60
Contents
PA300 POWER AMPLIFIER
T
here are several starting
points to the design of a
power amplifier: pure hi-fi without any compromise; simplicity
and reliability; high output
power. The design of the present
amplifier is a mixture of these.
The result is a unit that does not
use esoteric components, is not
too complex, and is fairly easily
reproduced. In fact, it could well
be named a ‘Hi-fi public address
amplifier’.
There will be a few eyebrows
raised at the power output of
300 watts (into 4Ω); it is true, of
course, that in the average living
room 30–40 W per channel is
more than sufficient. However,
peaks in the reproduced music
may have a power of 10–20
times the average level. This
means that some reserve power
is desirable. Also, there are loudspeakers around with such a low
efficiency that a lot more than
30–40W is needed. And, last but
not least, there are many people
who want an amplifier for rooms
much larger than the average living room, such as an amateur
music hall.
IC,
Design by A. Riedl
Taken by themselves, the properties of the
PA300 amplifier are not revolutionary. But
taken in combination, they show something
special: a robust 300 watt hi-fi power
amplifier that is not too difficult to build.
Straightforward design
Since every amplifier contains a certain
number of standard components, the circuit
of F i g . 1 will look pretty familiar to most
audio enthusiasts. Two aspects may hit the
eye: the higher than usual supply voltage
and the presence of a couple of IC s. The
first is to be expected in view of the power
output. One of the IC s is not in the signal
path and this immediately points to it being
part of a protection circuit. What is unconventional is an IC in the input stage.
Normally, this stage consists of a differential
amplifier followed by a voltage amplifier of
sorts, often also a differential amplifier, to
drive the predriver stages. In the PA300,
the entire input stage is contained in one
T e c h n i c a l d a t a (measured with power supply shown in Fig. 2)
Input sensitivity
Input impedance
Power output (0.1% THD)
Music power(500 Hz burst
5 cycles on, 5 cycles off)
Power bandwidth (90 W into 8 Ω)
Slew rate
Signal-to-noise ratio (referred
to 1 W into 8 Ω)
Harmonic distortion (THD+N)
(bandwidth 80 kHz)
Intermodulation distortion
(50 Hz:1 kHz; 4:1)
Dynamic IM (rectangular
wave + 15 kHz sine wave)
Damping factor (at 8 Ω)
a Type NE5534 (IC1).
The internal circuit of IC 1 is
shown in the box on further on in
this article. It may also be of interest to note that the NE5534 is
found in nine out of every ten CD
players (as amplifier in the analogue section). This is reflected in
its price which is low. Its only
drawback is that its supply voltage is far below that of the remainder of the amplifier. This
means an additional symmetrical
supply of ±15 V. Moreover, it restricts the drive capability of the
input stage. The supply requirement is easily met with the aid of
a couple of zener diodes and resistors. The drive restriction
means that the amplifier must
provide a measure of voltage amplification after the input stage.
1 Vrms
17.8 kΩ
164 W into 8 Ω
275 W into 4 Ω
176 W into 8 Ω
306 W into 4 Ω
7 Hz–67 kHz
20 V/µs
>96 dB (A-weighted)
at 1 W into 8 Ω: <0.004% (1 kHz)
at 150 W into 8 Ω: <0.001% (1 kHz)
<0.05% (20 Hz–20 kHz)
at 1 W into 8 Ω: <0.003%
at 100 W into 8 Ω: <0.0035%
1 W into 8 Ω: <0.004%
150 W into 8 Ω: <0.06%
<345 at 1 kHz
<275 at 20 kHz
Circuit description
The input contains a high-pass
filter, C5-R3 and a low-pass filter,
R2-C6. The combination of these
filters limits the bandwidth of the
input stage to a realistic value: it
is not necessary for signals well
outside the audio range to be
amplified – in fact, this may well
give rise to difficulties.
Opamp IC1 is arranged as a differential
amplifier; its non-inverting (+) input functions as the meeting point for the overall
feedback. The feedback voltage, taken from
junction D7-D8, is applied to junction R4-R5
via R9. Any necessary compensation is provided by C9, C12 and C14. The voltage amplification is determined by the ratio R9:R5,
which in the present circuit is ×40.
