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US005343159A
United States Patent [191
[11] Patent Number:
Butler
[45]
[54] DIRECI‘ BOX EMPLOYING HYBRID
VACUUIVI TUBE AND SOLID STATE
Date of Patent:
5,343,159
Aug. 30, 1994
Primary Examiner—James B. Mullins
Assistant Examiner-Jim Dodek
CIRCUITRY
[76] Inventor: Brent K. Butler, 6806 S. Norfolk St.,
Aurora’ Colo‘ 80016
Attorney, Agent, or Firm-Harness, Dickey & Pierce
[57]
ABSTRACT
The direct coupled solid state-vacuum tube hybrid cir
[21] Appl. No.: 17,764
cuit matches a high impedance audio source to a low
.
_
[22] Flled'
impedance audio destination. The resulting device
Feb‘ 16’ 1993
supplies essentially unit gain and has both single ended
[51]
[52]
Int. Cl.5 ............................................. .. H03F 5/00
US. Cl. ........................................ .. 330/3; 330/85;
381/120; 381/121
plied from a common low voltage DC power supply.
[58]
Field of Search ............... .. 381/120, 121, 122, 61;
The vacuum tube is incorporated directly in the feed
330/3, 85; 84/DIG. 10, 674, 677, 680
[56]
and balanced outputs. The solid state operational ampli
?er and vacuum tube devices are DC coupled and sup
back loop of the operational ampli?er and a suitably
References Cited
chosen dampening resistorallows the circuit to provide
U-s- PATENT DOCUMENTS
vacuum tube charactenstlc warming effects without
unwanted distortion or tube microphonics.
3,005,164
2/1956
5,022,305
6/1991 Butler .................................. .. 381/61
Newbold ............................. .. 330/91
9 Claims, 1 Drawing Sheet
L/NE OUT
(Unity)
SPEAKER/
BALANCED
(Microphone
Level Out)
GROUND ’ \64
1
5,343,159
DIRECT BOX EMPLOYING HYBRID VACUUM
TUBE AND SOLID STATE CIRCUITRY
BACKGROUND AND SUMMARY OF THE
INVENTION
The present invention relates generally to audio sig
nal processing circuitry and more particularly to a “di
2
because of its desirable, warm sound. Although the
precise physics of this phenomenon are not fully under
stood, it is believed that the warm sound is attributable
to the vacuum tube’s tendency to produce even har
5 monies, which can add a subtle warming effect that is
not normally found in solid state circuits. In contrast,
solid state circuits may tend to produce a larger number
of odd harmonics which to some sound somewhat stri
rect box” circuit for coupling a high impedance audio
dent or shrill.
source to a low impedance audio destination.
The present invention provides a circuit for coupling
Many popular musical instruments today have a
a high impedance audio source to a low impedance
pickup or transducer by which the sound of the instru
audio destination which employs at least one solid state
ment is converted into an electrical signal which may
operational ampli?er and at least one vacuum tube. The
then be electronically processed, ampli?ed and re
operational ampli?er and vacuum tube are connected
corded. Such instruments include electric guitars, 15 together so that the vacuum tube is in the feedback loop
which typically employ one or more magnetic pickups
and acoustic instruments, out?tted with contact micro
phones and the like. In addition, musical instruments of
all descriptions can be suitably played near a micro
phone to convert the sound into an electrical signal.
In the music and recording industries there is a wide
of the operational ampli?er. The feedback loop includes
a dampening impedance which permits the desirable
warming characteristics of the tube to in?uence the
output signal, but without inducing unwanted distortion
or tube microphonics. The circuit is thus suitable for use
in “direct box” applications particularly where the
variety of different pickups, transducers and micro
warm tube characteristic is desired.
phones in use today. Where the instrument will be situ
For a more complete understanding of the invention,
ated a long distance from the electronic processing,
its
objects and advantages, reference may be had to the
amplifying or recording equipment it is common to use 25
following speci?cation and to the accompanying draw
a low impedance transmission line or cable system,
mg.
often a balanced system, to minimize hum and ground
loop problems. It is generally recognized that a low
BRIEF DESCRIPTION OF THE DRAWING
impedance transmission line or cable system is less sus
FIG.
