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.
Try moving the amp to a different location. It could be induced by magnetic field nearby.
Sounds like you've investigated pretty thoroughly and logically. But I'm not sure what is meant by this statement:

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.

Does that mean that each cable remained connected to the speaker it was originally connected to, and you moved the left speaker and its cable to the right side of the room, and moved the right speaker and its cable to the left side of the room? Or does it mean that the cables remained on their original sides, but the speakers themselves were moved (with the cables being disconnected and then reconnected to the other speaker which was moved to that side)?

Also, I think that Sid's suggestion sounds well worth trying. And re your last question, I wouldn't worry about damage resulting from this hum. Whatever is causing it is producing the equivalent of a very low level signal, which would seem unlikely to be harmful in any way that I can envision.

-- Al
I would say if you haven't tried, see how the cables are oriented between the amp and the speaker. Any power supplys or AC cords near the right and not the left? Also have you checked the pre to amp cable routing? Another switch I would try is Pre to Amp reversale Lt for Rt and see if the hum switchs to the Lt channel. Also I will add that about 2 weeks ago I thought I have apower supply hum in my amp and determined that it was a wall wart powersupply for my Littlite TT lamp. You might check any of those plugged in to the same circuit. They are usually a pretty lower quaility power supply and can induce noise.
You've done a lot of good investigation without success. I think this dictates that you go back to the simplist way to solve the 'mystery hum' problem.

Disconnect everything. Unplug every thing. Then connect your speakers to you amp and plug the amp into the wall outlet. Any hum? No - then add one item, starting with the power conditioner. Listen. No hum - connect the pre-amp to the amp. No hum - plug the pre-amp in. No hum - turn it on. Etc thru all your stuff. When the hum reappears you've ID'd your problem.

PS Don't forget to turn the amp off between each change. It is easy to forget to do.
Thanks to everyone for your comments.

Yes, Sid. I was wondering about some weird magnetic field -- but affecting only the right channel? I think I'd have noticed this before as nothing in the house has changed and I've been in close proximity to both speakers as I've moved them around over the last few years. None the less, I will try later to see if moving the amp eliminates it.

Almarg - the latter...that is, I physically moved the left and right speakers but the cables that had fed them were not moved. I was initially worried I'd damaged a driver on the right speaker. Somewhat relieved that did not appear to be the case as the hum now also came from the speaker that had been on the left. Note too - before moving the speakers themselves, I had also switched connections at the back of the amp...connected the left cable/speaker to the right channel out on the amp and the right cable/speaker to the left channel out. When I did this, the hum moved to the left.

Theo - I did wonder about some possible interaction between the amp's MAC Sound Pipe power cord and right speaker cable. The power outlet for the amp is right behind the right speaker (but there is also an outlet behind the left speaker). I moved the power cord to the outlet on the left side of the equipment. The hum stayed with the right speaker and the left speaker remained quiet. It's an integrated amp so if I'm understanding your sugggestion correctly, I don't think I can change the pre-amp/amp wiring. I will take a look at the phono-preamp connection if that is what you intended, however. The power transformer for the phono-pre is on the right side of the equipment near that right speaker...maybe it has been inadvertently moved and gotten a little too close to the speaker. I will check it out.

Newbee - Good suggestion. I will try this. I did shut down and unplug the power conditioner (though I did not disconnect the inputs to the amp from phono-pre, TT & CDP when this was done) - the hum was still there.

Thanks to all once again for taking the time to reply.
Ghosthouse -- thanks for the clarification. You've gotten some good suggestions to pursue. I'll just add a couple of thoughts:

1)If the problem is caused by magnetic field induction, I doubt that it would be induction into the speaker cables, due to the low impedances which are present there (especially the amp output impedance, as well as the speaker load impedance).

It could, however, conceivably be magnetic field interaction directly into internal amp circuit points. It's probably worth trying to move the amp, even though it appears that nothing had changed in the house when the problem arose.

2) While you are pursuing Newbee's good suggestion, keep in mind that a very common cause of hum, assuming your interconnects are unbalanced rca's, results from the fact that the shields of those interconnects serve two functions. They serve as the return path for signal currents flowing between the connected components, and they also tie together the chassis of the connected components, which in turn are connected to ac safety ground (if the components have 3-prong plugs). AC safety ground, in turn, is connected to ac neutral back at the service panel in the house. What that all means is that if the ac safety ground potentials (voltages) to the two components are slightly different, unwanted currents will flow through the interconnect shields as a result (at the 60 Hz power line frequency and its harmonics), which the destination component will have no way of distinguishing from desired signal, thus resulting in hum.

The ways of minimizing that effect, if it is the cause of the problem, are to use interconnects with the lowest possible shield resistance (meaning they should be both short and high quality), and by keeping the safety grounds as common as possible (meaning don't have the power plugs isolated from each other by the power conditioner or by being plugged into ac outlets that are separated by significant wiring). You could also try isolating all of the safety grounds (other than the one for the amp, which should remain connected for safety purposes) using cheater plugs, or 3-prong to 2-prong adapters with the safety ground connection not connected.

There are advantages, as you realize, to having some degree of isolation between the ac to the amp and the ac to low-level components, and between digital and analog components, but that trades off against this increased possibility of hum. Trial-and-error is commonly the only way to optimize this trade-off.

Hope that helps,
-- Al
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.
Ghosthouse, is the preamp selector on phono when this hum occurs. If so check the wires on your cartridge are seated all the way on cartridge prongs. You may also want to try cheater plugs. Good luck...Bill
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
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?
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.

-- Al
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,


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.

Hi Richard,

Hope you have been well. Yep, I agree on the grounding. My ground is usually through the amps as well. Adding this second amp has turned out to be a chore. The new one even sounds different so I have to get in there at the coupling caps anyway. I'll give all of the caps a check.

Al, Thanks. I will look into shorting plugs.

Dan, Hope you figure out what is going on with your system. In my case, the hum is there with the input on amp switched to CDP. I also hear it with the input at position 2 which is not occupied by any source. Finally, I can hear it with all input cables to the amp disconnected. For me, fortunately, it isn't an obstacle to listening to music. Just something that didn't seem right and which I hoped to correct, if possible. Not so easy, perhaps!