The output of IC 1 is applied to drive
stages T1 and T3 via R6. These transistors
operate in Class A: the current drawn by
them is set to 10 mA by voltage divider
R 10-R 13 and their respective emitter resistors. Their voltage and current amplification is appreciable, which is as required
for the link between the input and output
stages.
The output amplifier proper consists of
drive stages T 6 and T 7 and power transistors T 8, T 9, T 14, T 15. which have been
arranged as symmetrical power darlingtons.
Because of the high power, the output transistors are connected in parallel. The types
used can handle a collector current of 20 A
and have a maximum dissipation of 250 W.
The output stages operate in Class AB
to ensure a smooth transition between the
n-p-n and p-n-p transistors, which prevents
cross-over distortion. This requires a small
current through the power transistors, even
in the absence of an input signal. This current is provided by ‘zener’ transistor T 2,
ELEKTOR ELECTRONICS NOVEMBER 1995
61
PA300 POWER AMPLIFIER
BC639
T13
AC
R40
33k
R38
D13
F
R41
F2
R42
950092 - 11
60V
As any reliable amplifier, the PA300 is provided with adequate protection measures.
These start with fuses F 1 and F 2, which
guard against high currents in case of overload or short-circuits. Since even fast fuses
are often not fast enough to prevent the
6A3 T
HP1
1N4004
D12
R35
47µ
25V
C20
I
C19
R50
470n
T12
BC337
47µ
50V
IC2 = LM393
J
2
3
C18
R34
T11
D9
1M
IC2a
1
B
1N4148 T10
2x BC337
4
IC2
R49
D14
R32
8
MJE15031
D
R39
B'
R45
B
A'
A
KTY81-122
R46
R47
C
100n
C
B
E
MJ15003
MJ15004
B
C
C
E
B
MJE15030
MJE15031
B
C
E
BC639
E
E
B
C
15V
1W5
BC546B
BC556B
BC337
MJE350
MJE340
BD139
-F
100n
C4
D2
50V
47µ
C7
R13 R15
D16
BAT85
C2
100n
R8
R1
2µ2
1n
R5
R3
C6
R4
A
A
C5 R2
2k2
J 0V76
1V4
E ≈2V3
68k
I
22k
D 1V5
100n
Clip
C8
47p
C12
22k
NE5534
R9
1
4
8
3
2
IC1
33p
5
7
C9
100n
C3
15V
1W5
1k
F
D1
560Ω
H 24V
R48
MJE340
T3
R12
C11
10k
R31
B
560Ω
6
R6
3k3
C 2V1
D3...D6 = 1N4004
D4
R19
10Ω
250Ω
P1
680n
C13
T2
470p
C14
R11
100n
560Ω
C10
D6
R21
1k
R23
150n
R24
C16
BD139
MJE350
D15
BC
556B T5
R17
BC
T4
546B
D3
R16
10Ω
R18
150n
C15
R22
1k
R20
D5
T1
D
15k
BAT85
15k
100n
560Ω
3k3
150Ω
C
27k
150Ω
C1
27k
680Ω
G 27mV
1k40
180Ω
27k
B 0V
3k3
27k
R10 R14
100k
MJE15030
56Ω
R7
6
5
R44
R36
R33
T7
G
G
IC2b
15V
1W5
D10
R37
T9, T15 =
MJ15004
33n
C17
T15
T9
R27
G
G
R28
D8
BY
254
R30
2Ω2
*
R29
L1
D7
R26
0Ω27
F 15V
100k
T8, T14 =
MJ15003
BY
254
0Ω27
R25
47k
0Ω27
T8
120k
0Ω27
T6
470k
A 10...30mV
47Ω
10Ω
T14
15k
Fig. 1. With the exception of an IC at the input, the circuit of the PA300 amplifier is conventional.
ELEKTOR ELECTRONICS NOVEMBER 1995
1k5
tekst
* zie
text
* see
siehe Text
* voir texte
*
7
H
B
LS–
LS1
LS+
1k5
D11
1N4002
R43
RE1
6A3 T
Protection circuits
3k3
F1
Contents
rectangular signals are not adversely affected by the inductor.
4k7
60V
nal level drops, the current will diminish
only slowly until it has reached its nominal
value.
