I'm trying to reverse engineer the amp circuits here, but I'm getting stuck at the RIAA amp. It's a differential amp with normal and inverted outputs so theres 2 RIAA circuits for 1 channel.
If you look at the SOT-89 output transistor pairs, at the top, right of the 3 relays are the pairs for the normal and inverted output.
And below them there's the rest of the amp circuit.
The CBL transistors are NPN 2SC3324
The ABL transistors are PNP 2SA1312
Here's as far as I've gotten with the circuit. I'm stuck at the weird current mirror thing consisting of Q22 Q24 Q33 Q31. etc.
The only connections to that circuit are the Q22 and Q31 bases/collectors; the 2 150 ohm junctions. And the 300 ohm resistors which are most likely tied to the power rails.
Then there's also the 2 2k resistors for no idea what.
Here's where I think the traces on the other side of the board are. There doesn't seem to be any parts on the other side since nothing seems to be missing... Apart for the other output amp, since it's a balanced output, but maybe it's just pseudo-balanced.
Here's the schematic for the output amp circuit which is on the lower left of the board. It's pretty much complete.
Maybe it's just a biasing circuit, but how does it handle both output stages then? And if so, why is it only there for the RIAA amp.
Here's also a very incomplete schematic of the input amp if it gives any ideas.
