Solid state circuit for emulating tube compression effect

A solid state amplifier for emulating the compression associated with an overbiased class-B push-pull tube amplifier at high input signal levels due to the flow of current into the grid of the output tubes resulting in a desirable output clipping characteristic with crossover distortion is disclosed. The invention includes at least one pair of class-B connected solid state devices, each having an input circuit and an output circuit. The output circuits are connected for mixing. A biasing element in the input circuit of each paired solid state device establishes a clipping level offset at the input circuit and at the output circuit of each device. A clipping element in the input circuit and the output circuit clips the offset at the input circuit and clips the offset at the output circuit of each respective solid state device. A charging element overbiases the offset in the input circuit whenever the input signal is greater than the input clipping element. The overbiasing causes crossover distortion for emulating the desirable compression associated with a tube amplifier.

BACKGROUND OF THE INVENTION 
The invention relates to replacement of tubes in power amplifiers with 
solid state devices. In particular, the invention is directed to a solid 
state circuit that duplicates tube power amplifier compression. 
Tube compression occurs whenever the tube power amplifier is driven into 
hard clipping. Normally, a solid state amplifier driven into hard clipping 
creates harsh odd-order harmonic distortion (square waves). In contrast, a 
tube amplifier compresses the signal so that the level decreases and it 
does not sound as harsh and strident. As a result, the sound is more 
subdued, but still has what the players call "punch". Thus, compression is 
a musical function that gives a tube power amplifier an edge over 
conventional solid state power amplifiers according to most heavy metal 
and bass guitar players, particularly at clipping conditions. 
The foregoing is a non-technical description of a phenomenon called 
increased crossover distortion. This function happens in all tube power 
amplifier designs whenever the output tube grid is driven positive with 
respect to the cathode causing it to become simply a forward biased diode. 
In a typical push-pull configuration, using two class-B biased tubes, the 
diode in each push-pull output stage causes the average bias level to 
increase at high signal levels and forces the class-B biased tubes to 
become over biased. Such condition causes the output signal to have severe 
crossover distortion, a condition where the signal zero crossing is 
delayed significantly. 
A typical tube power amplifier 10 which has been used on many popular 
models, is shown in FIG. 1. Typical circuit operation is described below 
followed by a description of overload (or tube compression) conditions. 
In FIG. 1, input signals are coupled via coupling capacitor 11 to the grid 
of vacuum tube 12 (e.g., 12AX7), which with tube 14 is half of what is 
called a long tailed phase inverter circuit. In this circuit, the cathodes 
of tubes 12 and 14 are connected together, as shown. Thus, tube 12 
operates in a grounded cathode mode; while tube 14 operates in a grounded 
grid mode with respect to the input grid of tube 12. Accordingly, equal 
but out-of-phase signals appear at the plates of 12 and 14. The purpose of 
the phase inverter is to supply two out-of-phase signals to class-B biased 
push-pull output tubes 16 and 18. 
Cathode resistor 20 sets the bias for each tube 12 and 14. Grid resistors 
22 and 24 are the respective grid bias resistors. Resistor 26 is a common 
cathode resistor. Resistor 28 is used to introduce feedback from the 
output to reduce overall distortion. The grid of tube 14 is shunted to 
ground (in this case, the low impedance feedback point) via capacitor 30, 
as is necessary for grounded grid operation. Load resistors 32 and 34 are 
the respective plate loads for tubes 12 and 14. The plate signals are 
coupled to the output tubes 16 and 18 via capacitors 36 and 38. 
Each output tube grid is connected to a negative bias source (e.g., -55 V) 
via bias resistors 40 and 42. This -55 V sources is generated externally 
from this circuit and is filtered adequately by capacitor 44. Negative 55 
volts is chosen as the appropriate value to bias the output tubes 16 and 
18 (e.g., 6L6GC) into good class-B operation with minimal crossover 
distortion at low signal levels. 
Completing the circuit, resistor 46 is a feedback resistor; resistors 48 
and 50 are power supply decoupling resistors; capacitors 52, 54 and 56 are 
filter capacitors for the various supply sources in the B+circuit. 
Finally, transformer 60 is a conventional tube push-pull output 
transformer, in this case with output taps for 8 and 4 ohms. The power 
amplifier 10 delivers approximately 50 WRMS to the matching load value. 
At all signal levels below output clipping (the output waveform being clean 
and free of distortion), the signal levels at the grid of each output tube 
16 and 18 is well below 55 volts peak swing, and the average DC bias level 
at each output tube grid is -55 VDC. However, at clipping and beyond, the 
signal levels at each output tube grid will exceed 55 volts peak swing. 