Diodes D7, D8 protect the output stages
against possible counter voltages generated
by the complex load. Resistor R 30 and capacitor C 17 form a Boucherot network to
enhance the stability at high frequencies.
Inductor L1 prevents any problems with capacitive loads (electrostatic loudspeakers).
Resistor R 29 ensures that the transfer of
3k3
which puts a small voltage on the bases of
T6 and T7 so that these transistors just conduct in quiescent operation. The level of
the quiescent current is set accurately with
P 1.
To ensure maximum thermal stability,
transistors T 1–T 3 and T 6–T 7 are mounted
on and the same heat sink. This keeps the
quiescent current within certain limits.
With high drive signals, this current can
reach a high level, but when the input sig-
AUDIO & HI-FI
power transistors giving up the ghost in
such circumstances, an electronic short-circuit protection circuit, based on T4 and T5,
has been provided. When, owing to an
overload or short-circuit, very high currents
begin to flow through resistors R25 and R27,
the potential drop across these resistors will
exceed the base-emitter threshold voltage
of T 4 and T 5. These transistors then conduct and short-circuit or reduce drive signal
at their bases. The output current then
drops to zero.
If a direct voltage appears at the output
terminals, or the temperature of the heat
sink rises unduly, relay Re 1 removes the
load from the output. The loudspeakers are
also disconnected by the relay when the
mains is switched on (power-on delay) to
prevent annoying clicks and plops.
The circuits that make all this possible
consist of dual comparator IC2, transistors
T10–T13, and indicator diodes D13 and D14.
They are powered by the 15 V line provided by zener diode D10 and resistor R42.
The ‘AC ’ terminal on the PCB is linked to
one of the secondary outputs on the mains
transformer. As soon as the mains is
switched on, an alternating voltage appears
at that terminal, which is rectified by D 12
and applied as a negative potential to T 12
via R50. The transistor will then be cut off,
so that C20 is charged via R36 and R44. As
long as charging takes place, the inverting
(+) input of comparator IC 2b is low w.r.t.
the non-inverting (–) input. The output of
IC 2b is also low, so that T 13 is cut off and
the relay is not energized. This state is indicated by the lighting of D13. When C20 has
been charged fully, the comparator changes
state, the relay is energized (whereupon
Contents
400V/35A
3A15 T
4x 10,000µ
100V
Mains
power-on
delay
60V
2x 42V5
625VA
60V
2x 1N4004
1k8
62
1W
relay
10µ
24V
63V
AC
950092 - 14
Fig. 2. The power supply is straightforward, but can handle a large current. Voltage ‘AC’
serves as drive for the power-on delay circuit.
D13 goes out) and the loudspeakers are connected to the output. When the mains is
switched off, the relay is deenergized instantly, whereupon the loudspeakers are
disconnected so that any switch-off noise is
not audible.
The direct-voltage protection operates
Fig. 3. This close-up photograph shows clearly how the transistors
are fitted to the heat sink via a rectangular bracket.
as follows. The output voltage is applied to
T 10 and T 11 via potential divider R 32-R 34.
Alternating voltages are short-circuited to
ground by C 18. However, direct voltages
greater than +1.7 V or more negative than
–4.8 V switch on T 10 or T 11 immediately.
This causes the +ve input of IC 2a to be
pulled down, whereupon this comparator
changes state, T13 is cut off, and the relay is
deenergized. This state is again indicated
by the lighting of D13.
Strictly speaking, temperature protection is not necessary, but it offers that little
bit extra security. The temperature sensor
is R 39, a PTC (positive temperature coefficient) type, which is located on the board in
a position where it rests against the rectangular bracket. Owing to a rising temperature, the value of R39 increases until the potential at the –ve input of IC2a rises above
the level at the +ve input set by divider
R45-R46, whereupon the output of IC2a goes
low. This causes IC 2b to change state,
whereupon T 13 is cut off and the relay is
deenergized. This time, the situation is indicated by the lighting of D14. The circuit has
been designed to operate when the temperature of the heat sink rises above 70 °C.
Any relay clatter may be obviated by reducing the value of R48.
The terminal marked ‘CLIP’ on the PCB
is connected to the output of IC1 via R31. It
serves to obtain an external overdrive indication, which may be a simple combination
of a comparator and LED . Normally, this
terminal is left open.
