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Need help finding cause of hum

I'd appreciate input on something I just noticed. The right channel of my system is producing a very soft, low level (low db) hum right on power up and with the volume all the way down. It's only noticeable with your ear an inch from the midrange/bass driver. It doesn't change level if the volume is increased. It is not audible when playing music. It has done this with 2 different amps (1 brand new). It does this whether or not input sources are connected. It does this whether the ground between TT and phono-pre is connected or not. When I switched the left speaker/cable to the right amp output and right speaker/cable to the left output - the hum moved to the left. So I thought, hmmm, problem with amp. But it is now doing this with a brand new amp. When I reversed speaker placement (Right to Left & Left to Right) - cables staying in place - the hum stayed on the right coming out of what had been the left channel speaker. The hum is only heard on the right. The left is totally silent. I usually run the amp plugged into one wall outlet and other gear out of an APC H10 power conditioner plugged into a different outlet on that wall. I tried plugging the amp into the amp power outlet on the back of the APC but the hum remained. I tried plugging both pieces (H10 & amp) into the same wall outlet. The hum remained. I thought this might be some type of ground loop hum but from what I've read, it doesn't seem consistent. It is only out of the right channel and barely audible. It is more a curiosity than annoyance at this point but I am concerned about the possibility of some sort of induced, stray current causing damage. Any ideas you might provide will be very much appreciated.
128x128ghosthouse
almarg's avatar
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almarg
9,670 posts
 
Al - Thanks for your reply. A lot to digest, but I'm thinking a ground loop isn't the cause. Wouldn't that cause hum in both channels? and at a higher volume? ALSO - I can hear the hum with only amp and speakers connected - no sources connected to the amp. Thanks again.

Both channels -- yes most likely. Higher volume -- not necessarily; it could be just about any level depending on the ground voltage differentials, impedance levels, etc.

Also, when you disconnect all the inputs it would be a good idea to put a pair of shorting plugs on the inputs. Otherwise your results may be confused by hum pickup from stray electromagnetic fields, to which the amp would be susceptible when the inputs are open and hence high impedance.

Basically, at this point I'd suggest disconnecting (and unplugging from the ac) everything but the amp and speakers, putting shorting plugs on the amp inputs, and plugging the amp directly into the wall, without the power conditioner. If you still hear the same symptoms, try moving the amp around physically. If the symptoms still remain the same, I would say that pretty much rules out everything other than the amp itself, even though the previous amp appeared to do the same thing.

And if that's how it turns out, I'd just leave it alone. As I said earlier, I can't envision any possibility of damage due to such a low level hum.

Hope that helps,
-- Al
dan_ed
2,451 posts
 
Almarg and Ghosthouse,

I may be hunting a similar issue, at least the descriptions sound similar to me.

I have an amp that has developed a hum in one channel only. It is this channel because I have isolated it by disconnecting all outputs and inputs. The hum is there as soon as the amp finishes the tube warm up phase and the outputs come on line. I am now running biamped and this hum is greatly amplified when I turn on the other amp. Sounds like 60Hz is bleeding back somehow.

I added a ground lift plug to the pc on the humming amp and that pretty much eliminates the hum with both amps on. The pre ground is already lifted to play nice with my phono stage. So good, so far.

I also run a pair of plate amps to power my bass horns. When I add the plate amp to mix on the same channel as the amp hum, I get audible 60Hz hum again. Lifting the ground on the plate amp does not solve the issue.

I also measured the open circuit hum at the speaker terminals, and with open circuit at the amp inputs.

Hummer: 1.2 volts left channel (grid voltage is about 10mV higher on this channel also)
.9 volts right channel

good amp: .6 volts right and left

Any ideas on how to further track this down?
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almarg
9,670 posts
 
Hi Dan -- That sounds like a tough one. If I understand correctly, the problem developed after a long period of use of exactly the same setup without any hum. Given that, and given the rest of your description, the one thing that comes to mind would be that the amp has developed a leakage path between ac neutral and chassis, within either a power transformer or a capacitor.

Not sure why only one channel would be affected, though, even if that theory is correct. But it seems to fit a lot of the other facts. Turning on the other amp would worsen the problem due to its current draw increasing the offset between ac neutral and ac safety ground (which presumably is tied to chassis). Lifting the ground on the humming amp would then let its chassis float to or toward the ac neutral potential.

Also, my understanding is that in general it is not a good idea to run a tube amp without a speaker load, which it sounds like you have done briefly while investigating the problem. It can result in higher than normal internal voltages, possibly over-stressing capacitors and other components.

Not sure what else to suggest.

Regards,
-- Al
dan_ed
2,451 posts
 
Thanks Al, you're circuit skills and experience far exceed mine.

I think the hum has been there for a while. I tried just that offending amp alone. The hum is very faint, but still there. I've only introduced the second tube amp in the last week, so the warts are coming to the surface, so to speak.

The amp is ok for a while with no loads, which I have to do to bias the tubes. Open circuit inputs, however, could really be asking for trouble. ;-)

This is an older amp, no longer in production, so I hope it is a cap and not iron. Caps are easy, replacing some iron could really be an issue.

Thanks again,

DAn
palasr
527 posts
 
Dan,

Most likely a power supply cap, especially given that its onset has been slow and insidious. Time to open the hood and check for bulges and leaks; I agree with Al on this and am uncertain why it would only affect one channel.

FWIW, I'm a strong adherent to grounding a system through one point only. Currently, that one point is via my power amp, though optimally the preamp (being the central nexus as it were) would be the best choice. Since I have no active device at the preamp position, and the phono stage floats, the power amp it is. Once ground potentials begin to compete, all hell can break loose.

-Richard
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