large number of musical excerpts to which the panel had been asked to listen or the
relatively short length of each trial, so the tests were repeated, with a larger (13
member) team of jurors, who were asked to adjudicate on their own, and were allowed
Contemporary Power Amplifier Designs
195
to increase the duration of each trial as wished. Once again there was no clear evidence
that there were audible differences between the units under test, apart from the
suggestion that two of the panelists were able to identify individual amplifiers, perhaps
because of small residual differences in their frequency response.
Speaking for myself, as an amplifier designer who listens to the results given by his
designs, I know quite well that there are audible effects which result from various
identifiable and measurable electrical design defects, and measurements on those
commercial designs to which I have had access have frequently revealed the presence
of uncorrected electrical defects of kinds which I have learnt to associate with certain
faults in sound quality. However, even when one has trained one's ears to recognise
these faults, this recognition may take a little while.
At this point, may I introduce Fingal, my Siamese cat, who had a great liking for Bach
organ music, and would climb onto my lap, and purr contentedly whenever I played
one of his favourite pieces from gramophone record. If I had made any change in the
reproduction system he would be aware of this, and signify his awareness by the angle
or alertness of his ears. If, after a minute or two, he was prepared to accept the change
he would settle down to (apparently) enjoy the music. On one occasion I was lent, for
lecture purposes, an expensive and impeccably specified Hi-Fi amplifier. As soon as I
began to play music through this new system Fingal leapt from my lap and fled from
the room. Actually, I agreed with his judgement.
My conclusion, clearly shared by Fingal, is that there are audible differences between
amplifiers, but, between good units, the differences are small, and not always easy to
identify in a short listening session. I also believe that most of the more experienced
Hi-Fi reviewers are sensitive to these differences, but tend greatly to exaggerate their
importance. In my experience there are greater differences between the sound of
comparable LS units, or phono cartridges, or even CD players, mainly for clear and
identifiable and measurable reasons. As engineers become better at identifying and
minimising measurable faults, then also these differences will diminish.
This Page Intentionally Left Blank
CHAPTER 10
PREAMPLIFIERS
The function of a preamplifier is to allow the user to select a signal from one or other
of a variety of sources- such as a radio, a tape recorder, a gramophone pick-up or a
CD p l a y e r - to offer an input impedance and signal level handling capacity which is
appropriate to the chosen source, to adjust the amplitude/frequency characteristics of
the input signal, if necessary, to give the required tonal quality, to amplify the signal,
if required, to the 0.5-5V rms level needed to drive an audio power amplifier, and,
finally, to provide this output signal at an adequately low impedance level.
In earlier, thermionic valve operated audio systems the minimal functions of a
preamplifier were often performed by a simple selector switch and volume control at
the front end of the power amplifier, or perhaps by an additional valve, with facilities
to switch between one or two alternative frequency response shaping networks. These
would be used to provide selective attenuation or gain adjustments, either by passive
RC networks or by negative feedback arrangements.
It was unusual at this period to employ a separate preamplifier unit, so where more
extensive signal manipulation circuitry was p r o v i d e d - as in the case of the
Williamson, or the Brimar preamplifier shown in Figure 5.16 - it was usually
incorporated within the main amplifier housing, an arrangement which is now
described as an integrated amplifier. It was not until the advent of the transistor, with
its small size, its low power requirements, its absence of AC operated heater circuitry
and its low heat generation, that the advantages of segregating the low signal level
input circuitry and the high signal level power stages and power supply wiring could
be seen as a normal feature in high quality systems.
It is also required that the signal manipulation and amplification carried out by the
preamplifier shall be done without significantly impairing the distortion or the
signal/noise ratio of the input signal, though the weight attached to the term
significantly will probably be a matter for debate between the designer and his critics
- who may be insulated by their armchairs from the need, in practice, to make
accommodations between desirable, but not always mutually compatible,
requirements.
A friend of mine, for whom the purchase, use, and subsequent resale of Hi-Fi exotica
appeared to be a major hobby interest, told me that in his experience there were greater
differences in the sound of different preamplifiers than there were between
comparably well-designed power amplifiers. If he was correct in this belief, it is
198
Valve and Transistor Audio Amplifiers
probable that the tonal differences he had noted had their origins in the various parts
of the circuit in which the gain/frequency response of the system had been deliberately
modified, for example to compensate for the skewed amplitude/frequency response
used in the recording of an LP or EP gramophone disc.
Gramophone Record Replay Equalisation Characteristics
Various combinations of recording pre-emphasis and replay de-emphasis have been
proposed for use in gramophone record reproduction. Of these, the most relevant is the
method recommended in the RIAA:BS1928/1965 specification, in that this has been
adopted, on a world-wide basis, for the manufacture of 33 and 45rpm vinyl discs.
The proposed pre-emphasis characteristics were based on a recognition of the practical
problems in disc manufacture and reproduction using velocity sensitive (e.g.
electromagnetic) cutting and replay heads. These realities also influence the extent to
which electrical headroom is needed- a matter which I will explore later on. In its
original form, the RIAA specification required a replay characteristic of the form
shown in Figure 10.1, in which the shape of the replay response was defined by three
time constants, 3180~ts, 318~ts and 75~ts. There has, however, been a recent
amendment to this specification, in acknowledgement of the inevitable VLF noise
present on LP disc replay, to include an additional LF roll-off defined by a 7950~ts
time constant, as shown in the dashed line curve in Figure 10.1.
Relative
20-~
'~ 1
0__[.'
-10-1
[
gain(dB)
+17dBat 50.05Hz
r__
'
"~..,.,...~
|
at
. . . . . .
f ' ~
-3dBat 2121.5Hz
Fre?uency
-20dBat 21.215kHz
~ J
Figure 10.1
RIAA response curve
The need for the rising replay gain characteristic below lkHz is because of the typical
maximum allowable LP/EP groove separation of 0.01 cm, which limits the magnitude
of the permissible recording cutter excursion (to avoid adjacent groove breakthrough)
with a consequent linear reduction in replay stylus velocity (and output voltage from
the pick-up) with decreasing frequency. The concept of HF recording pre-emphasis,
followed by replay de-emphasis, was initially adopted for 78rpm records, so that the
reduced level of treble, on replay, would lessen the irritating audible hiss due to the
emery powder loading of the shellac discs. This arrangement has been retained even
Preamplifiers
199
in the case of the much quieter vinyl surfaces. The most commonly used of the several
78rpm equalisation specifications was similar to the RIAA but with time constants of
31801as and 450~ts to lkHz, and then with a fall off in gain defined by a 501as time
constant to 21 kHz.
Replay Equalisation
Circuitry
There are a number of circuit arrangements which will generate the required RIAA
replay response curve for use with velocity sensitive gramophone pick-up transducers,
and I have shown the more commonly used circuits in Figure 10.2. Of these, the
Input
C
R1
Output
"W,"
-
--
C1
R2
ov
--->
C2
OV
o~
0
v
(b)
(a)
Input
R1
Output
Input
OV
v
0
R1
o
;',~
Output
>
Rin
OV~
(c)
Input
C
OV
0
C2
C1
~
.... 0
(d)
t
Rin
(e)
Input
A~
C
!
_
,I
..
~2
~n
Figure 10.2
RIAA replay circuits
OV
0
~
(f)
i
Output
2 0 0
Valveand TransistorAudio Amplifiers
simplest, and therefore, in the opinion of the purists, the most desirable options are the
two passive networks shown in Figures 10.2a and 10.2b. However, in order for these
to function correctly they must introduce an attenuation of20dB at lkHz, and this will
require some additional amplification to restore the original signal level. Also, for the
frequency response of the networks to conform accurately to the RIAA curve, it is
Input
Output
Rin:
Cfb
OV
OV
(g)
'nput
C
i
-
R3
-
Rin~
.
.
i>
I!
.
s
.
(h)
.
Input
C--
Output
-
R1
"~11 ]"
~
tCfb
OV
C
"Vb"
"= '
_
~
Output
R3
R2
R1
Nb
Rin
g!
.
.
.
.
(J)
Input
C~. . . .
R3
ov
-
>
-g
Rin
C
Output
R4
"
-
..--..
-
-
(k)
Figure 10.2 continued
RIAA replay circuits
I
- - -
ov
O
Preamplifiers 2 0 I
necessary for them to be driven from a very low impedance source and for the output
load to offer an impedance which is high in relation to R1. In practice, these
requirements imply the need for further amplification or buffer stages preceding or
following the passive network, so the presumed advantages of simplicity and absence
of (possible distortion introducing) active components are lost.
The layouts of Figures 10.2c, 10.2d and 10.2e can be arranged so that they can be fed
directly from the pick-up cartridge, with Rin chosen to provide the manufacturers
specified loading for the cartridge. This layout also offers a low output impedance to
any following stages which avoids the need for additional pre- and post-amplification.
All three of these circuits use shunt feedback, which has the relative disadvantage that
the input thermal noise is determined by the source impedance in series with Rin,
which will be worse, at low to mid-frequencies with an inductive transducer, than that
given by a equivalent series feedback layout. A simple calculation would suggest that
the source noise level for a series feedback circuit, such as that of Figure 10.2g, when
used with a pick-up cartridge having a DC winding resistance of 2200 ohms
(connected in parallel with a 47 000 ohms load resistor) will be 0.83gV. (This figure
is the room temperature value seen at the input of a feedback circuit of the kind shown
in Figure 10.2g, with a 20Hz-20kHz measurement bandwidth.) The calculated value
for a shunt feedback system, of the kind shown in Figure 10.2e, with an input (pickup load) resistor of 47 000 ohms, will have a noise voltage of 4.24~tV under the same
measurement conditions.
However, these figures are modified by the fact that the measurement bandwidth for
an RIAA equalised gain stage is not 20kHz, but less than a tenth of that figure, which
changes the noise output figures to 0.24gV and 1.22gV respectively. Also, pick-up
cartridges will have a significant value of inductance - the quoted value for a Shure
M75 cartridge being 450mH - and this will increase the source noise voltage at the
upper end of the audio pass-band. The effect of this is to alter the relative character of
the noise given by these two systems, so that the noise from a series-type circuit is a
high pitched hiss, while that from a shunt-type circuit is more of a low pitched rustle.
There are also a few other advantages offered by shunt feedback RIAA circuits such
as those shown in Figures 10.2c, 10.2d and 10.2e. Of these, the most important, in
practice, is that they are largely immune to very short duration input overload effects,
which can be caused by voltage spikes in the cartridge output arising from dust and
scratches on the surface of the record. These short duration overloads can greatly
exaggerate the clicks and plops which are such a nuisance on vinyl disc replay. Also,
they have, in theory, a lower level of distortion (see Taylor, E.F., Wireless WorM,
April 1973, p. 194)- although this is a matter only of academic interest in the presence
of the 0.5-1% THD generated at lkHz by a typical pick-up cartridge (Linsley Hood,
J.L., Hi-Fi News and Record Review, October 1982, pp. 59-68).
Shunt feedback layouts can also give a precise match to the RIAA specification, which
a series feedback circuit, as it stands, will not. This is because any series feedback
layout having a feedback limb which decreases in impedance with frequency will tend
202
Valve and Transistor Audio Amplifiers
to unity gain at high frequencies, whereas the RIAA specification stipulates a gain
curve which tends to zero. Unfortunately, in many preamplifiers of commercial origin,
which use series feedback RIAA stages in order to obtain the lowest practical (input
short-circuited) noise levels, this error is ignored, even though its effect can easily be
seen on an oscilloscope (Linsley Hood, J.L., Wireless WorM, October 1982, pp.
32-36) and the tonal difference which it introduces can equally easily be heard by the
listener.