1 is a schematic diagram illustrating the pres
ceptible to magnetically induced hum. Moreover, a 30
ently
preferred
embodiment of the circuit of the inven
balanced system, which conveys both in-phase and
tion.
out-of-phase signals has the recognized advantage of
eliminating common mode noise. Thus low impedance,
DESCRIPTION OF THE PREFERRED
balanced transmission lines or cables are frequently used
EMBODIMENT
in professional music and recording applications.
With the exception of professional low impedance,
balanced microphones, most musical instrument pick
35
ups and transducers are high impedance, single ended
Referring to FIG. 1, the presently preferred circuit
has an audio input jack or terminal 10 into which a high
impedance audio source may be plugged. In the pre
ferred embodiment input jack 10 may be a self-shorting,
pickup may have an impedance in the range of about 40 % inch phone jack. As illustrated, the ring of jack 10 is
connected to ground as at 12 and the tip is connected to
50K ohms and the more modern guitar pickups with
the input of the circuit as at 14.
active electronic circuits may have an impedance in the
range of 3K-4K ohms. When compared to a low impe
The presently preferred circuit has two output jacks,
dance system, which is nominally 300 ohms or less,
a single-ended output jack 16, which may be a nonshort
(unbalanced) devices. For example, the classic guitar
these guitar pickups are considered to be high impe 45 ing, l inch phone jack, and a balanced output jack 18,
dance devices. Being high impedance devices, these
pickups, and the transmission lines or cables connected
to them tend to be quite susceptible to hum and noise.
To deal with the bum and noise problem in high
which may be an XLR connector. Either of the output
jacks 16 and 18 may be coupled as an output terminal to
a low impedance audio destination device such as to a
impedance instruments it is common practice to use an
sound reinforcement or recording mixing console. As
will be more fully explained, the circuit couples or
impedance matching transformer or active electronic
matches the high impedance input to the low impe
circuit to couple the high impedance source to a low
impedance destination. Such matching transformers or
provides essentially unity gain.
dance output. In the preferred embodiment the circuit
circuits are sometimes referred to as “direct boxes”
Referring to the signal input end of the circuit, as at
since they allow a high impedance device to be con 55 input jack 10, the audio signal is coupled through resis
nected “directly” to a low impedance mixing console or
tor 20 and capacitor 22 to the noninverting input (+) of
other low impedance electronic device. Transformers
have been employed for this purpose for many years but
they are considered undesirable today, since transform
ers tend to degrade or color the sound by diminishing 60
operational ampli?er 24. A pair of oppositely poled
neers prefer using active electronic circuits to match or
diodes 26 provide overload protection of the noninvert
ing input. The diodes are respectively connected to the
positive and negative power supply rails (A+ and
A-). The diodes limit the audio input signal excursion
voltages to the supply rail voltages and thus protect the
couple the high impedance audio source to the low
operational ampli?er from excessively large signals.
the high frequencies. Thus, today, many audio engi
impedance audio destination.
The noninverting input of operational ampli?er 24 is
To a large extent, active electronic circuits are imple 65 provided with a DC reference through resistor 27
mented today using solid state transistor circuitry.
which is coupled to ground as illustrated. If desired,
There remains, however, a loyal group of audio engi
capacitor 28 can be provided to provide RF suppression
neers and musicians who favor vacuum tube circuitry
for the noninverting input. Referring to the output jacks
3
5,343,159
4
16 and 18, it will be seen that the singled-ended output
respective halves of a dual triode device such as a
(jack 16) is referenced to ground. Similarly, pin 1 of the
balanced output (jack 18) is also center referenced to
12AX7.
