Originally Posted by A9X-308
Oh, and Alan, if you're not a member of hifi-engine, join. It's basically a repository of owner and service manuals of tons of old and new gear. Free, and never had a spam email etc from them and I was a very early joiner.
I have been reading and analyzing the Bryston output stage for a few days. The more I doubt it's worth the while to even get into it. It's too complicate, usually from my experience is not good. Things are intertwined together, usually it's asking for instability. I attached the schematic. It's basically a Quazi-Complementary-Feedback Pair that is well studied. This is my observation, see whether you agree or not:
1) the signal from VAS(second gain stage) goes through Q27(inverter), to Q16(buffer), to Q18(phase splitter) before driving the output transistors Q2 and Q1. Then because Q2 is CE stage with gain, frequency response is a lot lower than Q1 which is EF. There is a big mismatch right there that might cause problem. Signal goes through 4 stages to the output, I don't like it.
2) My big concern is Q18 driving Q1. Input capacitance of Q1 increase with increase collector current. a pole is formed as shown in the RED markup with R15.
3) You can see the BLUE circles, there's a lot of BANDAID RC network around, must be last minute add on to stabilize the amp. You look at the RC value, the pole/zero are all in hundreds of KHz to MHz. Those are definitely not for audio frequencies, it's taming the oscillation.
4) Because of the Quasi Comp, TWO current limiting transistors needed Q16 and Q17, this further complicate the design.
5) The use of 2uH inductor L1 is a sure indication the amp cannot withstand higher capacitor load. An inductor has to be used to isolate the output from the capacitor load. Inductor should be avoid at all cost. I had a lot of test, that inductor destroy linearity when driving a non linear load.
Just too too much crap going on. Like I said, a good circuit is always very simple, you can see very distinct individual stage that perform a specific function. This one is all mumble jumbo together.
See what you think.