Bizarre buzz

I need some theories and hopefully answers.
This is a cituation, neither I, not anyone, who I asked, have encountered before.
It is very simple and puzzling at the same time.
Here it is:
There is a faint buzz coming from the amplifiers, NOT the speakers, barely audible from a few feet away.
Can also hear it with the ear on the transformer cover- sounds like a slight mechanical vibration.
The strangest thing is- you can only hear it with the amps and the whole system OFF, cold.
If the system is playing for some time, and then turned off- there is no buzz.
Only when the system been off overnight.
Disconnect the power cord from the amps- silent (obviously)
Again, only happens whith the amps OFF and the PC connected to the wall.
I'm not naming the brand on purpose, because I think it's completely irrelevant in this instance.
Please, throw in your theories and assumptions.
Thanks in advance.

Showing 1 response by gbart

I have a problem with the above theory of disconnected load. There is no practical reason for the amplifier's (main) transformer to remain energized when the amp is turned off. The transformers in some preamps and CD players remain energized 24/7 once plugged in, with only the signal output muted. I have not seen it done with any amp, where the main transformer remains energized all the time while the load is disconnected. Amps that remain in standby when "off" usually employ a separate tiny transformer that is always on, to furnish power to the triggering/soft-start/protection circuitry. Otherwise the main transformer and power supply remain de-energized.