I've had a problem with an unearthed Class II SMPS amp that would buzz when connected to an earthed audio source. The issue seems to be leakage current from the amp's ground to the earth through the aux cable shielding conductor. It is described more in depth here:
I'm trying to find a solution to stop the buzz without changing the type of input (3.5mm aux jack) and came to think of these small audio isolation transformers.
They don't seem to do the job, as there is still leakage current to earth between primary and secondary somehow. I wonder how that would work as I thought whatever might be common mode to a transformer side couldn't make it across to the other winding.
Could someone explain what's happening, or if there is a better way to insulate the aux input to eliminate the buzz? I'm attaching the schematic of the previous amp, which in theory should have worked fine.