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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
bigaud
8 posts
 
These will give you more data:

Try temporarily running a wire from the preamp case to amp case. This assumes both cases are connected to their respective ground paths.

Could also be a problem with a cable or jack. Any oxidized jacks? Does every cable between the components cause hum? Test with only one connected at a time.  If hum is only from one, try a different cable on that jack to jack path.
Heck if stumped  I might try reversing the cables, one at a time, to see what happens. That is mainly if the cables are directional.

If none of this makes any difference, then you know the problem is most likely internal to the preamp.


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almarg's avatar
Ag insider logo xs@2x
almarg
9,670 posts
 
There are amplifiers sold with ground lift switches, so I think this is a common problem and amp manufacturers think it a safe solution.
As a point of information, the ground lift switches that are provided in some components do not isolate AC safety ground from the chassis of the component, as a three-to-two "cheater plug" would do. Rather, they isolate the component’s internal signal ground from the chassis, which does not result in the safety hazard that isolating AC safety ground from the chassis would result in.

Regards,
-- Al


atmasphere's avatar
atmasphere
11,753 posts
 
As I get this:

The Rowland and Quicksilver work fine elsewhere in the house.

With no input connections at all, the Rowland and Quicksilver hum. By itself, the Quicksilver is silent.

By any chance do you have a Digital Voltmeter? There are some tests that you can do with the equipment unplugged that may shed some light on this.
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