+V
tR cl
_
A
1 c2
From FU I
O
0V
O
in
Pa
-
0V
9 O
(a) Livy
(b) JLH
+V
Figure 10.3
RIAA input stages
(C) J a m e s
Preamplifiers
203
This error has been corrected, in the case of the circuit layouts shown in Figures 10.2f,
10.2g and 10.2h, by adding a final RC lag network, R3/C3, in which the required
component values will depend on the gain of A 1 at 20kHz. The gain reduction in the
lkHz-21kHz part of the pass-band required by the RIAA spec. can be made more
simply by dividing the circuitry used to provide this response into two p a r t s - as in
Figure 10.2j - where R1, R2 and C 1 generate the frequency response curve from 30Hz
to lkHz and R3 and C3 provide the 75~ts time constant needed for the lkHz-2 lkHz
part of the spectrum. A possible drawback with this circuit (which will also apply to
any final frequency response correction lag network), depending on the nature of the
following circuitry, is that the time constant of R3/C3 will be affected by the circuit
load impedance, which is, for the purposes of this calculation, in parallel with R3. This
difficulty is removed by the use of an active output integrator, shown as A2 in Figure
10.2k, where R4/C2 provide a 75~ts time constant. The only likely problem with this
arrangement is that the circuit introduces a phase reversal, which may not be desired.
The required component values for these networks were originally calculated by Livy
and quoted in an article describing a simple RIAA equalising stage (Livy, W.H.,
Wireless WorM, January 1957, p. 29) shown in Figure 10.3a. The calculations were
recast by Baxandall in a slightly more accessible form, and quoted in a reference work
(Baxandall, P.J., Radio, TV and Audio Reference Book, S.W. Amos, ed., NewnesButterworth, Chapter 14), in the course of an excellent review of RIAA equalisation
options.
Using the formulae quoted by Baxandall, the required values for R1,R2,C1 and C2 in
Figures 10.2a, 10.2c and 10.2g can be determined by:
R1/R2 =6.818
C1 R1 = 2187~ts
C2 R2 = 109~ts
For Figures 10.2b, 10.2d and 10.2h the component values can be obtained from:
R1/R2 = 12.38
C1 R1 = 2937~ts
C2 R2 = 81.11as
Similarly, in Figures 10.2e and 10.2f, the values are given by:
R1/R2 = 12.3
C1 R1 = 2950~ts
C2 R2 = 78~ts
Practical Circuit Layouts
In valve amplifiers the most common RIAA equalisation circuitry was of the kind
shown in Figure 10.3a, due to Livy, in which the frequency dependent feedback
network is connected between anode and grid of a high gain amplifying v a l v e -
204
Valve and Transistor Audio Amplifiers
typically a pentode of the EF86 type. Translating this type of circuitry into a transistor
form using a single bipolar junction transistor (see Tobey, R., and Dinsdale, J.,
Wireless WorM, December 1961, pp. 621-625, and also Carter, E., and Tharma, P.,
Wireless WorM, August 1963, pp. 376-379), encountered the two problems that the
input impedances were much lower than was the case with a valve amplifier and that
the available gain was inadequate to achieve the required frequency response from the
stage, especially at the low frequency end of the RIAA curve where a gain of +20dB
with respect to that at l kHz was needed. The normal approach to this problem was to
increase the value of R1, which increased the LF gain at the expense of accuracy in
conforming to the lower end of the RIAA curve.
C4
Output
)u F
C1
OCA4
U?
CH
I
I
. . L C6
--1--15nF
,vv,~
~?~k
Input
I
i
i
r
C5
100uF
l~R350u
0V
0V
(a) Dinsdale
+V
,R3
'47k
~3
,C107
Output
C1
0.47uF
Input
82k
0V
OV
3k3
Figure 10.4
Further RIAA stages
3k3
(b) Bailey
Preomplifiers
205
In a rather later circuit of my own (Linsley Hood, J.L., Wireless WorM, July 1969, pp.
306-310), shown in Figure 10.3b, I elaborated the single transistor circuit into a triplet,
which greatly increased the available gain and also allowed a third order
(18dB/octave), 30Hz high pass rumble filter to be implemented using the negative
feedback path between the two BC 109 emitters.
The availability of junction FETs, which approached much more closely in their
operating characteristics to the thermionic valve- with the exception of a much lower
gain - encouraged their use in circuits based on their valve precursors, and an FET
based design due to James, using some early devices of this type (James, D.B.G.,
Wireless World, April 1968, p. 72), is shown in Figure 10.3c.
The circuit due to Dinsdale (Dinsdale, J., Wireless WorM, January 1965, pp. 2-9),
shown in Figure 10.4a, was one of the earliest of the transistor based RIAA
equalisation circuits to use series feedback around a gain block having a useful amount
of stage gain, and became the standard design adopted by a large number of Hi-Fi
manufacturers, though with silicon planar transistors rather than the germanium
diffused junction types used by Dinsdale. A problem with this design was that the
equalising network applied a capacitative load on the output of Q2, which reduced the
gain of Q2 at the upper end of the audio spectrum, although, in practice, this might go
some way to remedying the inadequate HF attenuation which is characteristic of the
series feedback RIAA system. This problem of falling HF gain was remedied by
Bailey (Bailey, A.R., Wireless World, December 1966, pp. 598-602) by the use of a
transistor based version of the valve Ring of Three shown in Figure 10.4b. This had a
low impedance, emitter-follower output stage, and also incorporated a second-order
high pass rumble filter, via R6-R10, to attenuate signals below 30Hz.
With the growing use of synmaetrical +/- power supply lines, the discrete component
version of the RIAA stage shown in Figure 10.5 has become popular, either in the
form shown, or in its complementary version, and usually with some elaboration of the
+V
0
CC SOURCE
R6
----~
R4
Input from PU
q~
q2
Output
C2
0V
Figure 10.5
Direct coupled RIAA stage
206
Valve and Transistor Audio Amplifiers
general structure. For a simple design with normal bipolar junction transistor inputs,
PNP input devices will normally be chosen because of their better noise performance,
although this design decision will be influenced by the availability of application
specific components such as the NPN super match pairs described below. This circuit
arrangement illustrates one aspect of modem design practice, which favours DC
coupling to minimise the use of capacitors in the signal path (except when their use is
forced, as, for example, in the RIAA frequency correction network). If Q 1 and Q2
were a matched pair it would be possible also to dispense with the DC blocking
capacitor (C2) in the feedback line. The capacitor, C 1, across the pick-up load resistor
will have a value recommended by the cartridge manufacturers to obtain the flattest
possible frequency response.
Most contemporary RIAA equalising circuits are based on versions of Figures 10.2g
or 10.2h, though there is an increasing trend to the use of versions of Figures 10.2c and
10.2d in which the output from the pick-up cartridge is amplified by a low noise, fiat
frequency response gain stage, and then presented to the RIAA stage at a signal and
impedance level at which the noise level differences between shunt and series
connected NFB networks are insignificant. The differences, at the high quality end of
the market, between one manufacturer's products and another, now relate to whether
integrated circuit or discrete component gain blocks are used as the amplifying stages,
and, if the latter, the type of component layout chosen.
Very Low Noise Input Systems
Towards the end of the vinyl era - to the great regret of many gramophone record
users, the manufacture of the 12 inch LP disc was discontinued in the late 1980s by
the major record manufacturers in favour of the more robust and less expensive
compact disc - the gramophone pick-up cartridge design which had come to be
preferred by the connoisseurs was one using a moving coil type of construction, rather
than the moving magnet or variable reluctance style of mechanism which had
dominated the high quality end of the audio market for the previous forty years. The
lower mass of the moving coil transducer, and its more effective linkage to the stylus
assembly allowed an improvement in the dynamics and stereo channel separation of
the system, and the much lower inductance of the pick-up coils greatly improved its
high frequency response. However, the typical mean signal level output voltage from
such a cartridge might well be only some 50-500~tV rather than the 3-5mV output
voltage which would be given by a moving magnet transducer, and this required a
different approach to RIAA equalised input stage design.
Moving coil pick-up cartridges had been manufactured for many years before there
were suitable components for the construction of electronic head amplifiers, and the
necessary amplification was provided by an input step-up transformer of the type
which had been employed for the same purpose with low output-voltage microphones.
However, the use of an electronic gain block offered advantages in terms of wider
bandwidth and greater freedom from unwanted hum pick-up.
The main approach employed by the audio circuit designers was to place this gain
block between the pick-up cartridge output and the input to the RIAA stage. This way
207
Preamplifiers
was chosen because it was easier at that time to design a flat frequency response head
amplifier than to make a switched gain RIAA stage with a noise level which was fifty
times lower than the current norm. For such a gain block to have such a low input
noise level it would need a lower input circuit impedance than would be found in a
normal small-signal transistor amplifier. There are various ways by which this could
be achieved. Of these, the most obvious was to connect a number of input devices in
parallel, as shown in the circuit of Figure 10.6, due to Naim. Here five transistors have
been arranged in this way, which would, other things being equal, reduce the input
base spreading resistance (that distributed resistance between the base connection on
the chip and the active base region of the transistor) to one fifth, and reduce the input
noise voltage to 1/~/5 or a little less than a half. Some of this input noise improvement
is lost, unfortunately, because of the need to include additional emitter resistors to
ensure current sharing between the unmatched input devices.
A
_ ....
,,
,~I~R12' - l ~ " ~ ~ o V
68R
~R2
I :~2ok
-
I a,
-.
.220R
'
Q1
Input from PU
~
OuF
214
'
~ "'~
..,_o,
.
9 v
!.
.
~R15
P4k7
r
t~ v
OV
Figure 10.6
Naim NAC 20 head amp
Another approach, rather more suited to the DIY constructor than the large-scale
manufacturer, is to use a small-power transistor as the input device, because these will
have a larger chip size and, in consequence, a lower base spreading resistance.
However, the use of such input transistors might require a degree of selection to find
those having the best noise level. An example of this type of circuit is shown in Figure
10.7, for a 30/60 ~ head amplifier, particularly suited to very low impedance, low
output voltage cartridges such as the Ortofon MC20. In this, Q1, a BD435 smallpower transistor, is operated as a grounded base amplifier with a load resistor (R3)
which is bootstrapped to increase the stage gain, while NFB is applied via R7, R4 and
Valve and Transistor Audio Amplifiers
208
R5. The output impedance at Q1 collector is reduced to a low level, as seen at the
output, by Q2 and Q3, which are connected as a high gain, compound
emitter-follower. The circuit is designed to be powered from a pair of 1.5V AA cells,
from which the current drain is 1.5mA/channel. The -~1dB bandwidth is 10Hz-40kHz,
and the THD is less than 0.02% at 1V output at l kHz.
+3V 1.5mA
R2
lk5
C4
=C5
r ,q3
BC214
lOOnF
Output
820R
Q1
18R
'
Input
,R8
' 330 R
R1
18R
Figure 10.7
Grounded base head a m p
l
+Vcc
R3
47k
,o . . . .
I
t _
~
R1
lOOR r" --~Op~47k| . /
I
ov
o
1 ~
I
o,o
~.
oo,o=
2u2F
I
"1
MC I L
I__.ll'q2
~MM
I
i
-
OV ~
-
9
I
~lbR4
~1~47k
Figure 10.8
Quad M ~ M M input circuit
;~4k7
I
/
I ,,,,
__~C30v
C5
,'t,~ " - - ' h ~ .
q IR6
9P2 20 R
"
"....
C.7
~
56k
2o~o
L
,~:,',~_.,~
t
I
..~F.=
!