Operational ampli?er 66 and vacuum tube 68 are
ground, unless the normally closed ground lift switch
30 is opened.
feedback loop, just as operational ampli?er 24 and vac
direct coupled to one another and share a common
The output of operational ampli?er 24 is connected
uum tube 34. In that regard, resistor 70 serves as the
directly to the grid 32 of triode vacuum tube 34. Vac
uum tube 34 may be % of a 12AX7 tube or the like. The
anode of vacuum tube 34 is connected to the positive
feedback resistor which couples the output of the oath
ode of tube 68 with the inverting (—) terminal of opera
tional ampli?er 66. It will be seen that the inverting
terminal of operational ampli?er 66 receives the audio
supply rail (A+) and the cathode is connected to the
output jacks 16 and 18 as illustrated and also through
resistor 35 to the negative supply rail (A—).
input signal, with the noninverting (+) terminal being
referenced to ground. The audio signal is supplied
through resistor 72 which is coupled to the cathode of
tube 34. In the presently preferred embodiment resistor
Notably, both the operational ampli?er 24 and the
vacuum tube 34 are supplied with power from the same
supply rails (A+ and A—). The presently preferred
15 72 may be on the order of 10K ohms as is resistor 70.
power supply for developing the A+ and A— power is
illustrated generally at 36. Preferably, an integrated
plug wall transformer 38 steps down the 110 volt AC
line voltage to nominally 13 volt AC. A 300 milliamp
The operational ampli?er and vacuum tube devices
thus operate as a hybrid, solid state-vacuum tube circuit
which effects impedance matching at substantially unity
gain. Operational ampli?er 24 and tube 34 form one
wall transformer is adequate for the presently preferred
circuit. The stepped down AC voltage is supplied
hybrid unity gain ampli?er circuit and operational am
pli?er 66 and tube 68 form another hybrid unity gain
through power supply jack 40 to the vacuum tube heat
ers 42 and also the DC recti?er circuit 44. The presently
ampli?er circuit. Because the inputs of operational am
pli?ers 24 and 66 are of opposite polarity, the respective
hybrid circuits supply an in-phase signal and an out-of
phase signal. The outputs of these hybrid circuits are
resistively coupled to the output jacks 16 and 18.
The presently preferred circuit derives the input of
operational ampli?er 66 from the output of the hybrid
circuit comprising operational ampli?er 24 and vacuum
preferred recti?er circuit uses a half wave diode recti
?er con?guration 46 with oppositely poled 16 volt
Zener diodes 48 and suitable resistors and ?lter capaci
25
tors to provide the A+ and A- DC supply rail volt
ages with a center reference ground. Nominally, the
supply rail voltages are about :15 volts.
By using the same low voltage DC supply rails to
tube 34. This has been found to be a quite suitable ar
rangement in that the number of active electronic com
ponents is kept to a minimum. The hybrid circuit com
supply power to both the solid state and vacuum tube
components, it is possible to directly couple the output
of operational ampli?er 24 to the grid of vacuum tube
prising operational ampli?er 24 and vacuum tube 34
34. This affords a number of advantages. First, the cir
generates very little noise or undesirable distortion.
cuit does not require coupling capacitors between the
output of the operational ampli?er and the grid of the
Hence the audio signal supplied through resistor 72 to
the inverting (—) input of operational ampli?er 66 is
vacuum tube. Coupling capacitors are a source of signal
quite clean. If desired, ‘however, it is possible to use
additional operational ampli?ers acting as buffers in a
circuit con?guration which provides the audio input to
degradation, since capacitors attenuate or block low
frequency audio signals. Second, being supplied by the
same low voltage rails, it is possible to include the cath
ode output of vacuum tube 34 in the feedback loop used
to establish the gain of operational ampli?er 24. More
speci?cally, resistor 50 couples the cathode of vacuum
tube 34 to the inverting (—) input of operational ampli
?er 24. If desired, a small capacitor 52 may also be
included in the feedback loop to suppress RF oscilla
trons.
Resistor 50 controls the dampening or degree to
which the vacuum tube characteristics are imported
into the sound. If a low resistance value or direct wire
the inverting (—) terminal of operational ampli?er 66
without passing it ?rst through the hybrid circuit of
operational ampli?er 24 and vacuum tube 34. In that
45
way, both the in-phase and out-of-phase components
would be derived directly from the input jack 10,
thereby eliminating the need to cascade the two hybrid
circuits together.