Power supply
As with most power amplifiers, the ±60 V
ELEKTOR ELECTRONICS NOVEMBER 1995
63
PA300 POWER AMPLIFIER
Contents
T15
F2
6A3/T
H2
R28
D8
R27
R8
R21
R23
C16
C2
C4
T5
T9
D2
D6
R3
R1
C6
AC Clip
C8
C1
C13
C10
C3
D1
C19
C14
A’ T10
D13
R38
R50
R33
D10
R45
R48
R46
R49
R47
T11
R43
R42
R40
C20 B’
D5
R35
R20
R34
C18
T13
R32
C17
R11
R10
R14
A
B
D3
R30
R41
R37
R36
R39
R44
IC2
C15
T4
D9
R18
T12
D14
D12
R16
R7
D15
R31
T1
R6
C9
T6
C5
R2
R4
C7
C11
T2
R17
R22
D7
R25
R26
H1
F1
6A3/T
T8
LS-
D11
R9
C12
R15
R13
R19
D16
R12
IC1
+
T14
RE1
P1
R24
D4
T7
T3
R5
1-290059
950092-1
LS+
L1/R29
T
ELEKTOR ELECTRONICS NOVEMBER 1995
0
Construction
Building the amplifier is surprisingly
simple. The printed-circuit board in F i g . 4
is well laid out and provides ample room.
Populating the board is as usual best
started with the passive components, then
the electrolytic capacitors, fuses and relay.
There are no ‘difficult’ parts.
Circuits IC1 and IC2 are best mounted in
appropriate sockets.
Diodes D13 and D14 will, of course, have
to be fitted on the front panel of the enclosure and are connected to the board by
lengths of flexible circuit wire.
Inductor L 1 is a DIY component; i consists of 15 turns of 1 mm. dia. enamelled
copper wire around R29 (not too tight!).
Since most of the transistors are to be
mounted on and the same heat sink, they
are all located at one side of the board.
However, they should first be fitted on a
rectangular bracket, which is secured to the
heat sink and the board—see F i g . 3. Note
that the heat sink shown in this photograph
proved too small when 4 Ω loudspeakers
were used. With 8 Ω speakers, it was just
about all right, but with full drive over sustained periods, the temperature protection
circuits were actuated. If such situations are
likely to be encountered, forced cooling
must be used.
As already stated, temperature sensor
R39 should rest (with its flat surface) against
the rectangular bracket. On the board, terminals ‘ A’ and ‘ B ’ terminals to the left of
R39 must be connected to ‘A’ and ‘B’ above
IC 2 with a twisted pair of lengths of insulated circuit wire as shown in Fig. 3.
The points where to connect the loud-
-
power supply need not be regulated. Owing to
the relatively high power output, the supply needs
a fairly large mains transformer and corresponding smoothing capacitors—see F i g . 2.
Note that the supply shown is for a mono amplifier; a stereo outfit needs two supplies.
The transformer is a 625 VA type, and the
smoothing capacitors are 10 000 µF, 100 V
electrolytic types. The bridge rectifier needs
to be mounted on a suitable heat sink or be
mounted directly on the bottom cover of the
metal enclosure.. The transformer needs two
secondary windings, providing 42.5 V each. The
prototype used a toroidal transformer with
2×40 V secondaries. The secondary winding
of this type of transformer is easily extended:
in the prototype 4 turns were added and this
gave secondaries of 2×42.5 V.
The box ‘Mains power-on delay’ provides
a gradual build-up of the mains voltage, which
in a high-power amplifier is highly advisable.
A suitable design was published in 305 Circuits
(page 115).
The relay and associated drive circuit is intended to be connected to terminal ‘AC’ on the
board, where it serves to power the power-on
circuit. If a slight degradation of the amplifier
performance is acceptable, this relay and circuit may be omitted and the PCB terminal connected directly to one of the transformer secondaries.
Fig. 4a. Component layout of the printed-circuit board
for the 300 W power amplifier.
speaker leads and power lines are clearly
marked on the board. Use the special flat
AMP connectors for this purpose: these have
large-surface contacts that can handle large
currents. The loudspeaker cable should
have a cross-sectional area of not less than
2.5 mm2.
