Jmho, it’s not DC offset on the AC mains. I agree it could be a ground loop hum.
For a test disconnect all source equipment interconnects from the inputs of the preamp. If you have a sub also disconnect any interconnect/s from the preamp.
Plug the preamp and power amp directly into the same AC wall duplex outlet.
Check for the hum and post back your results.
That's not unusual.
Amp off first wait about 45 to 60 seconds then shut off the preamp. If you still hear a slight thump wait a little longer after shutting off the amp before you shut off the preamp.
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For a test disconnect all source equipment interconnects from the inputs of the preamp. If you have a sub also disconnect any interconnect/s from the preamp.
Plug the preamp and power amp directly into the same AC wall duplex outlet.
Check for the hum and post back your results.
Oh, and it produces a thump when the amp and preamp are turned off together (I don't do this anymore).
That's not unusual.
Amp off first wait about 45 to 60 seconds then shut off the preamp. If you still hear a slight thump wait a little longer after shutting off the amp before you shut off the preamp.
.