Well, my hum/buzz is apparently not a ground loop hum. I installed a cheater plug yesterday after the hum/buzz came back, and it's still there. Must be the standing DC or there's some other issue with the amp.

Sorry the cheater plug didn't work.

Yeah, it seems to come and go. Used it all day yesterday and had a bit of buzz after it was on for about a half hour but then it went away. Hard to figure. Don't know if it's worth spending a couple hundred bucks for a line filter. I may just put up with the occasional buzz.

Blue Jeans has a Calrad transformer for 25 bucks. I've never used one, but it may be worth a try.

Could the noise be mechanical? Loose screw or a vibrating cooling fin? Use a mechanics stethoscope and list directly to the amp. Does the noise come from a specific part of the amp? Touch various areas to see if you can make it stop.

Maybe someone could check with Furman to see if any of their products could help this situation.

http://www.furmansound.com

Just thought I would share, might help someone?

I own a 7500. Recently I replaced my 970 with an Onkyo 807 for pre/amp duties. In doing so I used the same 3 pair of Blue Jeans analog cables to power the 5 speakers. After 4 weeks in the set up I noticed a very audible hum (ground loop) going through all 5 speakers. I tried to diagnose with the amp in the system and couldn't get it to stop (checking all cables for solid connections, etc). I never figured it out so I pulled the amp from the system about a month ago and the hum went away.

Only last night did I get some time to play with it some more. To troubleshoot, I tested all of the channels of the 7500 using my Outlaw RR2150 as the preamp and everything worked great, so I decided to hook it back into the 807 fearing that I had a possible faulty pre/out on the Onkyo. As I was connecting all of the connections I figured out the problem even before testing it - the extra analog cable (3pair but only using 5 channels - 1 extra cable hanging loose). The unused cable had to have been crossing another connection somewhere causing the issue - so I split the pair and no longer have the extra cable back there.

One stray cable, simple fix, problem gone.

Last May I purchased a 7500 to replace some older amps that I had built myself. Don't get me wrong these amps I built were awesome, just that they were too big to fit in the newly acquired equipment cabinet. In the process of doing the upgrade, I decided to replace the old audio cables. I acquired these cables from a major midwest distributer and spent a fair sum of money on them. When the 7500 arrived I hooked it up to the 990 and I had a constant low level hum from the speakers. If I muted the 990, the hum went away. I called Scott after trying to fix this for some time with no luck. He said that he would be willing to help me out, but I wasn't ready to do that yet and decided to try a number of things. Since I work in Seattle, this was limited to the occasional weekend that I was home and had no other chores to do. Long story short, I replaced the audio cables with Blue Jean's cables, and the hum went away. Out of curiosity, I did an autopsy on these other cables I purchased and found that they had a red wire connected to the center connector and a white wire to the common. The shielding was not connected at all. Go figure!