Re: AVO 163 amp board ~ transformers
I've stuck the raw amplifier circuit (without transformer and feedback) and the overall gain is 316.
Gain of the first stage is indeed 0.99
Gain of the third stage is 243, very close to the predicted 275.
The second stage gain is lower than I expected at 1.33. What I had not taken into account is the output resistance of the second stage is Rc = 10k, and the input resistance of the bias network of the third stage is 82k//20k = 16k. So the gain is reduced to 2x. Then it is further loaded by the input resistance of the third stage amp, which will be hfe x re. And that is why they chose the highest hfe grade for VT3
Substuting a BC550C with an hfe of 450-900 increases the amp overall gain to 333. The loading from T3 is still significant though. With an re of 80 ohms and an hfe of 400 the input resistance of the transistor is 32k. So the loading on the collector is 82k//20k//32k = 10k. so the expected gain of T2 is 3.3 x (10/(10+10) = 1.65. Spice gives 1.5 which now agrees well across the board.
Craig
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