I am looking at the attached schematic from Patrick Turner's (RIP) website. I have been racking my little brain trying to figure out the purpose of the circled components between the outputs of the 6CG7 LTP. Can anybody tell me? Thanks.
This is a compensation network to bring down the open-loop gain of the amplifier at high frequency, to improve stability (phase margin) with global feedback.
Thanks. I understand and use compensation networks. He has several compensation networks in the circuit that are straightforward. I just don't see how this one works. It is between the outputs, not to ground. Any references I can look at to explain it?
Because the voltage across R20 and R21 are 180 deg out of phase, he took advantage of that instead of having two such RC filters to ground. You can do it either way; his way saves a couple of components. Sorry I am unable to calculate the values of the 4 component solution, if different than the same.
The capacitor contributes to the rolloff (as noted). But resistor R17 adds a zero to the closed loop response, which provides phase lead as crossover approaches. If everything is correctly calculated and tested, the stability of the amplifier will be markedly improved.
I would use the network exactly as drawn. Using two single ended networks may allow parts tolerances to cause wholly avoidable differential phase errors.
I would use the network exactly as drawn. Using two single ended networks may allow parts tolerances to cause wholly avoidable differential phase errors.