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
Very puzzling!

Have you tried your preamp in another system to ascertain that it's not a problem with it?

Lifting grounds is a good diagnostic tool - obviously it's not a safe remedy - and if that doesn't cure the hum, then it's likely the hum is caused by something other than a ground loop.

Do you have any other equipment near the audio gear that could be causing the hum? Are dimmers turned off, LED lighting turned off, CATV disconnected from your gear?
Remove the Monster 2000.
Thanks to both of you for trying. I haven't tried the preamp in another system because I don't have a second one, but I'll rig something up in another spot and report back. Good suggestion.

No dimmers, LEDs, TVs on the same circuit.

Dill: I inserted the Monster because of the hum, hoping it might give everything a single ground spot. D'oh! :)

Thanks again. Keep 'em coming. This is a pain!
What about some history? Has this combo ever been hum free together? Is this pre new to you? Did you changed anything at all before the hum started? Sorry, I know these are basic questions, but if you're at all like me, you'll be searching down rabbit holes for days, just to realize the answer was right where you had started.
Unplugging the sub has little effect ....
Is the sub connected at speaker level (to the amps) or at line level (to the preamp)?  And if the latter, is it connected to the preamp via XLRs, while the power amps are connected to the preamp via RCAs as you stated?

In any case, if you already haven't, try disconnecting the sub from whichever component(s) it is connected to, in addition to unplugging it from the AC.

Regards,
-- Al
 
No dimmers, LEDs, TVs on the same circuit.

Disconnect any CATV hookup regardless, you may be surprised.

I fought a ground loop hum for a month or so, and ignored the CATV line because it was on a different circuit. Turned out to be the culprit anyway.
Ground loops find a way.
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.)
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

+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.
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.
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!
Again, thanks to everybody for taking the time. Here are new data as of this morning:

@cleeds I never think of my video stuff as a separate system, but it obviously is. I ran the preamp through a Meridian 551 integrated amp and then to a pair of Totems. All but dead silent, so it doesn't seem to be a noise problem with the preamp itself.

@almarg I was so hopeful it was the XLR/RCA adaptors, as Al suggested (the easiest fix, after all). Alas, unplugging them changed nothing. While I was at it, I switched the amp runs from the RCAs  to the XLR/adapter outputs. Again, no change.

@xti16 I did try lifting preamp ground only, with and without using the Hum-X on the preamp. Slight changes, nothing significant. I'm resisting the circuit-breaker dance for two reasons: first, see data below. Other circuits don't seem compromised; and, second, the breaker box is three stories down and while I'm in decent shape, I am no longer 25 years old. ;)

Okay, here are some more data.

Plugged the amp and preamp in two other parts of the house, using the same cabling each time. Here's what I found:

Location 1: Recent construction, new electrical service 2004.
Wiring is correct.
No hum at all.
Neutral/ground voltage: 0.1VAC

Location 2: Original construction, 1936 wiring but service upgraded, probably 1990s. Not the same circuit as #3 below.
Wiring is correct.
Barely audible (acceptable) hum
Neutral/ground voltage: 0.0VAC
Hot/ground voltage: 0.8VAC
Yes, those are right; I measured several times and re-checked wiring

Location 3: Listening room
Wiring is correct
Hummmmmmmmmmmming bird Farms, may we help you?
Hot/ground voltage: 0.9VAC

I'm now wondering if it's something to do with the circuit that feeds the listening room — but, again, that's just logical speculation, blissfully free from the constraints of actual circuit knowledge.

Anybody see anything helpful here? Thanks again!
Oh, and @mcroth — thanks for the suggestion, but you're right, when the sub is totally unplugged the hum is diminished slightly, but still pretty audible from several feet. I appreciate your raising the idea!
Hot to Ground 0.8 and 0.9VAC? Looks like an open ground. What's Neutral to Hot?
@xti16 Hell, sorry. That should be neutral/ground in #3 above. Anyway, it’s 124.5 in both spots. Also, the receptacle tester reports normal service in all three locations.
Okay, here’s a little more info. (I work at home. You can see I’m getting a lot done today. LOL.)