-|
_L.Ce
[ -
-1-
~ ov
5kO
~3kO
Preamplifiers
209
The other way in which the input resistance of the amplifier may be reduced is by
operating the input devices in push-pull, which will double the gain and halve the
effective noise resistance. An elegant way of doing this is that used by Quad, and
shown in Figure 10.8. This takes advantage of the fact that a silicon bipolar junction
transistor can operate as an amplifier, though admittedly only at relatively low input
signal levels, when its collector voltage is no greater than that on its base. Operation
at such a low collector voltage will help to reduce the shot noise component due to
collector-base leakage currents in the input devices. Quad used this layout in all their
pre-1994 RIAA input stages, with component values being chosen to suit their use
either as moving magnet or as moving coil input arrangements. Although the main op
amp gain block, A 1, is operated in the shunt feedback mode, the circuit as a whole uses
series FB because of the interposition of Q 1 and Q2. R10, R11 and C8 provide the
necessary additional HF attenuation to correct the inevitable series FB gain error.
Low Noise Devices
Bipolar junction transistors
The concept of using multiple, parallel-connected, input transistors as very low noise
input devices offered an opportunity to the IC manufacturers to use their existing
fabrication techniques to make low noise matched pair transistors where each
transistor was, in reality, a large number of parallel-connected devices - distributed
across the face of the chip to average out their characteristics. The National
Semiconductors LM194/394 devices were early examples of this type of construction,
and offered input noise resistance figures of the order of 40 ohms, and bulk (emitter
circuit) resistances of about 0.4 ohms. Recently, Precision Monolithics have offered
their MAT-01 and MAT-02 super-match pairs, and their SSM-2220 and SSM-2210
equivalents, which are respectively NPN and PNP matched dual transistors, which
have input noise resistance values less than 30 ohms. An ultra-low noise gramophone
pick-up input head amplifier stage, based on the SSM-22 l0 and SSM-2220 supermatch pairs, is shown in Figure 10.9.
This circuit makes quite an interesting design study, with the Q2 pair of transistors
acting as a cascode load for Q 1 to increase the input gain, with the Q3 pair acting as a
current mirror to sum the outputs of the two halves. Because the bases of Q2 are held
at approximately 3V, the collector voltages for Q 1 are clamped at about +2.4V, which
offers adequate input headroom while still being low enough to minimise collectorbase leakage currents. Constant current sources form tail loads for Q 1 and collector
loads for Q4. In addition to increasing the stage gain, this assists in rejecting signal
breakthrough .from the power supply rails. Switch S1 allows a choice of input
resistances and a choice of stage gain levels, 15 or 150", to suit MM and MC pick-up
inputs.
Junction FETs
Very low noise JFETs have been available for at least ten years from specialist
manufacturers such as Hitachi, Siliconix and Intersil, and, more recently, have also
been readily obtainable in dual matched pairs, specifically intended for use in
21 0
Valve and Transistor Audio Amplifiers
amplifier input stages. The availability of FETs in matched pair forms avoids two of
the problems inherent in these devices; that the drain current vs. gate voltage
relationship is influenced by so many factors, such as doping level, channel geometry
and the distribution of doping impurities, that their drain current cut-off voltage,
[Vgs(off)] and their drain current at zero gate voltage (IDss) are very variable from one
device to another, so that a considerable degree of selection would be necessary to
obtain a balanced pair.
+ Vcc
,R3
,
oouF
,d
Q3A
SSM2220
T ~
I
Q3B
OV
47q
C3
Q4
BC560
Q2A
SSM2210
R7
Output
Input
QIA
33~176176
OV
I
~
9~1a
|
I
SSM2210
~
ql
100~
I
I
~I"0R
9 ~1D
lOOuF
8
CC2
i
!
OV
"l"0.1u F
-Vcc
Figure 10.9
Super-match pair head amp
In terms of noise levels, JFETs traditionally suffered from a higher level of flicker (I/f)
noise than junction transistors. Also their channel resistance is much higher- because
of their method of construction- than those of BJTs. Improvements in 1/f noise have
been made by greater care in their physical construction and the channel resistance has
been lowered by connecting a large number of channels in parallel, in a similar way to
that used in making a junction transistor super-match pair.
A very high performance RIAA corrected input stage, due to Spectral Inc., is shown
in Figure 10.10. This also has a cascode connected input stage but with the bias offset
voltages of Q2a and Q2b used to provide the 2-3V drain voltage required by Q 1a and
Q lb. Q3a/Q3b provide a second gain stage, with Q4a and Q4b forming a current
mirror collector load for Q3. A symmetrical emitter-follower pair is used to lower the
Preamplifiers 2 1 1
preamplifier output impedance, to allow it to drive a low resistance RIAA equalising
network in the feedback path to Q 1b. As is current Hi-Fi practice in the USA, aimed
at lowering LF phase errors, there are no DC blocking capacitors in the signal path, so
RV1 is used to trim out any small residual DC output offset.
+15V
CC2
IOOR
r
c~
_i..C6
0"luF
v
100u F - -
RM IN
Q3A
q2A
Matched
q2s
pairs
Output
Z
Matched
pairs
Input
R2..
I
II ql A
q 1e
~4a
100R
10R
0V
0V
CCl
i
!
,R6
~100R
~.a,~-, _ L c4
-15V
Figure
10.10
Spectral RIAA input stage
Q3
5
2SK389
"O
~4
L
03
N--
~3
ltl
o
Z
LM 194
IS.D743
2
10
Figure 10.11
JFET/BJT noise figures
~o
1~o
~oo
Source
~k
resistance
~k
;ok
21 2
Valve and Transistor Audio Amplifiers
The input noise resistance of an FET, at drain voltages which are low enough to avoid
significant drain-gate leakage currents, is determined principally by the resistance of
the channel, and this is related to the slope (gfs) of the drain current/gate voltage curve,
in an ideal device, by the equation:
R(n) = 0.67/gfs (ohms)
As an indication of the relative performance of contemporary JFET and bipolar
transistors, the noise characteristics of LM 194 and 2SK389 monolithic matched pairs
are shown in Figure 10.11. This shows that, for input circuit resistances of above 100
ohms, the 2SK389 FET has a lower noise figure than the best of the bipolar devices.
Low noise op amps
Inevitably, all improvements in device manufacture, as applied to discrete
components, will find their way into the field of IC manufacture, and this results, in
the present case, in the availability of audio type ICs, such as the Analog Devices
AD743 bifet op amp, which has a 10kHz noise voltage of 2.8nV/~/Hz and an output
distortion figure of 0.0003% at lkHz and 5V rms. I have also plotted the noise figure
of the AD743 on Figure 10.11, on which it appears little worse than that of the
2SK389. Under the circumstances, there seems to be little point in using elaborate
discrete component circuits such as those shown in Figures 10.9 and 10.10, as an
altemative to an IC, except to provide greater headroom than that would be available
from an IC op amp operating from • 15V supply rails.
Headroom
Very few topics can have generated as much debate in the audio field as the extent to
which the designer should allow for possible input overload in any given circuit, a
factor generally referred to as headroom. Sadly, much of this debate has been entirely
misguided. The reason for this is that all the sources which provide signals to the
preamplifier do have specific output voltage limits. For example, in the case of a tape
recorder, either reel-to-reel or cassette type, the recording level is generally chosen so
that the peak output signal is not more than 3dB (~/2") greater than the normal span of
the recording range. At +6dB, the output THD would probably have increased, in the
case of a cassette recorder, from about 0.5% to some 3-5%, and at +12dB (4 ")
magnetic saturation of the tape would probably have clipped the peaks of the signal
anyway, preventing any further output voltage increase.
In an FM tuner, constraints on the permissible frequency excursions of the broadcast
signal impose stringent limits on the magnitude of the modulation, and this is rigidly
peak limited before being broadcast- again giving a +6dB maximum likely signal
over-voltage limit. In the case of CD the recording engineer will want to use the bulk
of the available 16-bit (65, 536) output levels, while there is an absolute prohibition
on any attempt to go beyond this, so, once again, there is an output voltage ceiling,
which is unlikely to be greater than +6dB above the intended maximum recorded
level.
Preamplifiers 2 1
3
This leaves only the vinyl gramophone disc as a source of possibly unlimited peak
output voltages, but, here again, at least for recorded signals, there are output level
constraints, as shown in Figure 10.12. The recording characteristics of the vinyl disc
were analysed by Walton (Walton, J., Wireless World, December, 1967, pp. 581-588),
to whom I am indebted for the data on which Figure 10.12 is based. Below about
l kHz, the electrical output from the pick-up cartridge is limited by the allowable
0.01 cm spacing between the grooves, and the resulting 0.005cm maximum permitted
cutter excursion. Above a turnover frequency of around lkHz the possible closeness
of succeeding groove excursions, which control the output voltage, is determined by
the shape of the rear face of the cutting head and the linear velocity of the stylus in the
groove (which will depend on the track diameter). Beyond about 3kHz there is a limit
imposed by the maximum groove accelerations which the cutter head can generate or
the pick-up stylus can follow. These are also limited by the groove velocity, and thus
depend on the track diameter.
r
Q.
~J
+50-
u +40- Maxa~udes
+30~
Max velocities
~~___~pMax accelerations
~+20
~+10 ~~(ac~
iticagl/lainiten~sa,
t~Oc?;
.
10/
-10.I
Figure 10.12
Maximum recorded levels
.
.
.
.
1"oo 3"00 1;
Frequency Hz
ik
X~5
d,am
26kHz
Walton considered that, under optimum conditions, and at a 7.5" groove diameter, a
groove modulation velocity equivalent to +30dB at 2kHz, with respect to 1cm/s, was
the maximum which would be cut. The Shure Corporation, which made a speciality of
the manufacture of pick-up cartridges with very high record groove tracking ability,
claimed that its best models were capable of following a groove modulation of +40dB
ref. 1 cm/s at 2kHz, decreasing with increasing frequency. They obviously considered
that the musical bells track on their stereo test record, which was cut at a recording
level of +25dB at 10kHz, would present a formidable challenge to their competitorsa consideration which lends support to Walton's curve of practical output limits in
Figure 10.12.
From this analysis, if a recorded velocity of 5cm/s is taken as the normal maximum
mid-band signal, the limitations of the recording process will ensure that the maximum
modulation velocity which the pick-up will encounter on a very heavily modulated
part of the recording will not exceed some 30cm/s. This practical limitation on the
21 4
Valve and Transistor Audio Amplifiers
pick-up cartridge output voltage was also shown by Wolfenden (Wolfenden, B.S.,
Wireless WorM, December 1976, p. 54) using data from Shure, and by Kelly (Kelly,
S., Wireless World, December 1969, pp. 548-555) who had confirmed, by experiment,
the magnitude of any feasible overload margin.
However, with the same kind of logic which persuades ordinary, sensible motorists
that a car which is capable of 150mph is a much better proposition- in a country
where the speed limit is 70mph- than one which will only do 90mph, many circuit
designers consider it necessary to provide RIAA stage voltage overload margins of
20-30 ", or greater. Unfortunately, by praising such achievements, reviewers
perpetuate the belief that this style of design is both good and necessary. Clearly, some
type of gain switching is necessary if the preamp is to cope with the voltage output
levels from both MM and MC type pick-up cartridges, but these cartridges will require
switch selection of their load resistance anyway, between, say, 47k and 100R, so the
provision of a switchable choice between low impedance/high gain, and high
impedance/low gain options has become normal practice. In reality, the highest output
voltage peaks are likely to be those associated with dust particles, scratches and other
blemishes on the surface of the disc, and while it is obviously desirable that such
I l controls
Tone
wer
j
amp
.~. OV
(a)
Other inputs
oo
PU i n p u t
RIAA
o_
o
Balance
b
D
To Power a m p
optional
subsonic
OV
OV
Figure 10.13
Basic preamp layouts
(b)
Preamplifiers
215
sudden input voltage excursions do not lead to a prolonged paralysis of the amplifier,
clipping which is very brief in duration may be acceptable to the listener.
