The symptom: Intermittent humming / buzzing coming from my Outlaw 7700. This is at the amp itself and not a ground loop hum, and no humming at the speakers themselves. Sometimes, the 7700 is dead silent. Other times it's incredibly annoying and can get loud. How loud? Try audible at 6-feet away. There didn't seem to be any pattern to the noise.

Reading through various forums, it sounded like DC in the power line. I do have florescent lights in the house, along with motors (refrigerator, forced air heating, bathroom fan, etc.) but even with those on, sometimes there would be humming, and other times it would be silent. Even when I had everything off in the house, it would still hum.

So, I tried out a used PS Audio UPC-200HB. They are extremely hard to find because (1) PS Audio has stopped making them, and (2) because when they show up on Audiogon, they are snatched up almost instantaneously. I was lucky enough to snatch one. I guess I could have tried a CIAudio XDC-2 which costs $299, but I wanted to get the power conditioning of the UPC-200HB. Also, unlike the XDC-2, the UPC-200HB is not current limiting. I guess I could have also tried a homemade DC filter, but since I don't know how to work with electricity very well, I figured it wasn't worth a fire risk.

How does it work? Absolutely kick a$$. The amp is now dead silent. Even more interesting is the fact it somehow eliminated my ground loop hum even though it shouldn't have. The source of the ground loop hum was through my satellite line, and I had been using a cheater plug. One of those cable line filter devices will not work on a satellite line. So when I hooked up the UPC-200HB, I decided to try it without the cheater plug just for the hell of it and sure enough, ground loop hum is gone!

Bottom line is... I LOVE THE UPC-200HB! It has made my 7700 much more enjoyable. It's too bad this model, along with the standalone Humbuster, have been discontinued. Perhaps it's to encourage the sale of PS Audio Power Plants? According to Edward, Outlaw will soon be selling a DC / RFI filter device. Perhaps to address the humming issue that appears to show up with some 7500 / 7700's?

Good to hear that the hum is now gone. I've had the 7125 and 990 for almost 2 years, but with no hum or noise. Now I'm expecting the 7500 next week. I'm going to use a MIT Z-Cord2 power cable and Z-Stabilize III strip with the 7500 and 990 to make use the power is clean.