I have a SS amp with balanced inputs as well. When I substitute it for the Quicksilvers, the hum disappears, even with the sub connected. This is true whether I connect the SS amp directly to the XLR outputs or to the RCA outputs via RCA-to-XLR cables.

This suggests some kind of issue between the Rowland preamp and the Quicksilvers, yes? It also suggests that the AC power by itself isn’t really a problem. But whaddo I know? Any ideas?

And to answer the obvious question: Yes, the Quicksilvers sound about a zillion times better, so keeping the SS amp instead isn’t really an option.
This suggests some kind of issue between the Rowland preamp and the Quicksilvers, yes? It also suggests that the AC power by itself isn’t really a problem. But whaddo I know? Any ideas?
pbraverman- I don't know nothing 'bout 'lectricity or circuit design ;-) but my powers of deduction say that, based on your posts of 11:55 and 2:30, it is possible/likely that the Quicksilvers either are more sensitive to a problem with the AC (in circuit 3) or there is an unusual aspect of the internal grounding of the Rowland and the Quickies that is incompatible. 

BTW, can you tell that I also work at home?



@swampwalker Good deductive powers, my friend, though the difference is pretty drastic and the sensitivity would have to be really, really high. With the Quicksilvers I can hear the hum from 15 feet. With the SS amp it's quiet as a tomb from 6 inches.

Others: I've searched online and wonder about grounding the chassis of the Quicksilvers to the chassis of the preamp. I understand that my lack of understanding may lead me to ask this question, and that may be the dumbest suggestion you've heard since breakfast (I mean since breakfast time, not the idea of breakfast). If so, feel free to put me straight.
"Location 1: Recent construction, new electrical service 2004.
Wiring is correct.
No hum at all.
Neutral/ground voltage: 0.1VAC"
This tells me the AC is a problem between the Rowland and the Quicksilvers. Question is what? The #2 location has minimal hum. It could be location 1 and 2 are on different phases. If so there may be something on the same phase as location #3 that's causing the hum but on a different breaker.

Does location 1 have its own service panel?
@xti16 No, all service is on the same panel; circuits were added in 2004.

Given the different dates of installation, I totally agree that it's possible that location 1 and 2 (and even 3, perhaps) are on different phases.

Um... what does that mean? (Sorry!)

Does the normal performance of the SS amp put any holes in that theory?

Is there something to try short of having an electrician check?

Thanks so much!
If you look at your breaker panel, there will be a column of breakers on the right and one on the left. Each column is a different phase. So it could be a problem if you had an outlet in your room connected to a circuit coming from the left column and a different outlet on a circuit on the right side and then you connected your components to both of those outlets, willy nilly. The power sine waves are shifted relative to each other (180* I think) so that they don’t line up...

Have you checked the connections in the panel (if you’re knowledgeable enough to do this without killing yourself)? It seems it could be a loose/dirty neutral or ground within the service itself.

I second the idea that it would be possibly useful to know which phase the three different locations are on. Particularly if location 1&2 are on a different phase than 3.
Did you try changing tubes?
An experiment I'd suggest at this point that might prove to be informative would be to see if either Quicksilver amp produces hum when it and the preamp are connected and powered up as usual, but with the other Quicksilver amp disconnected from the preamp and unplugged from the AC.

Regards,
-- Al
 
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!
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.
@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!
Are you sure it is a ground loop and not EMI/RFI?
@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.
There can be a combination or cumulative effect of issues.
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.
@edwyun Acknowledged, and thanks. What I should have said is that there's a little bit of RFI that I can hear, and a LOT of hum that sounds like a ground loop. I'm trying to attack one thing at a time. (I'm not that bright!).

