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Insane ground loop; anybody wanna try?

I have a ground loop that's been killing me for weeks. I've tried several things to limited or no success. I've written to Mike Sanders at Quicksilver, but I'm a little chagrined to keep asking him questions that aren't really the fault of his gear.

Anybody want to have a go at solving this puzzle? It's driving me nuts, and I'd be grateful for any help.

Relevant equipment:
Rowland Capri preamp
Quicksilver Silver 60 mono amps (EL34)
Sunfire True Sub

Amps, preamp, and sub are all plugged into a Monster 2000, so everything shares a common wall outlet.
Plugging the amps into separate wall outlets has little effect either way.
Amps are damn near dead-quiet with no input, so it's shouldn't be the transformers or the tubes.

Amps plugged in to the preamp (shielded DH Labs RCA cables) hum, and the sub does too. Swapping cables has no effect.
Unplugging and reconnecting sources (a turntable and a Mac Mini via a Schiit DAC) has no effect.
Unplugging the sub has little effect (except it eliminates the hum in the sub, haha).

Lifting the ground on the amps reduces the hum — by about half, but definitely not completely.
A Hum-X has no (or very little) effect, whether placed on the preamp, an amp, or the sub.

For obvious reasons I don't want to lift the ground on the amps permanently.

I'm not an electrical engineer, but I'm a logical guy.

Ideas? I'm open to any with two requests: First, if you don't know something for sure, please say so. I don't want to play in electron traffic because somebody just guessed at a solution. And second, if you disagree with somebody, don't call him names, okay? There's more than enough gratuitous meanness in the world right now without insulting people over stereo equipment. Thanks.
pbraverman
pbraverman OP
20 posts
 
@xti16 Hey... I think I agree with what you're saying but I'm not understanding the first line.

Are you saying: "Since locations 1 and 2 don't have a problem, but location 3 DOES..." etc.?

If that's what you're suggesting, it makes sense to me. I've tried nearly everything else and the fact that the equipment works in some locations certainly suggests that the equipment isn't the problem. Just confirm that I've got it right above.

All of that said, what continues not to make sense is that the voltage potential in location 2 and 3 is the same (and too high), but only location 3 has the killer hum, where location 2 is basically okay (at least sonically).

Thanks!
edwyun
196 posts
 
Are you sure it is a ground loop and not EMI/RFI?
pbraverman OP
20 posts
 
@edwyun Thanks for the input. I'm pretty sure it's a ground loop, since lifting grounds does have some clear effect. There is the slightest trace of RFI if I put my ear an inch or two from the speaker. I'm not sure I can slay that, but I can tame it a bit with cable choice.

If you think my analysis is off, by all means say so — I'm not an expert here.
edwyun
196 posts
 
There can be a combination or cumulative effect of issues.
toddverrone's avatar
toddverrone
838 posts
 
Man, what a brain trust!

When I mentioned loose or dirty connection, I was thinking of the connections inside the panel. So the connection where neutral connects to the neutral bus, hot to the hot bus, ground to the ground bus. If you open the panel and shut off the main service breaker, you’ll be able to loosen then re-tighten the connections.

It does sound like a ground loop, though one that is related to the branch circuit it’s on. Which is weird.
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