A typical op. amp. gain stage, fed from a +15V supply, is capable of a 9.5V rms
undistorted output voltage swing. It should therefore be capable of handling, without
clipping, the signal from any disc replay amplifier whose normal maximum output
level is less than 1.5V rms. For the greatest freedom from signal overload, the gain
control should be placed as far forward in the signal chain as possible, as shown in
schematic form in Figure 10.13a. Unfortunately, with this layout the total noise
contributions from the following stages will be present all the time, regardless of gain
control settings. The alternative option, shown in Figure 10.13b, is to place the RIAA
equalisation stage between the pick-up input and the input selector switching. Any
tone controls or filters, if used, can then be based on unity gain blocks, which if they
use the same type of gain stage (such as a low noise op amp) as that used in the RIAA
module itself, will not overload if the RIAA module does not. With this type of layout,
the preamplifier noise contribution will decrease to zero as the signal level is reduced.
Tone Controls and Filters
In the earlier days of audio, it was taken for granted that there would be shortcomings
both in the tonal quality and in the signal to noise ratio of all of the input signals. There
would also be deficiencies in the frequency response characteristics of both the input
transducers and the loudspeakers, and it was expected that any high quality
preamplifier unit would offer comprehensive facilities to allow correction of the
electrical output signal. However, there has been a continuous improvement in the
quality of the input signal, whether obtained from compact disc, LP record, FM tuner
or cassette tape. Similarly, there has been a continuing improvement in LS
performance, so that, for example, the bass output is only limited by the physical
dimensions of the LS cabinet, or of the listening room, and additional bass boost
would be pointless. Faced with this situation, many amplifier manufacturers no longer
offer anything other than fiat frequency response amplifier units.
While the flat frequency response approach is probably adequate for most purposes, I
feel that there are still a few remaining situations where, for example, high pass rumble
filters are useful- such as when a new CD has been mastered from an archive source,
and contains significant amounts of LF noise from mechanical sources, or where a
blameless recording suffers from low frequency traffic noise intrusions into the
notionally silent recording studio - or where the tonal balance chosen by the recording
engineer differs from that preferred by the listener. In this case some form of
frequency response tilt control would probably be useful. I have therefore shown two
design examples of these. The first of these, due to Quad, is shown in Figure 10.14,
and the second, due to Bingham (Bingham, J., Hi-Fi News and Record Review,
December 1982, pp. 64-65) is shown in Figure 10.15. I have also shown the layouts
for four steep cut filter circuits in Figure 10.16.
The filter circuits shown in Figures 10.16a and 10.16b use my own bootstrap design
(Linsley Hood, J.L., Electronic Engineering, July 1976, pp. 55-58) and give a
21 6
Valve and Transistor Audio Amplifiers
-18dB/octave frequency response. That of Figure 10.16a is a 50Hz high pass layout,
and that of 10.16b is a 10kHz low pass circuit. The filter circuits shown in Figures
10.16c and 10.16d have the same cut-off frequencies, but use the Sallen and Key
layout (Sallen, R.P., and Key, E.L., IRE Trans. Circuit Theory, March 1955, pp.
40--42) which gives a-12dB/octave response. For both filters the turnover frequency
is given by the formula:
fT = 1/[2n{(C1C2R1R2)]
Low Z
Input
J
- /
/
Output
(a)
+
O3
r
.o
u1 i i
v
.
E
e-
Frequency
(b)
Figure 10.14
The 'Quad' slope control
The turnover frequencies can be changed, up or down, by proportional alterations to
the resistor or capacitor values. For example, if the values of C 1, C2 and C3, in Figure
10.16a, are increased by the factor 5/3, the turnover frequency of this filter will be
reduced to 30Hz. Similarly, if the values of the resistors, R1, R2 and R3, are halved,
the turnover frequency of Figure 10.16b will be increased to 20kHz, and so on.
Since the bulk of modem signal sources are not much in need of remedial treatment, I
have not included technical descriptions of the wide variety of tone control
arrangements which have been evolved over the years. For those seeking further
information on these, I would recommend my book Audio Electronics, (ButterworthHeinemann, 1965, Chapter 4, pp. 169-189), or an earlier article of mine in Electronics
Worm + Wireless Worm (July 1990, pp. 636--639).
C2
Input
RV~
220k
..
220k
!~ 1
TL071
R2
,R6
'56k
RV1
R8
SLOPE
A2
,R7
'56k
+5-
220k
A3
~n
"0
v
cO
vt
C3
lOuF
TL071
.2 0
E
-
tn
Output
ov
C
.
.
.
.
.
J
v
RIO
220k
TL071
"[
v
0V
O
-5Frequency
~o
!
10o
I
lk
I
Frequency response
Figure 10.15
Bingham's tilt control
,
1Ok
,,,
1V
20k
21 8
Input
c
Valve and Transistor Audio Amplifiers
A1
il "
100nF
'
.L II II "
l~ ~
220nF
R3
OV
o
(a)
Output
(c)
A1
_~
R3
__ -.-
i
.,g,. i
cl +
1
i
ov - 12 d50Hz
B/octave
2u2F
50Hz
e -18dB/octave
Input R1L A
Cli ~ Cit . ~
22nFL 22nF [_ ~
R1 9 R; _ _ TL0
2k2 10kHz
-18dB/octave
(b)
Output
>
-]-
C3
-12nF
A1
,~
Input ~L1A _ AA
R2
~
--I
1c l
c'
270p 2
_
,,v
Output
27k
~ : - - J lOkHz
0v -12dB/octave
0V
O
(d)
Figure 10.16
Steep cut filter circuits
Signal Channel Switching and Remote Controls
The choice between input channels can be made by purely mechanical means, such as
a simple rotary switch or a suitable push-button sliding contact arrangement. The only
normal difficulty with this system is that the knob or the push-buttons which control
the channel selection will be on the front of the preamp box, while the signal inputs
between which the electrical choice is to be made all go to sockets on the back face of
the box, so that the intemal connecting wires which join the one to the other must pass
over other parts of the signal circuitry, and may need to be well screened. If the
switching element is a relay (preferably sealed, and inert gas filled), or a suitable
semiconductor switch, of the kind I have shown in Figure 10.17, the input switches
can be located close to the input sockets. It will also allow channel selection to be
made by an external voltage, and this will allow the convenience of remote control,
perhaps by the use of an infra-red transmitter/receiver assembly of the type widely
used for similar purposes with TV sets.
Most of the plain on/off types of signal switching, based on discrete components or
CMOS analogue switches, of the type I have shown in Figure 10.17 offer acceptably
low distortion characteristics up to, say, a 1V rms signal level. In the case of the series
connected FET switch, shown in Figure 10.17c, the value of Rin should be high in
relation to the 'on' resistance of the FET to swamp any signal induced changes in the
channel resistance. In the case of the CMOS analogue switches shown in schematic
form in Figure 10.17d, these devices normally are designed to operate with logic level
Preamplifiers 2 1 9
input control voltages, and do not significantly distort input signals so long as their
peak-to-peak values lie comfortably between the limits set by the DC supply rails. The
extent of the distortion introduced by these devices depends on the output load (which
should, ideally, be very high) and on the quality (and cost) of the component itself.
Input
Output
Input
Output
IJ
o,,
o-
. . . . .
ov
o
g_l__,
(a)
!
ov
o
(b)
+ Vcc
Input
Rin
Output
Output
2orrtrol V
__~V
(c)
-Vdd
(d)
Figure 10.17
Voltage controlled switches
Voltage Controlled Gain S y s t e m s
While there are a number of low distortion switching circuits available, the techniques
proposed for a continuous, voltage controlled, alteration of the amplifier gain are
usually much less satisfactory. For simple, low cost signal-level control the FET
arrangement of Figure 10.18a, in which the FET and the input resistance, R1, form a
simple resistive attenuator, is fairly widely used. As it stands, this would have a THD
figure, at 1V rms and lkHz output, of some 10-15%. In the Siliconix file note,
AN73-1, it is proposed that negative feedback should be derived from the drain
circuit, by way of R2 and R3 (Siliconix suggest that the THD is lowest when R2 = R3
10(R1//Rds//RL) ). This will reduce the THD to about 1-1.5% and although this figure
decreases as the signal level is reduced, it is still very much higher than would be
acceptable in any Hi-Fi context.
A commercially available alternative to the discrete component (e.g. FET) voltage
controlled attenuator is marketed as the Motorola MC3340P IC, a device which was
widely used in TV remote control systems. This uses two long-tailed pair circuits in
which the gain is controlled by the tail current. By connecting the non-inverting and
inverting outputs in antiphase the second harmonic distortion is partially cancelled.
The claimed performance offers a 90dB possible attenuation ratio, a bandwidth of
1MHz, and a THD of 0.2%, worsening to 3% as the attenuation is increased.
2 2 0
Valveand TransistorAudio Amplifiers
Input
R1
Output
o---~;
_
=
I>
Rload
Control V
C-
R3
=,V-
--'
I N FET
!1
(a) Simple LFET circuit
~_
put
0V
O"
Figure 10.18
-
w
0V
='O
(b) Basic control circuit
Attenuator circuits
I described the various other available techniques in an earlier article (Linsley Hood,
J.L., Electronics Worm + Wireless WorM, April 1995, pp. 320-321), and suggested
the use of a very low distortion discrete component design (intended for use in a
dynamic range control circuit (Linsley Hood, J.L., Electronics Worm and Wireless
WorM, November 1995, pp. 938-940). This gain control circuit was based on a
switch-mode attenuator, shown in schematic form in Figure 10.18b. In this circuit a
high frequency (180kHz) rectangular voltage waveform is used to control the
proportion of the time in which the channel is open. This, in turn, is controlled by the
'on to off' ratio of the switching waveform, which can be changed, within the range
5% : 95% to 95% : 5% by a DC input voltage to the waveform generator circuit. This
gives an attenuation ratio of about 30dB, and a T H D - at 1V rms and 10Hz-10kHzof less than 0.005%, mainly due to the steep-cut low pass filters used to remove signal
components within 20kHz of the switching frequency.
In their application note AN-105, PMI propose an improved version of the system
used by Motorola, of which I have shown the circuit in Figure 10.19. The THD
introduced by this circuit depends on the cancellation of the distortion, mainly second
harmonic, introduced by the controlled transistors, Q l-Q4. For this to be adequate,
there must be an exceedingly close match between all four transistors, and PMI show
2 21
Preamplifiers
this as an application for their MAT-04 monolithic transistor array. PMI claim a THD
value of <0.03% at 3V rms output, for this circuit. Inevitably, such a useful discrete
component layout will be produced in due course as an IC module, and this has been
done by Analog Devices, Inc., in their SSM-2108T and SSM-2118T ICs, which have
a claimed THD in the range 0.013-0.006% at 3V rms depending on operating
conditions.
+Vcc
3ok
I
I
:13ok
i
l:3ok ",,~ok
A3
Control V
,
-
. . . .
§
Output
c
,
a
v
_
v
,
'
v
Input
)
L
~a_~ I
A1
9
~
A
:,0J, .
'
C2
IOOuF
30"k
- - p .
.
.
.
.
.
.
-Vdd
PI
Figure 1 0 . 1 9
Voltage controlled gain stage, due to PMI
If a step-type gain adjustment is acceptable, a simple option is shown in the layout of
Figure 10.20, in which the output from a point on a resistive divider chain is selected
by one or other of a series of analogue switches. Since the sequential operation of these
switches can be done by the use of a voltage controlled dot/bar driver IC, such as the
LM3914, there would be little difficulty or expense in assembling the hardware for
this type of relatively low distortion voltage operated volume control.
