30W power am design (from Crafter DSP1 schematic)

[Synctime]

Attached is the power amp portion of the Crafter DSP-1 30W guitar amplifier.

It's an unusual design, as there's a long tail pair at the input. One input is the signal input and the other input is a compensated/equalised signal from the speaker output. I guess the latter is to balance the enclosure, but I suspect it's also providing positive feedback that amplifies the low frequency which is creating a constant 50Hz hum at the output. Am I right or wrong? Please advise.

I haven't opened the amp to investigate the source/cause of the  hum yet. It seems that it's a common problem with this Crafter amp because another one currently listed ebay UK has the same fault/problem. I am thinking of modifying the inout circuitry into a single input one without feedback, ie the classical design, or replace the entire power amp with a Class D mudule. Thoughts anybody?

[DrGonz78]

The way I interpret the speaker outputs signal return back to input, as a type of negative feedback. Thus reducing the signal upon itself using an inverted out of phase signal. In PA systems each gain stage is filled with negative feedback to keep the signal stable and less distortion. Well that's just a thought I had looking at this. I see where it might be positive feedback but I am pretty sure that the signal is phase-inverted which cancels out the source signal. To make the signal in phase with the source input signal would be positive feedback. I am guessing that would make the signals a tiny bit time delayed, milliseconds. Hence you delay the positive feedback more and more you can create a chorus effect from the two signals.

[Synctime]

I agree it's negative feedback, with some equalisation. I just couldn't figure out here the constant amplitude 50Hz hum is coming. It's present even with the input disconnect from the pre-amp output.

Maybe it's gain is too high, and it's picking up EMC from the transformer.

Usually power amps have low gain, no feedback, and no equalisation, so I am baffled by this design.

[g1]

Any kind of ground issue or ground loop will give you 50Hz hum, so a poor ground scheme could also be the source of your hum.