For the single-ended output jack 16, resistor 54 cou
pled to the cathode of vacuum tube 34 supplies the
output signal. For the balanced (XLR) output jack 18
connection is chosen, the operational ampli?er responds
resistors 56, 58, 60 and 74 distribute the cathode outputs
of tubes 34 and 68 to the balanced output terminals of
very quickly and, in effect, corrects for or eliminates
output jack 18.
any vacuum tube characteristics. On the other hand, a
In the illustrated embodiment capacitors 62 and 64
high resistance value allows the sound of the vacuum 55 are used to couple the balanced output terminals to the
tube to dominate, potentially resulting in unwanted
distortion and microphonics. Accordingly, the optimal
selection for resistor 50 is a tradeoff between these two
extremes. The presently preferred embodiment uses a
resistor of 10K ohms. It has been found that this inter
mediate value allows a subtle warming effect of the
vacuum tube to be heard without introducing unwanted
resistive network comprising resistors 56, 58, 60 and 74.
Capacitors are used in the illustrated embodiment to
block any DC voltage present on the balanced output
terminals due to the use of phantom power. In this
regard, some mixing consoles provide a 48 volt DC
phantom power that is used to energize condenser mi
crophones. Capacitors 62 and 64 will prevent any phan
distortion or microphonics.
tom power from entering the circuit via output jack 18.
In order to supply a balanced signal to the XLR
The use of capacitors in the output circuit may slightly
output jack 18 the circuit includes a second operational 65 degrade the low frequency response of the circuit.
ampli?er 66 and a second triode vacuum tube 68. In
Thus, capacitors 62 and 64 may be optionally provided
practice, operational ampli?ers 24 and 66 may be on the
with shorting jumpers to allow the output of the circuit
same chip and vacuum tubes 34 and 68 may be the
to be DC coupled or directly hard wired to the output
5
5,343,159
jack 18 in applications where phantom power is not
6
terminal being coupled to the cathode of said vac
being used.
uum tube;
To maximize the circuit’s usefulness, a level select
switch 76 is used to switch in an additional load resistor
feedback means coupled between the cathode of said
vacuum tube and the second input port of said
78 when desired. Switching the additional load resistor
in the circuit cuts the output level by 20 dB. To add
operational ampli?er thereby placing said vacuum
tube in the feedback loop of said operational ampli
further versatility, the circuit includes a speaker loop
?er;
jack 80 which can be optionally used as an audio input
further comprising direct current power supply and
wherein said vacuum tube and said operational
ampli?er share said direct current power supply.
4. A circuit for coupling a high impedance audio
source to a low impedance audio destination, compris
when the signal is derived from a musical ampli?er
speaker. More speci?cally, when-the speaker loop jack
80 is used, the input of the present circuit is placed in
series with the musical ampli?er speaker lead. The
speaker signal is fed through switch 82 and attenuating
resistor 84 to the noninverting input of operational am
ing:
at least one operational ampli?er with ?rst and sec
pli?er 24. Resistor 84 may be on the order of l meg ohm 15
and should be of suf?cient power rating to handle the
output current of a musical instrument ampli?er.
While the invention has been described in connection
ond input ports of opposite polarity and with an
output port and having a feedback loop to establish
the gain of said operational ampli?er;
at least one vacuum tube with cathode, plate and grid;
with the presently preferred embodiment, the principles
the grid of said vacuum tube being coupled to the
of the invention are capable of modi?cation and change 20
without departing from the spirit of the invention as set
an input terminal for coupling to said high impedance
forth in the following claims.