Digital Inputs
It has become customary for the manufacturers of compact disc players to provide a
direct digital output from the disc replay, usually as the RF signal derived from the CD
replay chain, shown, for reference, in Figure 10.21. This RF output will usually be
taken at a point following the 14-8 decoder, but before the Cross-Interleave ReedSolomon code (CIRC) error correction stage shown in the schematic replay layout of
Figure 10.21. This allows the manufacturer of specialist digital to analogue signalprocessing hardware to offer higher quality D/A conversion systems, more jitter-free
clock regeneration circuitry (which could be very important in high precision D/A
222
Valve and TransistorAudio Amplifiers
decoding), more competent error correction regimes and better anti-aliasing filtration
than might be offered in a budget-priced CD player. It is argued that these latter signal
handling stages probably contribute more to the overall tonal quality of the CD player
than the mechanical platform itself or its control hardware.
In general, the RF output signal will be presented as a simple coaxial cable output, but
sometimes in top of the range units, this signal may also be presented as an optical
fibre output to avoid possible electrical interference or signal degradation in the link
between the CD player and the A/D decoder.
Input
I J _1
~ , ~ i ! ..
I ,.__i I....
I
I
/
f -l
i
I ,-4,'!
!
~V
Figure 1 0 . 2 0
Step-type gain control
etc
ou,~u,
"'<J
Controlvoltage
+
~ll
-'~
.
.
n nn mmm
.
.
.
.
.
~
.
l
Disc drive
Front panel
and
Servo controls
ROM
RF amp
MUTE
14-8
decode
.
.....
.
I
I
I
~
r
I
CIRC
I
I~'F~.
!
CLOCK
REGEN.
RAM
DAC
DIGITAL
OVERSAMPLING
FILTER
DAC
'PF \
OUT LH
I -~lf
RO O
UT ~
Figure 10.21
CD replay schematic layout
roll
RAM
Motor
Head
OSition
cus
RF
detecti on
.
~
H
.dr
i
I
|
This Page Intentionally Left Blank
CHAPTER 1 1
POWER SUPPLIES
Active systems such as audio amplifiers operate by drawing current from some voltage
s o u r c e - ideally with a fixed and unvarying o u t p u t - and transforming this into a
variable voltage output which can be made to perform some useful function, such as
driving a loudspeaker or some further active or passive circuit arrangement. For most
active systems, the ideal supply voltage would be one having similar characteristics to
a large lead-acid battery: a constant voltage, zero voltage tipple and a virtually
unlimited ability to supply current on demand. In reality, considerations of weight,
bulk and cost would rule out any such Utopian solution and the power supply
arrangements will be chosen, with cost in mind, to match the requirements of the
system they are intended to feed. However, the characteristics of the power supply
used with an audio amplifier have a considerable influence on the performance of the
amplifier, so this aspect of the system is one which cannot be ignored.
High Power Systems
In the early days of valve operated audio systems, virtually all of the mains powered
DC power supply arrangements were of the form shown in Figure 11. l a and the only
real choice open to the designers was whether they used a directly heated rectifier,
such as a 5U4 or an indirectly heated one such as a 5V4 or a 5Z4. The indirectly heated
valve offered the practical advantage that the cathode of the rectifier would heat up at
roughly the same rate as that of the other valves in the amplifier, so there would not
be an immediate switch-on no-load voltage surge of 1.4" the normal HT supply output
voltage. With a directly heated rectifier this voltage surge would always appear in the
interval between the rectifier reaching its operating temperature, which might take
only a few seconds, and the thirty seconds or so which the rest of the valves in the
system would need to come into operation and start drawing current. Using an
indirectly heated rectifier would avoid this voltage surge and would allow lower
working voltage components to be used with safety in the rest of the amplifier. This
would save cost. On the other hand, the directly heated rectifier would have a more
efficient cathode system, and would have a longer working life expectancy.
Although there are several other reasons for this - such as the greater ease of
manufacture, by the use of modem techniques, of large value electrolytic capacitors,
or the contemporary requirement that there shall be no audible mains hum in the
amplifier output signal due to supply line AC r i p p l e - it is apparent that the
capacitance values used in the smoothing, decoupling and reservoir capacitors in
traditional valve amplifier circuits are much smaller than in contemporary systems
226
Valve and Transistor Audio Amplifiers
which operate at a lower output voltage. The main reason for this is that the stored
energy in a capacitor is defined by the relationship:
E c = ~CV 2
where E c is the stored energy, in joules, C is the capacitance, in farads, and V is the
applied voltage. This means that there is as much energy stored in an 81aF capacitor,
charged to 450V, as there is in a 400~tF capacitor charged only to 64V. Since the
effectiveness of a decoupling capacitor in avoiding the transmission of supply line
rubbish, or a power supply reservoir capacitor in limiting the amount of tipple present
on the output of a simple transformer/rectifier type of power supply, depends on the
stored charge in the capacitor, its effectiveness is very dependent on the applied
voltage- as is the discomfort of the electrical shock which the user would experience
if he inadvertently discharged such a charged capacitor through his body.
Solid State Rectifiers
The advent of solid state rectifiers - nowadays almost exclusively based on silicon
bipolar junction technology- effectively caused the demise of valve rectifier systems,
although for a short period, prior to the general adoption of semiconductor rectifiers,
gas-filled rectifiers, such as the 0Z4, had been used, principally in car radios, in the
interests of greater circuit convenience because, in these valves, the cathode was
heated by reverse ionic bombardment, so no separate rectifier heater supply was
required. The difficulties caused by the use of these gas-filled rectifiers were that they
had a relatively short working life and that they generated a lot of RF noise. This RF
noise arose because of the very abrupt transition of the gas in the cathode/anode gap
of the rectifier from a non-conducting to a conducting state. The very short duration
high current spikes this caused shock-excited the secondary windings of the
transformer- and all its associated wiring interconnections- into bursts of RF
oscillation, which caused a persistent 100-120Hz rasping buzz, called modulation
hum, to appear in the audio output.
The solution to this particular problem was the connection of a pair of capacitors,
shown as C 1 and C2 in Figure 11.1a, across the transformer secondary windings to retune any shock-excited RF oscillation into a lower, and less invasive, frequency band.
Sometimes these modulation hum prevention capacitors are placed across the
rectifiers or across the mains transformer primary winding, but they are less effective
in these positions. With modem, low conduction resistance, semiconductor diodes,
low equivalent series resistance (ESR) reservoir capacitors and low winding resistance
(e.g. toroidal) transformers, this problem can still arise, and the inclusion of these
capacitors is a worthwhile and inexpensive precaution. The circuit layout shown in
Figure 11. lb is the PSU arrangement used in most contemporary valve amplifiers. For
lower voltages a wider range of circuit layouts are commonly used, also shown in
Figure 11.1.
Power Supplies
57.4etc
TR1
+~VlIpC3~j
[~.
Reservoir
i
/
.
1
0V
c~
.
.
.
- O
(a)
TR1
D1
*!~!_
out l
.
.
input
A
N
Mainsinput
D2
ReservoirT
.... O
(b)
2
4
6
2
4
6
+V out
(c)
-_
ov
1
3
5
etc
+V out
(d)
OV
Outputwaveforrnwith f-w recification
F i g u r e 11.1
Full-wave rectifier systems
OV
227
228
Va!ve and Transistor Audio Amplifiers
TR1
0
Mains input
O
II IIICAPELEC
ov O
-V out O
'
.
.
IICAPELEC
~-9:=<:2
'
-
t
(e)
+V out
OV
Mains input
-V out
(f)
+V out C
C2
t-
O
Mains input
0vC
-
-
v
|
v
(g)
D1
+V~
C~ f ~
T
--~1
C1
' klTr~ ''~O
~L.,c2
(h)
Figure 11.1 (continued)
Full-wave rectifier systems
i
Mains input
PowerSupplies 2 2 9
Music Power
In their first flush of enthusiasm for solid state audio amplifiers, their manufacturers
and their advertising copy writers collectively made the happy discovery that most
inexpensive audio amplifiers powered by simple supply circuits- such as that shown
in Figure 11. lb - would give a higher power output for short bursts of output signal,
such as might, quite reasonably, be expected to arise in the reproduction of music, than
they could give on a continuous sine-wave output. This short-duration, higher output
power capability was therefore termed the music power rating, and, if based on a test
in which, perhaps, only one channel was driven for a period of 100 milliseconds every
second, would allow a music power rating to be claimed which was double that of the
power which would be given on a continuous tone test, in which both channels are
driven simultaneously (the so-called rms output power rating).
Influence of Signal Type on Power Supply Design
Although this particular method of specification enhancement is no longer widely
used, its echoes linger on in relation to modern expectations for the performance of HiFi equipment. The reason for this is that, in the earlier years of recorded music
reproduction there were no such things as pop groups, and most of those interested in
improving the quality of recording and replay systems were people such as Peter
Walker of Quad or Gerald Briggs of Wharfedale loudspeakers, whose spare-time
musical activities were as an orchestral flautist and a concert pianist, and whose
interests, understandably, were almost exclusively concerned with the reproduction, as
accurately as possible, of classical music. Consequently, when improvements in
reproduction were attempted, they were in ways which helped to enhance the
perceived fidelity in the reproduction of classical music, and the accuracy in the
rendition of the tone of orchestral instruments. In general, this was easier to achieve if
the electronic circuitry was fed from one or more accurately stabilised power supply
sources, though this would nearly always mean that such power supplies would have,
for reasons of circuit protection, a fixed maximum current output. While this would
mean that the peak power and the rms power ratings would be the same, it also meant
that there would be no reserve of power for sudden high level signal d e m a n d s - a
penalty which the tonal purists were prepared to accept as a simple fact of life.
However, times change, and Hi-Fi equipment has become more easy to accommodate,
less expensive in relative terms, and much more widely available. Also, there has been
a considerable growth in the purchasing power of those within relatively youthful age
bracket, most of whose musical interests lie in the various forms of pop m u s i c preferably performed at high signal levels - and it is for this large and relatively
affluent group that most of the Hi-Fi magazines seek to cater. (If an example of the
balance of contemporary musical taste is needed, consider the musical programme
chosen by Martin Colloms and Hi-Fi News for the amplifier comparison trial referred
to in Chapter 9, in which ten of the sixteen test pieces were drawn from the pop idiom.)
The ways in which these popular musical preferences influence the design of audio
amplifiers and their power supplies relate, in large measure, to the peak short-term
output current which is available, since one of the major instruments in any pop
230
Valve and Transistor Audio Amplifiers
ensemble will be a string bass, whose sonic impact and attack will depend on the
ability of the amplifier and power supply to drive large amounts of current into the LS
load, and it must do this without causing any significant increase in the tipple on the
DC supply lines or any loss of amplifier performance due to this cause.
A further important feature for the average listener to a typical pop ensemble is the
performance of the lead vocalist, commonly a woman, the clarity of whose lyric must
not be impaired by the high background signal level generated by the rest of the group.
Indeed, with much pop music, with electronically enhanced instruments, the sound of
the vocalist- though also electronically enhanced- is the nearest the listener will get
to a recognisable reference sound. This clarity of the vocal line demands both low
intermodulation distortion levels and a complete absence of peak-level clipping.
The designer of an amplifier which is intended to appeal to the pop music market must
therefore ensure that the equipment can provide very large short duration bursts of
power, that the power supply line tipple level, at high output powers, must not cause
problems to the amplifier, and that, when the amplifier is driven into overload, it copes
gracefully with this condition. The use of large amounts of NFB, which causes hard
clipping on overload, is thought to be undesirable. Similarly, the effects of electronic
(i.e. fast acting) output transistor current limiting circuitry (used very widely in earlier
transistor audio amplifiers) would be quite unacceptable for most pop music
applications, so alternative approaches, mainly based on more robust output
transistors, must be used instead.
