Speaker hum problem - need help

Very interesting.
Thank you for your willingness to report your findings back to the forum.
I wish everyone would have such good follow through.

You said that you sent the amp back to the factory and they couldn't find anything wrong with it. Assuming the amp is OK, I would try 2 things. The first would be to connect a different pair of speakers just to rule them out as the problem. If the hum is still there, try the amp in a different location. Maybe in a friends system. That will tell you a lot about what is causing the hum.

One thing you really need to do before making any decisions or spend money is to hear the amp working without the hum. If not, you can't rule it out. The thing you say about the AC being the problem that gives me pause is that you tried different power products and there was no change whatsoever. If your AC is at fault, I would expect you to hear something if you tried different solutions, like a line conditioner. Even if it didn't fix the problem, I would expect to hear some difference; whether it be better, different or worse. I could be wrong, of course, and maybe the AC is the problem, but I would need to hear the amp working properly before I tried fixing anything else.

Try another amp/preamp in your house. It may have really crappy AC outlets with wires just plugged in and old ones can make intermittent contact. Wiggle the plug in the wall and see if the hum changes. Usually a hum equals a ground loop. This is a mystery. Take your Coda to a friends house. If it (coda)works there, and something different in your house doesn't work, your wiring/outlets is suspect. If the Coda hums at a friends, send it to Coda again.

Well, I wasn't able to get to RS today (did a nearly 4 hour bike ride in 85 degree heat.....kinda beat), but, I was able to do the extension cord test. Used a (probably) 14 gauge 25 foot Home Depot type extension cord. Tried it in two separate outlets: one across the living room from the outlets I've mentioned, one down the hall. NO CHANGE!

I then tried changing the inputs on the amp (not that I have anything plugged in aside from my XLR and speaker cables), and ran through them all, with each remote outlet (with extension cord attached), and then again with amp plugged into the power strip I've been using. NO CHANGE on any input.

You know.....despite what they said at the factory, I am beginning to wonder about this amp. But, the argument against that was the experience with the previous (tube) amp, with different speakers and cabling. Same hum, except variable based on volume of amp.

I tried one other experiment.....with the amp plugged into it's usual power strip, I tried some other speakers I have in the house....a set of Monitor Audio Silver 7's. Believe it or not.....still heard the same hum!!

I live in a semi-rural neighborhood in the SF Bay Area. There's low density housing and no industry locally. You'd think that would create for some pretty high quality electrical sources? Sounds like I need to buy that outlet checker.

I have contacted the salesperson from the shop at which I purchased the DAC....he said he'd be able to come by this week and try to sort out the situation.

Any other ideas aside from bringing my amp to another location (I could probably do that with the shop at which I bought the DAC)??

This situation is really perplexing and annoying....

Matt

Been doing a bit more reading elsewhere......would an "isolation transformer" help in this situation? Is it possible to install one immediately after the wall outlet (i.e., can I plug my power strip into the transformer)? Am I shooting from the hip?