audio source and an output terminal for coupling to
I claim:
1. A circuit for coupling a high impedance audio
source to a low impedance audio destination, compris 25
mg:
at least one operational ampli?er with ?rst and sec
ond input ports of opposite polarity and with an
output port and having a feedback loop to establish
the gain of said operational ampli?er;
operational ampli?er output port;
said low impedance audio destination;
said input terminal being coupled to said ?rst input
port of said operational ampli?er, and the output
terminal being coupled to the cathode of said vac
uum tube;
,
feedback means coupled between the cathode of said
vacuum tube and the second input port of said
30
at least one vacuum tube with cathode, plate and grid;
the grid of said vacuum tube being coupled to the
operational ampli?er thereby placing said vacuum
tube in the feedback loop of said operational ampli
?er;
operational ampli?er output port;
an input terminal for coupling to said high impedance
wherein said vacuum tube and said operational ampli
?er are powered by the same DC voltage rails.
audio source and an output terminal for coupling to 35
5. A circuit for coupling a high impedance audio
said low impedance audio destination;
source to a balanced low impedance audio destination,
said input terminal being coupled to said ?rst input
port of said operational ampli?er, and the output
comprising:
?rst and second operational ampli?ers each with ?rst
and second input ports of opposite polarity and
terminal being coupled to the cathode of said vac
uum tube;
each with an output port and each having a feed
back loop to establish the gain of each of said oper
feedback means coupled between the cathode of said
vacuum tube and the second input port of said
ational ampli?ers;
operational ampli?er thereby placing said vacuum
?rst and second vacuum tubes each with cathode,
tube in the feedback loop of said operational ampli
?er;
plate and grid;
45
wherein said feedback means is of a preselected impe
dance suf?cient to cause sonic effects of said vac
uum tube to be imported onto sound of an audio
signal transmitted through said circuit from input
terminal to output terminal.
2. The circuit of claim 1 wherein said grid of said
vacuum tube and said output port of said operational
ampli?er are direct coupled to one another.
3. A circuit for coupling a high impedance audio
source to a low impedance audio destination, compris 55
ing:
at least one operational ampli?er with ?rst and sec
ond input ports of opposite polarity and with an
output port and having a feedback loop to establish
the gain of said operational ampli?er;
at least one vacuum tube with cathode, plate and grid;
the grid of said vacuum tube being coupled to the
operational ampli?er output port;
an input terminal for coupling to said high impedance
audio source and an output terminal for coupling to 65
said low impedance audio destination;
said input terminal being coupled to said ?rst input
port of said operational ampli?er, and the output
the grid of said ?rst vacuum tube being coupled to the
?rst operational ampli?er output port;
the grid of said second vacuum tube being coupled to
the second operational ampli?er output port;
an input terminal for coupling to said high impedance
audio source and a pair of balanced output termi
nals of opposite polarity for coupling to said bal
anced low impedance audio destination;
the input terminal having means for communicating
an input signal to said ?rst input port of said first
operational ampli?er and to said second input port
of said second operational ampli?er;
a ?rst one of said pair of balanced output terminals
being coupled to the cathode of said ?rst vacuum
tube and a second one of said pair of balanced
output terminals being coupled to said cathode of
said second vacuum tube;
?rst feedback means coupled between the cathode of
said ?rst vacuum tube and the second input port of
said ?rst operational ampli?er thereby placing said
?rst vacuum tube in the feedback loop of said ?rst
operational ampli?er;
second feedback means coupled between the cathode
of said second vacuum tube and the second input
7
5,343,159
8
7. The circuit of claim 5 wherein said grid of said ?rst
vacuum tube and said output port of said ?rst opera
tional ampli?er are direct coupled to one another; and
port of said second operational ampli?er thereby
placing said second vacuum tube in the feedback
wherein said grid of said second vacuum tube and
loop of said second operational ampli?er.
said output port of said second operational ampli
6. The circuit of claim 5 wherein said ?rst and second
?er are direct coupled to one another.
8. The circuit of claim 5 further comprising direct
feedback means are each of a preselected impedance
current power supply and wherein said vacuum tubes
sufficient to cause sonic effects of said vacuum tubes to
and said operational ampli?ers share said direct current
power supply.
be imported onto sound of an audio signal transmitted
9. The circuit of claim 5 wherein said vacuum tubes
and said operational ampli?ers are powered by com
through said circuit from input terminal to output termi
nals.
monly shared DC voltage rails.
is
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