In view of the normal lack in much pop music of any identifiable reference sound
source - such as would be provided by the orchestral or acoustic keyboard instruments
in classical music f o r m s - a variety of descriptive terms has emerged, to indicate the
success or otherwise of the amplifier system in providing attractive reproduction of the
music. Terms such as exciting or giving precise image location or vivid presence or
having full sound staging or blurred or transparent are colourful and widely used in
performance reviews, but they do not help the engineer in his attempts to approach
more closely to an ideal system performance - attempts which must rely on
engineering intuition and trial and error.
High Current Power Supply Systems
In order for the power supply system to be able to provide high output currents for
short periods of time, the reservoir capacitor, C3 in Figure 11.1b, must be large, and
have a low ESR (equivalent series resistance) value. Ideally, the rectifier diodes used
in the power supplies should have a low conducting resistance, the mains transformer
should have low resistance windings and low leakage inductance, and all the
associated w i r i n g - including any PCB tracks - should have the lowest practicable
path resistance. The output current drawn from the transformer secondary winding, to
replace the charge lost from the reservoir capacitor during the previous half cycle of
discharge, occurs in brief, high current bursts, in the intervals between the points on
the input voltage waveform labelled 1 and 2, 3 and 4, 5 and 6 and so on, shown in
Figure 11.1 c. This leads to an output tipple pattern of the kind shown in Figure 11.1 d.
PowerSupplies 2 3 1
Unfortunately, all of the measures which the designer can adopt to increase the peak
DC output current capability of the power supply unit will reduce the interval of time
during which the reservoir capacitor is able to recharge. This will increase the peak
rectifier/reservoir capacitor recharge current and will shorten the duration of these
high current pulses. This increases the transformer core losses, and both the
transformer winding and lamination noise, and also increases the stray magnetic field
radiated from the transformer windings. All of these factors increase the mains hum
background, both electrical and acoustic, of the power supply unless steps are takenin respect of the physical layout, and the placing of interconnections - to minimise it.
The main action which can be taken is to provide a very large mains transformer,
apparently excessively generously rated in relation to the output power it has to
supply, in order that it can cope with the very high peak secondary current demand
without mechanical hum or excessive electromagnetic radiation. Needless to say, the
mains transformer should be mounted as far away as possible from regions of low
signal level circuitry, and its orientation should be chosen so that its stray magnetic
field will be at fight angles to the plane of the amplifier PCB.
Half-wave and Full-wave Rectification
Because the reservoir capacitor recharge current must replace the current drawn from
it during the non-conducting portion of the input cycle, both the peak recharge current
and the residual tipple will be twice as large if half-wave rectification is employed,
such as that shown in the circuit of Figure 11.1h, in which the rectifier diode only
conducts during every other half cycle of the secondary output voltage, rather than on
both cycles as would be the case in Figure 11.1b. A drawback with the layouts of both
Figure 11.1a and 11.1b is that the transformer secondary windings only deliver power
to the load every other half cycle, which means that when they do conduct, they must
pass twice the current they would have had to supply in, for example, the bridge
rectifier circuit shown in Figure 11.1e. The importance of this is that the winding
losses are related to the square of the output current (P = i~R) so that the transformer
copper losses would be four times as great in the circuit of Figure 11.1b as they would
be for either of the bridge rectifier circuits of Figure 11.1 f. On the other hand, in the
layout of Figure 11.1b, during the conduction cycle in which the reservoir capacitor is
recharged, only one conducting diode is in the current path, as compared with two, in
the bridge rectifier set-ups.
Many contemporary audio amplifier systems require symmetrical +ve and-ve power
supply rails. If a mains transformer with a centre-tapped secondary winding is
available, such a pair of split-rail supplies can be provided by the layout of Figure
11.1 e, or, if component cost is of no importance, by the double bridge circuit of Figure
11.1 f. The half-wave voltage doubler circuit shown in Figure 11.1g is used mainly in
low current applications where its output voltage characteristic is of v a l u e - such as
perhaps a higher voltage, low-current source for a three-terminal voltage regulator.
232
Valve and Transistor Audio Amplifiers
,,,
DC Supply Line Ripple Rejection
Avoidance of the intrusion of AC ripple or other unwanted signal components from
the DC supply rails can be helped in two ways - by the use of voltage regulator
circuitry to maintain these rails at a constant voltage, or by choosing the design of the
amplifier circuitry which is used so that there is a measure of inherent supply line
signal rejection. In a typical audio power amplifier, of which the voltage amplifier
stage is shown in Figure 9.11, there will be very little signal intrusion from the +ve
supply line through the constant current source, Q6 and Q7, because this has a very
high output impedance in comparison with the emitter impedance of Q 1 and Q2, so
any AC tipple passing down this path would be very highly attenuated. On the other
hand, there would be no attenuation of rubbish entering the signal line via RS, so that,
in a real-life amplifier, R5 would invariably be replaced by another constant current
source, such as that arranged around Q7 and Q8.
For the negative supply rail, the cascode connection of Q 10 would give this device an
exceedingly high output impedance, so any signal entering via this path would be very
heavily attenuated by the inevitable load impedance of the amplifier. Similarly, the
output impedance of the cascode connected transistors Q3 and Q4 would be so high
that the voltage developed across the current mirror (Q5 and Q6) would be virtually
independent of a n y - v e rail tipple voltage. In general, the techniques employed to
avoid supply line intrusion are to use circuits with high output impedances wherever
a connection must be made to the supply line rails. In order of effectiveness, these
would be a cascode connected FET or bipolar device, a constant current source, a
current mirror or a decoupled o u t p u t - such as a bootstrapped load. HT line
decoupling, by means of an LF choke or a resistor and a shunt connected capacitor,
such as R2 and C2 in Figure 4.2, was widely used in valve amplifier circuitry, mainly
because there were few other options available to the designer. Such an arrangement
is still a useful possibility if the current flow is low enough for the value of R2 to be
high, and if the supply voltage is high enough for the voltage drop across this
component to be unimportant. It still suffers from the snag that its effectiveness
decreases at low frequencies where the shunt impedance of C2 begins to increase.
Voltage Regulator Systems
Electronic voltage regulator systems can operate in two distinct modes, each with their
own advantages and drawbacks: shunt and series. The shunt systems operate by
drawing current from the supply at a level which is calculated to be somewhat greater
than maximum value which will be consumed by the load. A typical shunt regulator
circuit is that shown in Figure 11.2a, in which the regulator device is an avalanche or
Zener diode, or, for low current, high stability requirements a two-terminal band-gap
element. Such simple circuits are normally only used for relatively low current
applications, though high power avalanche diodes are available. If high power shunt
regulators are needed a better approach is to use a combination of avalanche diode and
power transistor, as shown in Figure 11.2b. The obvious snag is that in order for such
a system to work, there must be a continuous current drain which is rather larger than
the maximum likely to be drawn by the load, and this is wasteful. The main advantages
PowerSupplies
233
of the shunt regulator system are that it is simple, and that it can be used even when
the available supply voltage is only a little greater than the required output voltage.
Avalanche and Zener diodes are noisy, electrically speaking, though their noise can be
lessened by connecting a low ESR capacitor in parallel with them. For applications
where only a low voltage is needed, its actual value is not very important but a low
circuit noise is essential, a simple arrangement is to use a string of silicon diodes, as
shown in Figure 11.2c. Each of these diodes will have a forward direction voltage drop
of about 0.6V, depending on the current flowing though them. Light emitting diodes
have also been recommended in this application, for which a typical forward voltage
drop would be about 2.4V, depending on the LED type and its forward current. All of
these simple shunt regulator circuits will perform better if the input resistor (R1) is
replaced by a constant current source, shown as CC 1.
R1
V+ inC
V+ ir~
"eta,; - ~
Zener
OV 0
out
0 V
-
=~/~'--
--
"~
Zener diode
OVO
_
(a)
out
1
diode
---00V
o V
-
00V
(b)
CC1
V+ ino-~-~
--.~
V+ out
D1
1N4148
D2
1N4148
OVo
Figure
11.2
Simple
shunt
.
.
.
.
D3
1N4148
00V
(c)
regulators
Series Regulator Layouts
The problem with the shunt regulator arrangement is that the circuit must draw a
current which is always greater than would have been drawn by the load on its own.
This is an acceptable situation if the total current levels are small, but this would not
be tolerable if high output power levels were involved. In this situation it is necessary
to use a series regulator arrangement, of which I have shown some simple circuit
layouts in Figure 11.3. The circuit of Figure 11.3a forms the basis for almost all this
type of regulator circuit, with various degrees of elaboration. Essentially, it is a fixed
voltage source to which an emitter-follower has been connected to provide an output
voltage (that of the Zener diode less the forward emitter bias of Q 1) at a low output
234
Valve and Transistor Audio Amplifiers
impedance. The main problem is that, for the circuit to work, the input voltage must
exceed the output voltage - the difference is termed the drop-out voltage - by enough
voltage for the current flow through R1 to provide both the necessary base current for
Q 1 and also enough current through D1 for D 1 to reach its reference voltage. Practical
considerations require that R1 shall not be too small. In a well-designed regulator of
this kind, such as the 78xx series voltage regulator IC, the drop-out voltage will be
about 2V.
V+ in~
0V C~-
T
-
~.~tf--"~V+
"-
out
V+ inC
_
__QI(PNP) .
...~ V + o u t
Zener diode
R2
O 0V
R30
~"
.a.
ovo
i
R2
-
i ov ou
-
ov
(b)
Zener diode
-
Figure
11.3
Simple series regulators
--
0 0V
(c)
This drop-out voltage can be reduced by reversing the polarity of Q 1, as shown in
Figure 11.3b, so that the required base input current for Q 1 is drawn from the 0V rail.
This arrangement works quite well, except that the power supply output impedance is
much higher than that of Figure 11.3a, unless there is considerable gain in the NFB
control loop. In this particular instance Q2 will conduct, and feed current into Q 1 base
until the voltage developed across R3 approaches the voltage on the base of Q2, when
both Q2 and Q1 will be turned off. By augmenting Q2 with an op amp, as I have
shown in Figure 11.4, a very high performance can be obtained from this inverted type
of regulator layout.
Over-current Protection
A fundamental problem with any kind of solid state voltage regulator layout, such as
that of Figure 11.3a, is that if the output is short-circuited, the only limit to the current
which can flow is the capacity of the input power supply, which could well be high
enough to destroy the pass transistor (Q1). For such a circuit to be usable in the real
world, where HT rail short-circuits can, and will, occur, some sort of over-current
protection must be provided. In the case of Figure 11.3c, this is done by putting a
resistor (R2) in series with the regulator output, and then arranging a further transistor
(Q2) to monitor the voltage across this. If the output current demand is enough to
develop a voltage greater than about 0.65V across R2, Q2 will conduct, and will
progressively steal the base current from Q 1.
Power Supplies 2 3 5
In the inverted stabiliser circuit shown in Figure 11.4, R1 monitors the output current,
and if this is large enough to cause Q1 to conduct, then the output voltage will
progressively collapse, causing the PSU to behave as a constant current source, at
whatever output voltage causes the load to draw the current determined by R 1. (I know
this protection technique works because this is the circuit I designed for my workshop
bench power supply twenty years ago (Linsley Hood, J.L., Wireless WorM, January
1975, pp. 43-45), and it has been in use every working day since then, having endured
countless inadvertent output short-circuits during normal use, as well as surviving my
son having left it on overnight, at maximum current output, connected to a nickelplating bath which he had hooked up, but which had inadvertently become shortcircuited.) In the particular layout shown, the characteristics of the pass transistors
used (Q3 and its opposite number) are such that no current/voltage combinations
which can be applied will cause Q3 to exceed its safe operating area boundaries, but
this is an aspect which must be borne in mind. Although I use this supply for the initial
testing of nearly all my amplifier designs, it would not have an acceptable
performance, for reasons given above, as the power supply for the output stage of a
modem Hi-Fi amplifier.