Thus, the grid will be biased positive with respect to the cathode at each 
positive peak signal swing. Whenever the grid is driven positive with 
respect to the cathode, it becomes a simple forward biased diode. With the 
positive peak swing clipped, the average negative DC bias voltage level at 
the grid of each output tube 16 and 18 is increased in proportion to the 
overload input value above the clipping value. Thus, the output tubes 16 
and 18 become over biased beyond class-B and at severe output clipping 
significant crossover distortion is generated as well. Consequently, at 
overload, the output signal of tube amplifier 10 will be clipped at the 
peaks. However, it will not be as "dirty" as a typical solid state power 
amplifier operating under the same conditions, because a large portion of 
the overloaded output waveform is forced or compressed into the severe 
crossover distortion region. To a musician, such a waveform is much more 
musical in nature and "cleaner" (i.e., less harsh) than a solid state 
amplifier at overload. Due to the compression (i.e., distortion near the 
zero crossover), the actual peak output clipping is reduced and is far 
more tolerable than that of the solid state amplifier. This phenomenon is 
thus, tube power amplifier compression. 
SUMMARY OF THE INVENTION 
The present invention is directed to a solid state amplifier for emulating 
the compression associated with an overbiased class-B push-pull tube 
amplifier at high input signal levels due to the flow of current into the 
grid of the output tubes resulting in a desirable output clipping 
characteristic with crossover distortion. The invention includes at least 
one pair of class-B connected solid state devices. Each device has an 
input circuit and an output circuit. The output circuits are connected for 
mixing. Biasing means in the input circuit of each paired solid state 
device establishes a clipping level offset at the input circuit and at the 
output circuit of each device. Clipping means in the input circuit and in 
the output circuit clips the offset at the input circuit and the offset at 
the output circuit of each respective solid state device. Charging means 
overbiases the offset in the input circuit whenever the input signal is 
greater than said input clipping means, said overbiasing causing crossover 
distortion for emulating the desirable compression associated with a tube 
amplifier.

DESCRIPTION OF THE INVENTION 
A solid state emulator 100 of the invention is shown in FIG. 2. Input 
signal is coupled to an operational amplifier (OP AMP) 102 via coupling 
capacitor 104 with resistor 106 providing a reference to ground. The 
output of amplifier 102 drives upper and lower circuits U and L including 
class-B biased, push-pull connected emulator operational amplifiers 110U 
and 110L. Each 0P AMP circuit 110U and 110L is a unity gain stage that 
duplicates one of the output tubes 16 and 18 in the push-pull tube power 
amplifier 10 shown in FIG. 1. The OPAMP emulator circuits 110U and 110L 
are identical except for the diode directions discussed hereinafter. Thus, 
the reference numbers and designations U and L will be used only where 
necessary. The upper circuit U is discussed below followed by a discussion 
of the differences in the lower circuit L. 
In the upper circuit U, the output of amplifier 102 is coupled to amplifier 
110 via resistor 112 and capacitor 114. A diode 116 is coupled to ground 
at the input of amplifier 110. A resistor 118 is coupled to an upper bias 
circuit 119 comprising the parallel combination of diode 120 and resistor 
122 to ground, in series with resistor 124 to the -15 volt supply. The 
output of amplifier 110 is applied to diode 126 via resistor 128. The 
signal at diode 126 (i.e., the output of the upper circuit U) is mixed 
with signal from the lower circuit L via resistors 130U and 130L. The 
mixed outputs are then amplified by output amplifier 132 which is a 
non-inverting gain stage with a feedback resistor 134, a ground circuit 
including capacitor 136 and series resistor 138, and output coupling 
capacitor 140. In order to provide a greater offset voltage the diodes 120 
and 126 may be multiple diodes in series (not shown). 
In the exemplary embodiment illustrated, the upper bias circuit 119 creates 
-0.6 volts at the cathode of diode 120), and this bias is applied to the 
input of amplifier 110 via resistor 118. This -0.6 volt input bias offsets 
the output of amplifier 110 at the same amount. Further, this offset is 
applied to diode 126 through resistor 128. Thus, output circuit diode 126 
is biased into slight forward conduction at idle. The lower emulator 
circuit L is identical to the upper circuit U except that the direction of 
diodes 116L, 120L and 126L are reversed or complimentary to the diodes 
116U, 120U and 126U. All other elements are the same. 
A low level input signal, e.g., a 1 volt peak sine wave, is coupled in the 
upper circuit U via resistor 112 and capacitor 114 to the input circuit of 
amplifier 110U. The input is offset -0.6 VDC. The applied signal has a 
negative peak value of -1.6 volts and a positive peak value of +0.4 volts. 
Diode 116, whose cathode is at ground, is reversed biased at the negative 
peak swing, and is forward biased at the positive peak swing. However, 
diode 116 does not conduct in the forward direction because the peak swing 
is only +0.4 volts and diode conduction begins at +0.6 volts. The same 
signal swing occurs at the output of amplifier 110 because it has a unity 
gain. The output signal is then applied to diode 126 in the output 
circuit, which as noted above, is already biased at idle into a slight 
forward conduction. Hence, diode 126 clips the negative swing because it 
is forward biased for this swing, and it allows the positive swing to 
pass, because it is biased below 0.6 volts forward and is in effect 
ultimately reverse biased. The resulting waveform is shown in FIG. 3A as 
curve IU. The waveform is a clean half sine wave in the positive direction 
and a clipped half sine wave in the negative direction. 
