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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
21 posts
 
Hi, folks. Sorry to be unresponsive; had some actual work to do there for a while. Thanks as always for the input.

Phase: All three circuits I checked are on the same phase — if I understand correctly — meaning all three are on the left half of the panel.

@toddverrone I think that answers your first question, yes? As to the dirt or loose contact: Are you talking about a contact at the breaker in the panel? The breaker has only one leg, doesn't it — is there a neutral or ground lead there? If you're talking about a problem at the outlets themselves, the problem is the same on two different outlets in my listening space so it seems unlikely that's it.

@gs5556 Yes I did, but thanks for the suggestion.

@almarg Yes, either amp behaves the same way with the other amp clear of the system.

Thanks, fellas!
xti16
1,525 posts
 
Since location 1 does not have the problem but locations 2 & 3 are not tells me the problem is not with your equipment but with your wiring or something plugged into the wiring. So I would suggest either start unplugging devices/appliances etc and see if the hum goes away. If it doesn't I would suspect a corroded or loose connection in the wiring on that circuit.
pbraverman OP
21 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!
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edwyun
244 posts
 
Are you sure it is a ground loop and not EMI/RFI?
pbraverman OP
21 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.
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