Figure 11.4
SeriesstabilisedPSU
However, there is no such demand for a completely unlimited supply current for the
voltage amplifier stages or the preamplifier supply rails, and in these positions, a high
quality regulator circuit can be of considerable value in avoiding potential problems
due to hum and distortion components breaking through from the PSU rails. Indeed,
there is a trend, in modem amplifier design, to divide the power supplies to the
amplifier into several separate groupings, one pair for the gain stages, a second pair
for the output driver transistors, and a final pair of unregulated supplies to drive the
236
Valve and Transistor Audio Amplifiers
output transistors themselves. Only this last pair of supplies need normally to be fed
directly from a simple high current rectifier/reservoir capacitor type of DC supply
system.
A further possibility which arises from the availability of more than one power supply
to the power amplifier is that it allows the designer, by the choice of the individual
supply voltages which are provided, to determine whereabouts in the power amplifier
the circuit will overload, when driven too hard, since, in general, it is better if it is not
the output stage which clips. This was an option of which I took advantage in my 80
watt power MOSFET design of 1984 (Linsley Hood, J.L., Electronics Today
International, June 1984, pp. 24-31).
Integrated Circuit (Three Terminal) Voltage Regulator ICs
For output voltages up to +24V, and currents up to 5 amperes, depending on voltage
rating, a range of highly developed IC voltage regulator packages are now offered,
having over-current (s/c), and thermal overload protection, coupled with a very high
degree of output voltage stability, coupled with a typical >60dB input/output line
tipple rejection. They are most readily available in +5V and +15V/-15V output
voltages because of the requirements of 5V logic ICs and of IC op amps, widely used
in preamplifier circuits, for which + 15V supply rails are almost invariably specified.
Indeed, the superlative performance of contemporary IC op amps designed for use in
audio applications is such an attractive feature that most audio power amplifiers are
now designed so that the maximum signal voltage which is required from the pre amp
is within the typical 9.5V rms output voltage available from such IC op amps.
Higher-voltage regulator ICs, such as the LM337T and the LM317T, with output
voltages up t o - 3 7 V and +37V respectively, and output currents up to 1.5A, are
available, but where audio amplifier designs require higher voltage stabilised supply
rails, the most common approaches are either to extend the voltage and current
capabilities of the standard IC regulator by adding on suitable discrete component
circuitry, as shown in Figure 11.5, or by assembling a complete discrete component
regulator of the kind shown in Figure 11.6.
In the circuit arrangement shown for a single channel in Figure 11.5, a small-power
transistor, Q 1, is used to reduce the 55-60V output from the unregulated PSU to a
level which is within the permitted input voltage range for the 7815 voltage regulator
IC (IC2). This is one of a pair providing a +15V DC supply for a preamplifier. A
similar 15V regulator IC (IC 1) has its input voltage reduced to the same level by the
emitter-follower Q4, and is used to drive a resistive load (R7), via the control
transistor, Q5. If the output voltage, and consequently the voltage at Q5 base, is too
low, Q5 will conduct, current will be drawn from the regulator IC (IC 1), and, via Q4,
from the base of the pass transistor, Q2. This will increase the current through Q2 into
the output load, and will increase the output voltage. If, however, the output voltage
tends to rise to a higher level than that set by RV 1, Q5 will tend towards cut-off, and
the current drawn from Q2 base will be reduced, to restore the target output
voltage level.
BR1
2
:: 45-0-45V
C3
4700uF
R3
Power output
+35-50V
To -ve supply
R4
0R22
IVU2501
R9
820R
2k7
C10
Oul F
C14
Oul
i
i•4C770u
+15V to preamp
C11
Oul F
OV#2
Figure 11.5
Stabilised PSU (one half only shown)
C12
F
OV#1
238
Valve and Transistor Audio Amplifiers
Over-current protection is provided by the transistor Q3 which monitors the voltage
developed across R4, and restricts the drive to Q2 if the output current is too high. Safe
operating area conformity is ensured by the resistor R3, which monitors the voltage
across the pass transistor, and cuts off Q2 base current if this voltage becomes too
high.
In the circuit of Figure 11.6, which is used as the power supply for an 80-100W power
MOSFET audio amplifier- again only one channel is s h o w n - a P-channel power
MOSFET is used as the pass transistor, and a circuit design based on discrete
components is used to control the output voltage. In this, transistor Q21 is used to
monitor the potential developed across R33 through the R35/RV3 resistor chain. If this
is below the target value, current is drawn through Q19 and R29, to increase the
current flow through the pass transistor (Q17). If either the output current or the
voltage across Q 17 is too high, Q7 is cut off and there is no current flow through Q 18
into Q 17 gate.
+ 70V in
Q17 P-MOSFET
0R15
+ 55V to
power amp
D1
0.47t
D2
I
~
l
t g:=,~ I
_===C9:
.12k
Set V out
Q7
R13
15k
ov
I
I
I
'T"
To LS
Figure
I
I
I
ov
-12V ref.
11.6
S/c protected PSU
This regulator circuit allows electronic shut-down of the power supply if an abnormal
output voltage is detected across the LS terminals (due, perhaps, to a component
failure). This monitoring circuit (one for each channel) is shown in Figure 11.7. This
uses a pair of small-signal transistors, Q 1 and Q2, in a thyristor configuration which,
if Q2 is ttLrned on, will connect Q 1 base to the 0V rail, which, in turn, causes current
to be drawn from Q2 base, which causes Q2 to remain in conduction even if the
original input voltage is removed. The trip voltage will arise if an excess DC signal
(e.g. >10V) appears across the LS output for a sufficient length of time for Q1 to
Power Supplies
239
charge to +5V. Returning to Figure 11.6, when the circuit trips, the forward bias
voltage present on Q 19 base is removed, and Q 17 is cut off, and remains cut off until
the trip circuit is reset by shorting Q2 base to the 0V rail. If the fault persists, the
supply will cut out again as soon as the reset button is released. An electronic cut-out
system like this avoids the need for relay contacts or fuses in the amplifier output lines.
Relays can be satisfactory if they are sealed, inert gas-filled types, but fuseholders are,
inevitably, crude, low cost components, of poor construction quality and with a
variable and uncertain contact resistance. These are best eliminated from any
signal line.
Trip volta~
C1
IOOuF
Figure 11.7
Trip circuit
q
OV
Typical Contemporary Commercial Practice
The power supply circuit used in the Rotel RHB 10 330 watt power amplifier is shown
in Figure 11.8 as an example of typical modern commercial practice. In this design,
two separate mains power transformers are used, one for each channel (the drawing
only shows the LH c h a n n e l - the RH one is identical) and two separate bridge
rectifiers are used to provide separate +70V DC outputs for the power output
transistors and the driver transistors. This eliminates the distortion which might
otherwise arise because of breakthrough of signal components from the output
transistor supply rail into the low power signal channel. Similarly, the use of a separate
supply system for each channel eliminates any power supply line induced L-R crosstalk which might impair stereo image positioning.
Battery Supplies
An interesting new development is the use of intemally mounted rechargeable
batteries as the power supply source for sensitive parts of the amplifier circuitry- such
as low input signal level gain stages. Provided that the unit is connected to a mains
power line, these batteries will be recharged during the time the equipment is switched
off, but will be disconnected automatically from the charger source as soon as the
amplifier is switched on.
+70V
8A
BR2
-' ....
-70V
C1
C3
10nF
2200uF
C8
10nF
F2
8A
+70V
6k8 2W
. ~ ~" ~
,.
~
C5
6800uF
c6
6800uF
6k8 2W
-70V
Supply to driver transistors
LH channel. Other channel identical
Figure
11.8
Rotel r h b l O PSU (only o n e channel s h o w n )
L
-'1-- 10nF
2200uF
Supply to o u t p u t transistors
LH channel
Power input
C9
10nF
Thermal fuse
PowerSupplies 2 4 1
Switch-mode Power Supplies
These are widely used in computer power supply systems, and offer a compact, high
efficiency regulated voltage power source. They are not used in Hi-Fi systems because
they generate an unacceptable level of HF switching noise, due to the circuit operation.
They would also fail the requirement to provide high peak output current levels.
This Page Intentionally Left Blank
INDEX
A
Absolute zero, 11
Amplification factor (~t), 4
Amplified diode, 153, 175-6, 190-91
Amplifier stages, valve, 41-2
Anode, 1, 3
Anode current resistance (Ra), 4
Anode follower, 61
Anode-grid capacitance, 6
Apparent loudness, 148
Armstrong, E., 71
Asymmetry,
in output stages, 145-6
in PNP/NPN pairs, 156
Attenuator systems, 219-221
Audio signals, 189
Avalanche breakdown, 14
Avalanche diode, 232
Bailey, A.R., 156-61
audio amplifier, 156-8
RIAA stage, 204
Battery PSUs, in audio amps, 239
Baxandall, P.J., 150
audio amplifiers, valve, 101-2,
104
RIAA calculations, 203
tone control, 84-5
transistor output stage, 150-3
Beam tetrodes, 7
Bias,
optimum value, BJTs vs
MOSFETS, 135
output stages, effect on THD, 149
Biasing, transistor, 119-20
Bifilar/trifilar windings, 94-5
Bingham, J., slope control, 215
BJTs vs. MOSFETs, 134
Blomley, P., 163
Bode diagram, 134
Boltzmann's constant, 16, 174
Bonjiorno, A., 162
Bootstrap filter, 217-8
Borbeley, E., 162
Breakdown (dielectric), 31
Briggs, G., 229
r
Capacitance,
junction diode, 14
valve intemal, 6
Capacitors,
ceramic, 34
temperature coefficient, 34
electrolytic, 30-1
Cascode,
FET-bipolar, 129
gain stage, 122
isolation, 130
output impedance, 130
RIAA stage, 87
stage gain, 55
Cathode, 1-2
biasing, 44-8
damage due to ionic
bombardment, 6, 16
directly vs. indirectly heated, 2, 8,
10
emissivity, vs. temperature, 10
244
Valve and Transistor Audio Amplifiers
,,
materials, 2
operating temperature, 2, 9
CD, replay improvements, 221-2
Channel resistance (MOSFET), 23,
131
Charge on electron, 16
Chip dimensions, in BJT, 126
Choke, swinging, 30
Class A operation,
amplifier output stages, 155
J.E. Sugden, Ltd, 155
Clipping, output, on overload, 215
selective, in audio amps, 236
CMOS switches, 218
Colloms, M., 194, 229
Compound emitter follower, 125,
143
voltage offset, 126
Constant current source, 158
Control grid, 4
Crossover distortion,
in Q/C output stages, 145-7
MOSFET vs. BJT, 135-42
Current mirrors, 138, 158
circuit symbol, 138
Current gain, transistor, 119
Current limiting, in voltage
regulators, 234
Diode, 1
Disc cutting problems, in vinyl
records, 198
Distortion (see also THD),
beam tetrode, 51, 93
disc replay, 198-9
effect of NFB, 181
effect of stage gain, 121
feedback pair, 122
junction transistor, 119
PU cartridge, 201
transformer core, 68
valve amplifier, 93
Distributed load operation,
tetrode/pentode, 98
Dominant lag HF compensation, 153,
160
Doping levels, non-uniform, in PN
junction, 133
Doping of semiconductors, 11
Doping agents, valve cathode, 2
Drain gate capacitance, MOSFET 23,
135
Drop out voltage, 234
Dummy loads, 149
E
D
Darlington pair, 125-6, 143
DC offset, output, 156, 191-2
DC bootstrap circuits, 153
Decoupling, HT line, 61
Depletion zone, junction, 12-13
Design errors, in valve audio amps,
115
Dielectrics, capacitor, 34-5
Dinsdale, J. Transistor RIAA stage,
204-5
Eddy current losses, 27, 68
Efficiency, valve output stages, 98-9
Electret formation, 35
Emitter follower, output impedance,
124
Enhancement mode MOSFETs, 131
Epitaxial transistors construction,
118
Equivalent series resistance (ESR),
230
Evolution of trapped gases, in valve
anodes, 126
Excess noise, 6, 210
resistor, 38
Index 2 4 5
F
Feedback capacitance, transistor, 186
Fermi level, semiconductor, 12
Ferrite bead, cause of distortion, 29
FET (JFET),
attenuator systems, 219-21
construction, 18-20
input voltage operating range,
127
long tailed pairs,
PMI, 220
analog devices, 221
low noise, 209-10
matched pairs, 210
Filament, thermionic valve 2
Filters,
bootstrap, 217-8
HP/LP, 87-9
Sallen and Key, 217-8
slope control, Bingham, 215-7
Williamson, 87-8
Fixed bias, effect on valve output
power, 44, 109
biasing arrangements, 128
Fleming, Sir A., 1
Frequency response, V-MOS vs.