The lower emulator circuit L using lower amplifier 110L is identical except 
all the diodes are reversed and lower the bias circuit 119L consisting of 
diode 120L, and resistors 118L, 122L and 124L therein creates +0.6 volts 
at the anode of diode lower 126L (0.6 volts being the typical forward drop 
of the diode). In the lower circuit L the bias is applied to the input of 
lower amplifier 110L via resistor 118L. This +0.6 volt input bias then 
also offsets the output of amplifier 110L by the same amount. Further, 
this offset is applied to diode 126 through resistor 128. Thus, diode 126 
is biased into slight forward conduction at idle. A 1 volt peak sine wave 
applied to this lower emulator circuit L is thus opposite the upper 
emulator circuit U. As a result, a clean half sine wave is produced in the 
negative direction and a clipped half sine wave is produced in the 
positive direction. This waveform is shown in FIG. 3A as curve IL. The two 
emulated waveforms IL and IU are mixed together at node 131 creating a 
relatively clean sine wave as shown in FIG. 3C. To appreciate how these 
combine FIG. 3B shows IL and IU superimposed. 
At high level signals in the upper circuit U, e.g., at a 3 volt peak sine 
wave, the input signal is coupled via resistor 112 and capacitor 114 to 
the input of upper amplifier 110. The input is offset -0.6 VDC. If diode 
116 were not present, the applied signal would have a negative peak value 
of -3.6 volts and a positive peak value of +2.4 volts. However, with diode 
116 present and with its cathode grounded, it is reversed biased at the 
negative peak swing, and forward biased at the positive peak swing. Thus, 
diode 116 conducts in the forward direction because the peak swing is 
greater than +0.6 volts. Accordingly, diode 116 limits the peak swing to 
+0.6 volts and clips the positive waveform somewhat. Capacitor 114 charges 
in the negative direction to allow the 3 volt peak sine wave to pass with 
a positive peak value of 0.6 volts and a negative peak value of 
approximately -4.6 volts. At this condition, the average bias is -1.6 VDC 
rather than -0.6 VDC. Hence, the upper emulator circuit U is over-biased 
for these signal conditions. 
As noted above, the same signal swing occurs at the output of amplifier 110 
as is at the input, because the amplifier is a unity gain stage. This 
signal is then applied to output diode 126, which is already biased at 
zero crossing into a heavy forward conduction due to the over-biased 
conditions. Hence, diode 126 clips the negative swing, (because it is 
forward biased for this swing) and it clips that portion of the positive 
swing for which it is over-biased. Diode 126 then allows the remaining 
positive swing to pass because it is biased below 0.6 volts forward and 
then is ultimately reverse biased. The resulting signal is thus 
asymmetrical, having spent more time in the negative swing than the 
positive swing. This waveform is shown in FIG. 3D as curve II U. The 
signal is a partial clipped half sine wave in the positive direction and a 
fully clipped half sine wave in the negative direction with significant 
asymmetry. 
The lower emulator circuit L using lower amplifier 110 is identical except 
all the diodes are reversed. Thus, it should be clear that a 3 volt peak 
sine wave applied to the lower emulator circuit L will be opposite the 
upper one. A partially clipped half sine wave in the negative direction 
and a fully clipped half sine wave in the positive direction with 
significant asymmetry results. This waveform is shown in FIG. 3D as curve 
IIL. Mixing these two emulated waveforms together at node 131 creates a 
clipped sine wave with considerable crossover distortion as shown in FIG. 
3F. To appreciate how these combine, FIG. 3E shows the signals IIU and IIL 
superimposed. 
It is useful to point out the components in the circuits of FIGS. 1 and 2 
that perform the same functions or act in the same manner: 
1: Resistors 32 and 34 (Tube) and Resistors 112U and 112L (SS) are source 
resistors for the clipping function. 
2: 36, 38 (Tube) and 114U, 114L (SS) are the coupling capacitors that 
charge to overbias. 
3: 40, 42 (Tube) and 118L, 118U (SS) are the bias source resistors. 
4: Grid of 16, grid of 18 (Tube) and diodes 116U, 116L (SS) provide the 
input clipping mechanism. 
5: 16, 18 in push/pull (Tube) and diodes 126L, 126U (SS) correspond as 
follows, in the tube amplifier, each output tube supplies one polarity 
signal swing to the output. In the solid state amplifier, the diodes 
remove the unwanted polarity output swing. In the tube amplifier, the 
input signal is split into two out-of-phase signals to drive identical 
output tubes in push-pull via the output transformer. In the solid state 
amplifier, identical input signals are applied to two emulators which are 
polarity reversed, and the output signals are summed. 
Finally, tube compression has a certain attack and decay which is how fast 
the compression happens and how long it takes to stop. The solid state 
emulator 100 acts in a similar manner. Additionally, depending upon input 
waveform, different overbias conditions can occur on each signal half 
cycle in the tube amplifier. Similarly, the solid state emulator 100 can 
also overbias in a similar manner. 
While there have been described what are at present considered to be the 
preferred embodiments of the present invention, it will be apparent to 
those skilled in the art that various changes and modifications may be 
made therein without departing from the invention, and it is intended in 
the appended claims to cover such changes and modifications as fall within 
the spirit and scope of the invention.