Recently I developed a pesky problem of an apparent ground loop. From all I read about the symptoms, that was the diagnosis.
It seemed to be associated with the phono stage. No hummmmmmm with hi level inputs.
Following standard advice from the web, getting all components fed from one outlet didnt help. Lifting the ground on the phono and/or pre amp with a cheater plug didn't help. Making a star ground for all components didn't help. (Darn, it worked so well with my ham gear!)
I'd recently purchased a high quality interconnect, and thought I'd put it where it would do most good: between step up transformer and phono stage. This, in plac of a Blue Jeans interconnect ( shielded coaxial cable by Belden manufacture ).
So I've already tipped my hand here.
I put a Blue Jeans cable back there, and moved the new unshielded cable to go between phono stage and pre amp. "Ground loop" cured!
My conclusions are that, while the fragile low level signal input to the phono might have benefitted from the fancy new cable, its low levelness was swamped by interference from all the power cables, power supply magnetism etc back behind the rack because of it being unshielded.
Perhaps there are other explanations. But information on the web about how to resolve hummmmm is woefully inadequate. If ypu have this problem, begin by reading everything you can on the net, but be prepared for frustration when it doesnt work.
Get all your gear on one outlet. Next, make a star ground. Next, unplug your cable tv router. Next, disconnect you interconnects to try to isolate the culprit. Next--here's a new one--consult a ham radio operator. Why? Because they have these problems all the time. Because they have technical training you don't... Often to an extraordinary extent. Because, as a group, they are cheap soms of guns who take pride in scrounging and, as a result, performing miracles with scraps. Because they collectively embrace an ethic of charitable service to the community-- i.e., so you're essentially consulting a lay electrical, or even actual engineer, who will be happy to help you for free!
Maybe not least, but surely last, hire an electrician to consult.