@toddverrone Got it. But wouldn't the acceptable performance in locations 1 and 2 seem to indicate it's not the entire electrical service?

Doesn't seem to be any harm in pulling the panel, shutting off the main breaker, and having a look tomorrow. I'll disconnect and reconnect the relevant breaker (#3) while I'm in there. If I don't reply further — well, you'll know I skipped the second step.
Edwyun 2-22-2017
There can be a combination or cumulative effect of issues.
+1. Specifically, it’s possible that there is nothing "wrong" with the AC wiring to the listening room, but it may have different characteristics than the more recent wiring to location 1, which in turn are bringing out an inherent susceptibility of the power amp/preamp combo to ground loop issues.

More specifically, see pages 31 to 35 of this paper, written by a renowned expert on such matters, in which he describes "what drives 99% of all ground loops." As you’ll see, Romex (which perhaps was used in the relatively recent wiring of location 1) is much less likely to bring out the susceptibility to ground loop issues that is inherent in some designs than wires randomly positioned in conduit (which perhaps was used in the older wiring to the listening room).

If that is what is occurring, and it seems like a reasonable bet at this point, a solution may be to have an electrician install new wiring to the listening room, using Romex. Alternatively, inserting a suitably chosen Jensen transformer between the preamp and each of the amps would probably resolve the problem, while also perhaps reducing other forms of noise, at a cost of around $300 plus some additional cabling.

Reports that have been provided here regarding the transparency of Jensen transformers have generally been very positive, although I can recall a couple of people who have provided comments that are a little bit mixed.

A suitable Jensen model would be the PI-RR, which would accept an RCA cable at its input and provide an RCA output, or the PI-XR, which would accept an XLR cable at its input and provide an RCA output. You would need two of them, one connected via a short RCA cable to the input of each amp. The cable from the preamp to the Jensen transformer could be whatever length is necessary. Here is a good supplier, although they can also be ordered directly from Jensen.

Also, although it is apparently unrelated to the problem I would still strongly advise that at some point soon you eliminate the XLR-to-RCA adapters, that are shorting the signal on pin 3 of the preamp’s XLR outputs to ground. If the sub does not provide XLR inputs you could either special order adapters from Cardas which leave pin 3 open, or have a cable supplier custom make an XLR to RCA cable which leaves pin 3 open, or purchase a Jensen PI2-XR transformer which would provide two channels of balanced to unbalanced conversion. Or you could use the preamp’s XLR outputs for the main signal path, connecting them to the amps via the Jensen PI-XR transformers, and connect RCA cables from the preamp to the sub.

Good luck. Regards,
-- Al


@pbraverman
Yes that is what I was trying to say. The Voltage between neutral and ground means there is resistance in the wiring. But at less than 1V is not bad.
Thanks to all of you, especially @toddverrone, @almarg, and @xti16, I think I have it figured out.

The original circuit feeds the two outlets at the system end of the room. I don’t think that was installed in 1936, but it’s in metal conduit, so it definitely isn’t too recent. Not surprisingly, one outlet box connects to the other, so any shortcomings in the circuit are shared.

Mais voilà. There is a four-gang outlet at the opposite end of the room that we added upon buying the house in 1998. (The room doubles as my office.) It’s Romex-fed, and I think it even has its own circuit. There’s some other stuff on it (most unfortunately a laser printer), but when I run a long extension cord from the system to the 1998 outlets, the hum is almost totally gone. At any rate, it’s acceptable.

From there isn’t not too hard for me to run some Romex about 20 feet and install a new outlet at the other end. Some day somebody is going to ask what the hell went on, but for now it should solve the mystery.

If anybody thinks I’m wrong, please let me know. Otherwise, for now, I think it will work.

Thanks again for all the time and advice. I’m truly grateful.
Sounds good!  Glad I was helpful.

Regards,
-- Al
 
Well done! Hunting these things down can be epic.. Enjoy the silence.
If this helps it will be less work.

