Hello, I have just rebuilt an old Playmaster Electronics Australia Mosfet amp (circa 2002 or so). Firstly I am looking for a circuit diagram, It has 6 power mosfets each channel and from memory it's 200watts RMS into 8 ohms. However I found some old circuits using 4 power mosfets that also claims to be 200 watt RMS and these have similar topography but not exactly the same . They all run on +- 70vDC rails. Anyway it basically works perfectly except when I join the minus of the inputs at any point I get a very low transformer buzz from both speakers. You have to stick your ear right to the speaker to hear it. I am on a mission to make it perfectly quiet! Chassis is grounded to mains, centre tap of both transformers goes separately to the PCB ground (0v) tracks (that track also connects directly the centres of an array of 2 x 2 x 6800uf caps per side), then single ground (0v) wires from those tracks to the centre bolts of each of the bridge rectifiers (chassis). One diagram I have has those grounds connected to chassis via a 0.1uf cap?? The signal input minus passes through a 10 ohm resistor to ground (0v) and as I mentioned above any joining at any point of that left and right signal minus creates the exact same level of buzz. Secondly the bias: currently at idle I have set about 12mv across the .22 ohm FET source resistors , though some of them are showing about half that. So I figure that is about 50ma per power FET except the ones that are about 7mv. Just curious. It is driving a pair of 1988 vintage AE1's sounds totally awesome.
That is s ground loop.
There are a few established techniques to solve them, search and test which one helps you.