Slight hum from left speaker -- Help appreciated

I'm assuming that when you say you "changed right and left amp-preamp interconnects (no change)," you mean you changed them only at the amp end, and that the hum stayed on the left. If so, then you've got the problem narrowed down to two suspects: the amp and the left speaker cable. To eliminate the cable as a suspect, switch the cables at both the speaker and amp ends. If the hum is still on the left, then the problem is the amp. If it moves to the right, then you've got a bad cable.

I tried it all. I floated the ground, exchanged the right and left speaker wires (the hum moved to the right speaker), changed right and left amp-preamp interconnects (no change), and moved wires and interconnects around. Nothing solved the problem. My preamp is passive so I'm only turning the amp on for this testing. The only thing that was proven was that the left speaker wire seems to tbe the trouble. Whichever speaker it was connected to was humming. Is this an amp problem? I am using the RCA inputs. The amp also contains balanced inputs and there are "short jumpers" between pin 1 and 3 on each channel. I checked them, but they seem firmly placed. The hum is very slight. Thanks!

Your plug has three prongs, the positive, the negative, both of which are flat, and the ground which is round. You will in essence be disconnecting the round ground prong. To do this go to you hardware store and get a "cheater plug" this is a plug which has three slots for your plug but only has the two flat prongs - these you plug into your outlet. Do not connect the ground wire on the cheater plug to anything.

Uh, excuse my ignorance, but how does one "float the ground"? (Thanks for the useful tips!)

easy and fastest is to float the ground on the new edition components as mentioned by Newbee. kurt

First thing to do is figure out where the hum originates. To do this, you have to switch channels, first on your speaker cables, then working your way up the chain of interconnects. Connect your left-hand cable into your right speaker, and vice versa. If the hum stays on the left, the problem is that speaker. If not, revert the cables and switch the I/Cs between amp and preamp. Again, if the hum stays left, the problem is in the amp (or the left speaker cable, but you said you switched those out already). Keep trying this, and you will at least know what piece is giving you a problem. If you start from the speakers and work backward, the problem will be in the component or wire immediately downstream from whatever wire change DOESN'T cause the hum to shift to the right.

which component is causing your hum? Just reverse interconnects, one at a time, starting at the source, and when the hum moves to the other speaker you have found the source. Then see if the interconnects are near a power source. You can also try to float the ground on that component. you could have hum induced by the location of a nearby component (i.e. amp). Move your equipment and see what happens....