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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
22 posts
 
Great — again, indebted to all of you for taking the time. Please don't apologize for asking basic questions. I agree that it's easy to overlook something obvious and I take no offense. It also occurs to me that I never actually tested the outlets with a receptacle tester. I'll do that as well — having met the previous owners of this house, anything is possible.

I got the preamp a while ago and honestly don't remember if it ever played nice, though obviously Rowlands have a reputation for being pretty tightly engineered. I like the earlier suggestion to try it elsewhere and I'll figger how do that tomorrow.

The preamp has one RCA and one XLR output (one pair of each, but you knew that's what I meant). The amps are connected to the RCAs; the sub is connected to the XLRs via adapter and then by RCA cables. I'll disconnect it from the preamp and try again — another good suggestion — and I'll try switching the two sets of outputs.

There is no CATV in the house at all. (I have two teenagers. They think I'm Stalin.)
almarg's avatar
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almarg
9,670 posts
 
the sub is connected to the XLRs via adapter and then by RCA cables.
I wouldn’t be surprised if that turns out to be the key to the problem. While the post I made earlier today in this thread addresses a somewhat different situation, it provides what may very well be relevant background.

The following paragraph in that post may be particularly relevant:

Most XLR-to-RCA (and RCA-to-XLR) adapters short the inverted signal on XLR pin 3 to ground (XLR pin 1), although I believe Cardas can supply adapters on special order that leave pin 3 unconnected. Shorting XLR pin 3 to ground is appropriate when adapting RCA outputs to XLR inputs, and may be appropriate when adapting a transformer-coupled XLR output to an RCA input. However, at best it is poor practice when adapting an actively driven XLR output to an RCA input. And while many and perhaps most components would be able to tolerate that, with some designs shorting that signal to ground may result in major performance issues (see this thread, for example), or conceivably even damage, eventually if not sooner.
The likelihood of a problem arising as a result of shorting an output signal on XLR pin 3 to ground with an adapter figures to generally be greatest when the output impedance of the circuit providing the signal is low.  That is the case with your Capri preamp (80 ohms balanced output impedance, presumably corresponding to 40 ohms for each of the two signals in the balanced signal pair).  In the case of the thread linked to in the paragraph I quoted above the result was buzzing, but hum seems very conceivable as well.

Regards,
-- Al

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cleeds
5,943 posts
 
+1 to what Al sez. ^^^^^^^^^

also:
No dimmers, LEDs, TVs on the same circuit.
These devices can leak noise to neutral and ground, which are bonded together at your service panel.  You'll want to defeat those devices to make sure they're not contributing to the problem.
xti16
1,525 posts
 
If I understand when only the pre is connected to the amps you have the hum. Have you tried listing the ground on the pre only? Another thing I would try to further what cleeds is saying is to turn off all circuit breakers in the house except the one for your system to insure it's nothing else in your home (yep you'll have to reset all you digital clocks). If the hum is gone with all the other breakers off add everything back one at a time. If still no hum turn on the breakers one at a time until the hum comes back. Then turn off that breaker and continue to turn the rest of the breakers only to insure it's not from multiple locations.

Also if you have a volt meter measure the voltage between neutral and ground. I get 0.15 VAC. I would start when all your breakers are off except at the system with nothing connected. Keep measuring as you turn on the breakers too.
mcroth
81 posts
 
Not too sure if this will help since you said you unplugged the sub.  My hum problem was coming from the Sunfire True Sub.  Try to carefully tighten up all the nuts that make ground contact to the plate on the amp. Especially the contour and volume pot nuts. I now these hums can drive you crazy!  Good luck!
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