Before running Romex, I would open the outlet box, the closest electrically to the incoming power, and check all the connections. Then follow the lines to boxes going back to the panel, or coming from the panel, and check those connections. Tighten or re-do when in doubt. if there is no separate ground wire, a poor conduit connection somewhere may be the cause.

 I had one customer where the cable TV ground where the cable entered the house LOOKED proper but did not actually electrically contact the pipe it was on, and the system got a hum (though that may have been only when using TV. ) I fixed the connection so it actually grounded and the hum disappeared.

Another had a cable box and a modem in proximity, and I ran a wire from the cable box case to the outlet box (ground) to kill the hum.

A friend ran a wire between two of his components' chassis to kill a hum or noise.

bigaud brings up a good point. Try and isolate all the receptacles on that circuit and check/tighten all the connection at the receptacles and at the breaker (after turning it off of course). That would cost a little time.
@jmcgrogan2 wrote:
Disconnect any CATV hookup regardless, you may be surprised.

I fought a ground loop hum for a month or so, and ignored the CATV line because it was on a different circuit. Turned out to be the culprit anyway.
Ground loops find a way.


This happens due to a different grounding point. I discovered this at my house also, but with DSS/DirectTV back in the early days. I had the dish and coax grounded outside right below the dish with a separate copper grounding rod, and the electrical service in the house was grounded 35 feet away, at the service entrance. I didn’t realize it at the time, but those two different grounding points are a difference in potential, as well as creating a 35 foot "loop" if the ground from the satellite were somehow introduced into the audio system (which is what was happening with the DirecTV receiver). When I unplugged the satellite coax, the humming went away.

While this doesn’t quite apply in the case of the OP’s question, it makes me wonder what the grounding situation is for the wiring in the house. If everything is grounded through the circuit breaker panel (which in turn is grounded to a single point in the house such as a water pipe, or outdoors to a stake in the ground), then it should be OK. But if somehow the newer wiring is grounded in a separate place, then that could cause a problem.

I have an odd hum issue in this house as well. My equipment is in an addition built in the mid 1980s, where the original part of the house was built in 1940. I found out that the ground wire isn’t connected in some of the outlets in the addition, so I have my own dilemma to attend to shortly...
If you are using inter-connects with arrows on them , the arrows must point to the Pre-Amp .......Most of the time people think they should point to the way the signal goes, which is wrong........The arrow points to the grounded end of the cable (which is only grounded on one end)  This will cause this type of problem..........autospec
Consider trying an Equi=Core PC that is actually a Balanced Power device using a proprietary transformer to split the 120V into 2 out of phase 60V halves, and when they are recombined the non zero summed noise is drained to ground, lowering the noise floor dramatically. Tim Stinson of Luminous Audio couldn't get rid of a ground loop hum at the 2016 Capital Audiofest that was preventing Tim from using his Arion Phono Stage in a system, Sunil of CARE Audio loaned him an Equi=Core 300 and solved the hum instantly.It's worth giving Mark a call at Core Power Technologies.
In my best system I run AG horns, my current Trios are something like 102 db sensitivity or even higher.  They pick up tiny hums, noise, sometimes I think even insects walking around stealthily in my basement. I've run many amps, low and high power tubes and low and high power solid state, and always encounter what I call ground loop issues.  Get out your cheaters and lift all the grounded power plugs, disconnect unused circuits throughout the house, etc, to no avail.  Still the damn hum and noise.
My cure, courtesy of Steve McCormack, now running SMc Audio.
The music source is not the problem.  Make a triangle from your pre-amp to both mono blocks, all three legs.  Ground them to each other, all three legs. Unhook your sub if need be.  Voila, hum is gone, as is any stray noise.  Now, you would think the third leg is redundant.  ie the Pre separately linked to each block, and thus the blocks have a hardwire ground to each other via the Pre.  Well, I did that, two legs instead of three,rid myself of most hum and noise, yet still had some audible hum and noise.  Connecting all three legs equals dead quiet, and perfect source music.
Sound illogical?  Too much work? Hey, it blueprints perfectly in the ground plane, and I use solid copper wire, like 14 or 16 gauge, doesn't need insulation.  Simple really. Avoid 12 gauge solid, not malleable enough, and any stranded copper, not worth the fuss for grounding.  I don't know how you feed music to your sub, but I'm sure you can hook in after you know the noise is gone.  Good luck.  Best regards, Joel
I've been busy but have read through everything each of you contributed. I'm very grateful to everybody for taking the time to offer thoughts. Thank you.

