Help identifying a ground loop hum

If you have cable TV, disconnect it and see what happens. Also, if your preamp is tubed, change it (them) and see if hum is gone. Good Luck!

Mezmo: A little clarification, please. Are you saying that the "hum" is physically generated by the external power supply and not heard through the speakers? If not through the speakers, then the transformer in the external power supply is the likely suspect. You can try spikes under the unit and weight on top, as long as you don't cover vent holes. If necessary, the transformer can be suspended from the chassis by rubber grommets to minimize vibration coupling. If the hum is simply due to a cheap transformer or an undersized transformer, a good tech should be able to recommend an appropriate replacement (probably toroid) to eliminate the hum. Sorry for the long answer. Hope it helps.

Mezmo : from your note it seems that the noise comes from the power supply. If caused by a ground loop or if it´s the transformer itself, just disconnect everything from the preamp and if it still "hums" as you described then you know the problem is isolated in this unit meaning is not a ground loop because there´s no loop. So the transformer as JCBtubes suggested is the source. On the other hand you don´t mention if you have power conditioning. You might consider this as a possible cause too (lack of it). Your comments