T-MOS, 135
G
Gain control, position, 215
Gain,
blocks,
typical designs, 212-17
low noise, 206
FET voltage amplifier, 129
margins, in NFB system, 160
Gas, effect within valve, 7, 9
Gate capacitance (MOSFET), effect
on gm,174
Gate stopper resistor, power
MOSFET, 169
Gate-drain capacitance, MOSFET,
23
GEC,
audio amp designs, 107-10
metal cone LS, 110
Gettering, 9
Gramophone p.u., maximum output
voltage, 214-5
Gramophone recording
characteristics, 213
Grid alignment, beam tetrode, 51
Grid current biasing, 42-3
H
Hailer and Keroes, (see also Ultralinear), 98
Heater circuit wiring, 10
Heater, valve, 2
Heater voltage, operating range, 9
HF instability, effect on sound
quality, 104
Hitachi Ltd., gain stage design, 160
Hole storage,
in BJTs, 135
in transistor output stages, 191
MOSFET freedom from, 169
Holes, in doped semiconductors, 13,
17
Hysteresis loss, 27, 68
Inductors, air cored, 29
Input capacitance, MOSFET, 135
Input impedance,
in circuits using NFB, 77-8
junction FET, 30, 97
Input signal, voltage limits, 212-13
Input stages, discrete components vs.
IC, 212
Input switching systems, 218-20
246
Valve and Transistor Audio Amplifiers
Instablity, audio amp. DC level, 113
Intrinsic semiconductors, 11, 117
Increased voltage/current ratings in
V. regs, 236
J
James, D.B., FET based RIAA stage,
205
Johnson (thermal) noise,
in FETs, 20, 128
in resistors, 38
Jung, W.C., 160
K
Kelly, S. PU output overload
characteristics, 214
L
Latch-up, avoidance, 135, 148
Leak power amplifier, 106
Leakage currents, capacitor, 33
Leakage inductance, 67
LF choke, as anode load, 45-6
Life expectancy, valve cathode, 41
Light emitting diode (LED), 233
Lin, H.C., 143
Listening trials, audio amplifier, 105,
193-5
Livy, L.H. RIAA calculations, 203
Locanthi, B.N., 156
Long tailed pair, 123,220-1
Loop stability,
Baxandall power amp., 102,
104, 160
GEC 9-12+ amplifier, 110
Williamson power amplifier, 97
Loudspeaker,
GEC metal cone, 110
protection, 190-2, 238-9
variability of load
characteristics, 149
Low-level signal loss, 148
LS load, effect of non-linearities,
84-5, 113
M
Magnetic field, stray, 231
Magnetic flux (B), 26
Magnetic hysteresis, 26, 174
Mains transformers, design
requirements, 231
Majority carriers,
in BJT operation, 156
in MOSFET, 169
Marantz, audio amp. circuit, 190
McIntosh amplifier, 94-5
Microphony, 7, 10, 11, 20, 145
Miller effect, 6, 186
Modulation hum, 226
Moir, J., 194
MOSFETs,
as output devices, 190
construction, 20-21
gate protection, 132-3
majority carrier operation, 135
on resistance, 131
relative cost, 169
relative linearity, 21, 171
similarity to pentode valve, 131
Vg / Id relationships, 132
Mullard (valve) audio designs, 107
Music power ratings, 229
Mutual conductance (gin),
BJT, 121
JFET, 127, 124
MOSFET, 133
valve, 8,16
Index 2 4 7
N
Negative feedback (NFB),
control of gain, 82
derived from output
transformer, 103
dominant lag, 160
effect on distortion, 81, 101,
104
gain/phase margins, 134, 160
HF loop compensation, 105
local, 61,105
series vs. shunt, 75-6, 199-201
via internal capacitance, 186
Nelson-Jones, L., 176-7
Noise,
l/f, 210
diode, 6
flicker, 6, 210
input resistance, 209
reduction by parallel
connection, 207, 209
thermal, in JFET, 128, 212
MOSFET vs. JFET, 131
O
Oscillation, in output stages, 169
Otala, M., 160
Output,
biasing, transistor, optimum
value, 147-8
DC offset, 191-2
Output characteristics,
effect of grid bias, 113
effect of load impedance, 99
power, in battery operated
amps., 59
power requirements, 'pop'
music, 229-32
waveform distortion, effect of
NFB, 187
Output impedance,
effect of NFB, 77-8
effect of fixed bias, 113
Output stages,
choice of valve type,
Continental practice, 106
fixed bias, 113
impedance, BJT vs. MOSFET,
131
instability, 169
malfimction, 145-6
overload characteristics, 148
transistor, 191
transistor,optimum bias value,
147-8
Output transformer, effect on sound
quality, 65
Over-current protection, voltage
regulator, 234
Overload, desirable response, 168
Overload protection,
Bailey circuit, 156-8
effects of, 134, 230
not required for valve amps.,
134
typical circuitry, 190-1
voltage regulator, 234
Oxide coated cathodes, 2
P
Parasitic oscillation, in power
MOSFETs, 23, 25
Peak clipping, effect on amplifier
loudness, 148
Pentode, 6-7
stage gain, 52-3
Performance shortcomings, audio
amps., 215
Permeability (~), 25-6
Phase margin, in NFB amplifier, 160
Pick-up inputs, gramophone, 86-90
Pinch-off, in junction FET, 18
Planar process, 117-8, 145
4 8
Valveand TransistorAudio Amplifiers
Plate (anode), 3
Poisoning of valve cathode, 10
Positive feedback (PFB),
effect on gain, 100
effect on THD, 158
Potential barrier, in PN junction, 126
Power supply requirements, for 'pop'
music, 230
Preamplifier, noise level, 213-4
Presence control, 110
PU cartridge,
amplifier systems, 206-11
moving coil, 206
Push-pull layout, 59
Q
Quad (Acoustical Manufacturing
Co.),
303 amplifier, 150
current dumping system, 178-9
listening trials, 193-4
MM/MC RIAA input stage,
208-9
output triples, 150-3
valve output stage design, 93-4,
99-101
Quasi-complementary emitter
followers, 143-5, 150-3, 158
Quiescent current,
control, Pioneer circuit, 177
setting, output stages, 146-7,
150, 153, 162, 167
R
Radford, A., 105
Rectifiers,
semiconductor, 11-14, 226
valve, 6
OZ4, 226
directly vs. indirectly
heated, 226-7
Relay, LS protection, 190, 238-9
Reservoir capacitors, influence,
225-9
RF instability, 6
RIAA equalised stages,
Brimar, 89
cascode, 87
circuit layouts, 199-202
component value calculations,
203
direct coupled, 205
frequency response, 198
Ring of three, 83
RMS output power rating, 229
Rotel, power amplifier design, 191
Rumble filter,
Bailey, 204
JLH, gramophone pu input, 205
Williamson, 87-8
$
Safe operating area, transistor
(SOAR), 133
Sallen and Key, filters, 217-8
Sandman, A.M., 174, 180
Screen grid, 6, 50
cross-coupling, quad system,
100
decoupling, 100
Secondary emission, 7, 50
Secondary breakdown, 127-34
Self, D., 175-7
Self-healing, capacitor, 34
Semiconductor rectifiers, 13-14, 226
Settling time, 190
Shaw, I.M., 150
Shock excitation, RF, in power
transformer, 226
Shure, pick up groove trackability,
213
Shut-down of PSU, on overload, 238
Signal to noise ratio,
valve vs. transistor preamps.,
90-1
Index
Silicon,
P-type, N-type, 11
semiconductor junction, 12-13
breakdown voltage, 14
temperature coefficient,
170
Slew-rate limiting, 188
avoidance, 153, 161
distortion, 160-1
Slope control,
Bingham, 215-7
quad, 215-6
Sonab valve amplifier, 113-14
Space charge, 3, 5, 6, 92
Spectral low-noise input stage,
210-11
Split-rail power supplies, 191-2
Square wave test signal, 102
Square-wave response, power
amplifiers, 158
Stabilising techniques, HF, 182
Stability margins, in NFB amplifiers,
134, 155, 167
Stage gain,
BJT, 121, 181-2
Brimar preamp., 90
cascode, 55
DC bootstrap, 153
grounded grid vs. RF pentode,
47, 50
JFET, 128
valve, 45-53
Super-match pair transistors, 206-9
Supply line tipple rejection, 232
Suppressor grid, 7, 51
Swinging choke, 30
Switch-on noise avoidance, 192
T
Technics Class AA amplifier, 180
Temperature sensitivity, Ge devices,
141
MOSFETs vs. BJTs, 168
249
Testing audio amplifiers,
at low signal levels, 148
listening trials, 193-5
use of square-wave test signal,
102
Tetrode, 1, 6
THD vs. output power, 148
Thermal compensation,
inadequacy of specification,
148
output transistor, 144
Thermal diffusion, 141
Thermal dissipation limits,
JFET, 127
resistor, 37
Thermal noise,
long tailed pair, 124
resistor, 38
Thermal overload protection, in IC
V. regs, 236
Thermal runaway, 126, 144, 170
Thermions, 1
Tolerances, in resistors, 37
Tone controls,
core losses, 231
distortion, 65-9
inter-stage coupling, 46
mismatch, 58
phase-splitter, 47
winding/lamination noise, 231
Transient intermodulation distortion
(TID), 160
Transistor circuit symbols,
BJT, 17
JFET, 18
Transistor HF performance, 15
Transistor sound, 148
Transition frequency, 15
TRENCHMOS MOSFET
construction, 174
Triode valve, 1
Triode connection,
effect on R a, 65
tetrode/pentode, 64, 97
Tungsten (bright emitter) cathode, 2
Tunneling, in JFET, 18
S0
Valveand TransistorAudio Amplifiers
Tum-off time, in output transistors,
191
IJ
Ultra-linear output valve operation,
circuit examples, 125
compromise load values, 98-9
Hailer and Keroes circuit, 98
I/
Valve operated amplifier stages, 41-2
RIAA, 202
Valve parameters, 8-9
Van de Gevel, Iq control circuit, 178
Virtual earth, 79
feedback circuit, 113
mixer layout, 79
W
Walker, P.J., 98,229
Walton, J.,
gramophone recording
characteristics, 213
White cathode follower, 113
Williamson, D.T.N., 64, 86-7, 95-6,
107-8
Wolfenden, B.S. Recording
characteristics, 213
u
Zener diode, 14, 232-9
Zobel network, 105, 153
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