I am running a Dodd tube preamp and Dodd Hypex amp. I have a loud hum emanating from my Polk SDAs. Symptoms are:
The hum is not affected by the volume control on the preamp.
The hum stops when I mute the preamp.
The hum stops when I mute the amp.
The hum is independent of the input selector on the preamp.
Hum with or without SDA cable connected.
Hum whether Polks or other speakers are connected.
Hum with no inputs connected to preamp.
Hum with Hafler amp (a spare) or Dodd amp (although significantly louder with Dodd)
Hum will go away but soon return following what seems like any change: new outlet (tried upgrading to a hospital grade outlet), different amp (Dodd/Hafler, as mentioned above), disconnect/reconnect inputs, etc.
This has been especially frustrating as I can, for example, disconnect/reconnect ICs from inputs, enjoy no hum for up to 45 minutes with music playing, go to sleep, and wake up in the morning with hum having returned.
I don't think it's a ground loop problem.
I'm ready to call an exorcist!
Any advice is greatly appreciated. Thank you.