I've an audio amplifier that was built from spares from an old kit from 19-something. The power supply for this amplifier is the simplest it could possibly be, consisting of a transformer, bridge rectifier and bulk capacitor.
Even when receiving no signal i can hear a humming coming from the speakers. i have a spare EMI filter circuit that came from some consumer appliance, and my hope is that it will solve the issue, but that's beyond the scope of this question. My question is where on the power supply circuit should I put the EMI filter - on the primary, secondary or the rectified output?