Class A and B power amplifier

The question requires
a Class A and B power amplifier.
The question analyzes
a Class A and B power amplifier built with discrete components, with temperature compensated constant voltage bias and speaker protection circuit. It is powered by mains power and uses dual 18V ring bull transformers + rectifier filtering to obtain dual 26V DC power supplies. The output is added to the Zobel network. To compensate for the impedance changes of the speaker at high frequencies. 
Schematic design description
The schematic diagram refers to the big guy TJR2005, with some modifications;
the filter cut
-off frequency is about 3Hz -200k, and the human ear recognizes sound at 20-20kHz;
the input stage
long-tail differential amplifier + simple current source, suppresses common mode and suppresses temperature Drift;
The VB voltage doubler circuit
makes the composite tube composed of the latter stage in micro-conduction, eliminating crossover distortion; by adjusting the rheostat, the composite tube of the latter stage is even fully conductive, forming a Class A power amplifier; the
output stage Darlington structure, Parallel management of
 
Zobel network
Zobel network: There are coils inside the speaker, that is, inductive components. The capacitive reactance and speaker inductive reactance in the Zobel network are compensated so that the load works as resistive as possible, that is, the stability is improved when the load is parallel. I am in audio It is indeed called Zobel Network in the design manual, and the value used here is also the commonly used recommended experience value in the book.
The feedback part
controls the amplification factor and is coupled to the ground through a capacitor, that is, only the AC part is amplified. By changing the value of the coupling capacitor, the frequency of the signal to the ground is changed, that is, the frequency of the amplified signal is changed, so the amplified tone is different;
speaker protection Circuit
 
 
 
PCB design instructions:
Pay attention to large currents and signal loops; the problem is not big, just remember to reserve the position of the heat sink.
The physical display explains
the precautions.
If you make AC/DC by yourself, pay attention to safety;
the current problem
is that there is a problem with the VB amplification circuit of the left channel. The potentiometer cannot be used to adjust the voltage. The C pole of the triode in the VB voltage doubler part has always been about 0.6V. , B pole is also about 0.6, md, transistor, and two resistors are fine. They have been replaced. It should be caused by other places;
so the left channel will be distorted. I don’t know what caused it. The circuit principle should be fine. The right channel It's normal; I haven't had time recently. I'll check it later. Maybe it's a welding problem or a certain component problem.