Need help finding cause of hum

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