Here's the update:

I used what time I had to reroute and reconfigure the two electrical circuits in the house per my last posting. My back is sore, my fingertips roughed up. And when I finally had everything put back together... Can you wait?! ... No change. I kinda expected that, being a pessimist at heart, but was still disappointed. Hey, at least I lowered the ground resistance to near zero in the system's circuit.

So I kept poking and think I have probably found the real culprit: the Rowland preamp.

I should have tried some of these things first, as they're simpler, but alas, we all learn by experience, yes?

Turns out that when I run my DAC straight into the amps, the hum is barely audible. (It's a Mac server, so I can control the volume through iTunes.) That also seems to eliminate that digital equipment as the cause per se.

But here's the salient clue, something I hadn't tried before: If I disconnect all source cables from the preamp, and then disconnect the preamp from the AC power, leaving the preamp and amp connected ... the hum remains. Yikes.

I opened the preamp and looked at the power board. There is a cap with one leg connected to the ground screw via a PCB trace, but the solder connection is hair-thin. I've contacted Rowland to ask if that trace is supposed to be stronger, which I'm capable of addressing myself. But if that's not it, I'm awaiting their response, which I assume will include sending the preamp to them.

The greatly reduced hum has also allowed me to identify a little RFI in one channel, but I'll take one thing at a time.

If this stimulates any thoughts, I'd be glad to read them. Otherwise, this feels more solid to me and I hope it's the beginning of the solution.

Thanks again, all.
pbraverman, thanks for the update!
Thanks for the update.  When I get a chance, I'll check my Capri to see if it is an issue as well.  But, I don't have any ground hum (except for a little EMF hum from the phono input only at max gain).
@edwyun I haven't received a response from Rowland, so that would actually be super-helpful if you'd be willing to check. (I'll holler at them if I don't hear by tomorrow.)

The thin trace is on the top side of the power board, in the corner where the ground ring connects to the AC receptacle two inches away. (Since the cover is on the bottom, you can't see the top side without unscrewing the board, but it's super-easy, just four hex bolts.) Here's a photo of the board and the ground I'm wondering about:

https://www.dropbox.com/s/xkhm3lnsog45auy/Capri-ground-7D2_4666.jpg?dl=0

If you get a chance, let me know if yours looks the same. Thanks a lot!
Here's mine.  It looks the same.  BTW, that trace is not for the ground but for a capacitor.  So I don't think it is an issue.

http://www.audiokarma.org/forums/index.php?media/20170308_193253.9518/full
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.


I have a Jolida 302 brc. I love the amp but when I first got it I had a ground loop hum. I tried everything couldn't get rid of it. Finally I purchased a PS Audio power cord that has a ground pin that can be removed. I removed the ground pin and the hum disappeared. I know you don't want to do this but you could get a three to two adaptor they are sold everywhere (buy two one for each amp). That will tell you if it's the problem. There are amplifiers sold with ground lift switches, so I think this is a common problem and amp manufacturers think it a safe solution. I bought an outlet tester three LED lights will tell you if the outlet is wired correctly. My outlets are wired correctly,  I am confident my amp will be fine. Love the Jolida but when I'm ready the Quicksilvers will be my next purchase!   
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