The relay is powered from a standalone transformer putting out 10v ac under load. I didn't use the heater filaments for the supply as I'm using an elevated heater supply and although the relay contacts can withstand the elevated supply, it's only a few volts below the specified maximum.
The transformer goes directly to a bridge rectifier and then into a 7805 circuit with a 2200uf and 0.1uf cap on the input and the same on the output.
The wires from the 7805 circuit to the relay are not short because I wanted the transformer as far away from the signal circuitry as possible and I wanted the relay as close to the signal as possible.
The whole relay switching circuit is floating and not connected to the same ground plane as the rest of the amp.
The dc power wires going from the 7805 to the relay are standard gauge wire as often found in amps... no shielding, etc etc.
I have tried moving the relay power wires but no luck. Disconnect the transformer from the mains and the hum disappears... the circuitry still works noise free so it's the relay or relay psu that's inducing hum.
The attached picture shows the internal circuitry (I had little choice with regards to the layout - I was donated the chassis). The small transformer on the right is the relay pt. Next to it is the 7805 circuit. The other end of the chassis just below the preamp valves is the relay pcb.
I'm going to disconnect the relay supply wires tomorrow but leave the transformer on and see if the hum still occurs. Before I do that though, am I missing something obvious?
Cheers!