Finally
How the amplifier and power supply are assembled is largely a question of individual
taste and requirement. The two may be
combined into a mono amplifier, or two
each may be built into a stereo amplifier
unit. Our preference is for mono amplifiers,
since these run the least risk of earth loops
and the difficulties associated with those. It
is advisable to make the ‘0’ of the supply
the centre of the earth connections of the
electrolytic capacitors and the centre tap of
the transformer.
The single earthing point on the supply
and the board must be connected to the enclosure earth by a short, heavy-duty cable.
This means that the input socket must be
64
AUDIO & HI-FI
Contents
950092-1
20 Hz to 20 kHz.with a bandwidth of
80 kHz and a power output of 150 W into 8
Ω. Up to 1 kHz, the distortion is very low
and then increases, which is usual and
caused by the inertia of the semiconductors.
Figure 5b shows the distortion at 1 kHz
as a function of the output level at a bandwidth of 22 Hz to 22 kHz. The dashed
curve refers to a load of 4 Ω and the solid
curve to a load of 8 Ω. The irregularities
between 10 W and 100 W are not caused
by the amplifier but by the limits of the
measuring range of the analyser. From the
clipping points, the curves rise almost vertically.
Figure 5c shows the maximum for a distortion of 0.1%. The dashed curve (4 Ω
load) is very close to the 300 W line. The
small reduction at low frequencies is caused
by the imperfectness of the electrolytic
buffer capacitors in the power supply.
Figure 5d shows the Fourier analysis of a
1 kHz signal for a power output of 1 W into
8 Ω. The fundamental frequency is suppressed. The 2nd and 3rd harmonics are
down by 110 dB and 120 dB respectively
referred to the fundamental frequency. The
THD + N figure at this measurement was
0.0009%.
Parts list
Fig. 4b. Track layout of the printed-circuit board
for the 300 W power amplifier.
an insulated type. This socket must be
linked to the input on the board via
screened cable.
To test the amplifier, turn P1 fully anticlockwise and switch on the mains. After
the output relay has been energized, set the
quiescent current. This is done by connecting a multimeter (direct mV range) across
one of resistors R 25–R 28 and adjusting P 1
until the meter reads 27 mV (which corresponds to a current of 100 mA through each
of the four power transistors). Leave the
amplifier on for an hour or so and then
check the voltage again: adjust P 1 as re-
quired.
Test results
The technical data given on page0 0 were
verified or obtained with a power supply as
shown in Fig. 2. They show that in spite (or
because?) of its simple design, the amplifier
offers excellent performance. The distortion
figures are particularly good.
Measurements with the Audio Precision
analyser are illustrated in F i g . 5.
Figure 5a shows the total harmonic distortion (THD+N) over a frequency range of
R e s i s t o r s:
R1 = 68 kΩ
R2 = 2.2 kΩ
R3, R9 = 22 kΩ
R4, R22, R23 = 1 kΩ
R5, R6, R10, R13 = 560 Ω
R7, R8, R42 = 3.3 kΩ, 5 W
R11, R12, R37 = 15 kΩ
R14, R15 = 150 Ω
R16 = 680 Ω
R17 = 180 Ω
R18, R19 = 10 Ω
R20, R21, R46, R47 = 27 kΩ
R24 = 56 Ω
R25–R28 = 0.27 Ω, 5 W
R29 = 2.2 Ω, 5 W
R30 = 10 Ω, 5 W
R31 = 10 kΩ
R32, R34 = 100 kΩ
R33 = 47 kΩ
R35 = 1.5 kΩ
R36 = 470 kΩ
R38, R49 = 3.3 kΩ
R39 = sensor Type KTY81-122
R40 = 4.7 kΩ
R41 = 33 kΩ
R43 = 1.5 kΩ, 5 W
R44 = 47 Ω
R45 = 1.40 kΩ, 1%
R48 = 1 MΩ
R50 = 120 kΩ
P1 = 250 Ω preset
C a p a c i t o r s:
C1–C4, C8, C10, C11 = 100 nF
C5 = 2.2 µF polypropylene, pitch 5 mm
C6 = 1 nF
C7, C18 = 47 µF, 50 V, bipolar, radial;
C9 = 33 pF, 160 V, polystyrene
C12 = 47 pF, 160 V, polystyrene
ELEKTOR ELECTRONICS NOVEMBER 1995
65
PA300 POWER AMPLIFIER
Contents
a
Elektor DEFAULT
b
THD+N(%) vs FREQ(Hz)
AUDIO PRECISION THDVSLVL
1
1
0.1
0.1
0.010
0.010
0.001
0.001
.0005
20
100
1k
10k
.0005
10m
20k
0.1
THD+N(%) vs measured
1
LEVEL(W)
10
100
c
AUDIO PRECISION PWR-BAND
d
LEVEL(W) vs FREQ(Hz)
300
950092 - 16b
950092 - 16a
Elektor GB2FFT
AMP1(dBr)
vs FREQ(Hz)
0.0
500
-20.00
-40.00
100
-60.00
-80.00
-100.0
10
-120.0
-140.0
1
-160.0
20
100
1k
10k
20k
0.0
500.0
1.00k
1.50k
2.00k
2.50k
3.00k
950092 - 16c
I n t e g r a t e d c i r c u i t s:
IC1 = NE5534
ELEKTOR ELECTRONICS NOVEMBER 1995
IC2 = LM393
M i s c e l l a n e o u s:
L1 = see text
Re1 = 16 A, 24 V, 875 Ω relay (e.g.
