Strange Hum from Right Pre Channel Only


My tube preamp has developed a peculiar hum problem only in the right channel, and it began only after I switched to XLR cables running from the preamp to the XLR inputs on my Krell FPB 300cx amp. I never had this problem when I used RCA’s from the pre into a RCA/XLR adaptor attached to the amp inputs.

Strangely, when I switch L/R channels for the XLR cables attached to the pre, the problem goes away, but now I have Left pre out to Right amp in, and Right pre out to Left amp in, not an optimal arrangement.

Please advise what could be causing this right channel hum and how I can remedy it, other than by reversing the channels on the pre end.

Thank you.

Neal
nglazer
Ground loop. Try using a two prong adapter on your power cables until you find it.
Strangely, when I switch L/R channels for the XLR cables attached to the pre, the problem goes away, but now I have Left pre out to Right amp in, and Right pre out to Left amp in, not an optimal arrangement.
Doesn't sound like a ground loop. I would expect a ground loop hum to be in both channels. Almost sounds like a bad connection somewhere and by reversing the cables you got lucky. I would first try wiggling the connectors to see if the hum comes back. If nothing try putting tension on the cable close to the connector. I would do this to both cables at both the pre and the amp side. I'm guessing since this happened with the new cables it's a cable problem. But it could also be a connector on the device that had the rca/xlr adapter.

Just saw the duplicate post.
if your preamp is old, than it may need service.
An intermittent connection in the XLR cables sounds the most plausible from everything presented so far as Xti16 suggested.
Thank you. I do not think it is a ground loop. I suspected an issue with the XLR cables. These are HiDiamond top of the line XLR's IC's and I have to see if there is anywhere in the USA I can have them looked at.

Very mysterious. In 40+ years of this mania I have never had this occur.

Neal
I've had it happen exactly that way a few times. Next time you have a problem, see if moving the cable at the input to the amp can make it come and go.
Thanks for suggestions. I will try Atmasphere's suggestion. In the meantime, some other pieces of evidence:

When I use the XLR cables directly from DAC outputs to amp XLR inputs, i.e., using the DAC as a preamp, there is no hum.

When I switch the cables completely from pre to amp, i.e., what was left to left now is right to right, and vice versa, the hum REMAINS ONLY in the right speaker. This leads me to believe the cables are not the problem.

Last, when I reverted to my RCA cables into RCA/XLR adaptors on the amp end, no hum whatsoever.

Does this help at all in solving the mystery?

Thanks again,

Neal
Were you using the same XLR cables with the DAC as you were with the preamp?
Atmasphere,

Yes, same cables. No hum. Thanks.

Neal
OK. Leave the cables connected at the preamplifier end.

Connect the other end of the cables to the amp. Do you get the hum?

If yes, swap the interconnect cables left for right **at the amplifier end only**. Did the hum move or stay put?
Thank you, Atma. I will try that. It is the one variation I have not yet tried.

Neal