Siemens V23056-AO105-A101*)
F1, F2 = glass fuse, 6.3 A, slow complete
with PCB type holder
Loudspeaker and mains connectors for
board mounting (AMP - see text)
Mica washers for T1–T3, T6–T9, T14 and
T15
Rectangular bracket e.g. SWP40, 20 cm
long (Fischer 40×30×5**)
Heat sink <0.4 K W–1
PCB Order no. 950092
Mains transformer, 2×42.5 V, 625 VA (see
text)
Fuse (power supply) 3.15 A, slow, I2t≥ 400
Bridge rectifier 400 V, 35 A
4 off electrolytic capacitors, 10,000 µF, 100
V
PCB Order No. 924055
[950092]
* ElectroValue 01784 33603 or
0161 432 4945
** Dau 01243 553 031; trade only, but information on your nearest dealer will be
4.00k
950092 - 16d
Fig. 5. Curves obtained during measurements on the amplifier with an Audio Precision Analyser (see text).
C13 = 680 nF
C14 = 470 pF, 160 V, polystyrene
C15, C16 = 150 nF
C17 = 33 nF
C19 = 470 nF
C20 = 47 µF, 25 V, radial
S e m i c o n d u c t o r s:
D1, D2, D10 = zener, 15 V, 1.5 W
D3, D6, D12 = 1N4004
D7, D8 = BY254
D9 = 1N4148
D11 = 1N4002
D13, D14 = LED
D15, D16 = BAT85
T1 = MJE350
T2 = BD139
T3 = MJE340
T4 = BC546B
T5 = BC556B
T6 = MJE15030
T7 = MJE15031
T8, T14 = MJ15003
T9, T15 = MJ15004
T10, T12 = BC337
T13 = BC639
3.50k
given by telephone.
66
AUDIO & HI-FI
Contents
The NE5534
The NE5534 is a good quality, versatile, lownoise operational amplifier which is excellent
value for money.
Compared with older types, it has better
noise figures, small signal performance, power
bandwidth, and output drive capability.
These characteristics make it ideally suited
to high-end audio applications. It is found
even in the most expensive CD players.
The adjacent simplified diagram gives an
idea of the internal structure of this versatile
device. It consists of a number of differential
amplifiers that are set with the aid of current
sources and current mirrors. Well-designed compensation circuits result in excellent linearity and very low distortion.
The standard design gives an amplification
of ×3. The frequency response can be optimized
for various applications with the aid of an external capacitor. It may be adjusted for a capacitive load, high slew rate, low overshoot or
for application as a unity amplifier.
Some technical data
Small-signal bandwidth
10 MHz
Output voltage (at Ub = ±18 V)
10 Vrms across 600 Ω
Input noise
4 nV Hz–1
DC voltage amplification
105
6×103 at 10 kHz
AC voltage amplification
Power bandwidth
200 kHz
Slew rate
13 V µs–1
Supply voltage range
±3 V to ±20 V
ELEKTOR ELECTRONICS NOVEMBER 1995
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