Well, you are not alone. I finally got the chance tonight to hook up my new 990/7500 combo and I have spent the last three hours trying everything to get rid of the hum. It is equally as bad as you described.

I just tried the same plug adapter solution you used and the hum is gone, but I am pretty certain this is not a good fix for either of us. On the other hand, I least i can go to sleep now.
Ed

Quote:
Originally posted by dgilley:
Hello,

I just hooked up my new 7700 today and I am so disappointed! Major hum. Present all the time and audible over low volume parts of music. Really bad - definitely not tolerable. While the amp sounds great, I can't take the hum so it must be solved or I'll have to return the 7700 and go back to my Denon 4306's built-in amp which had ZERO hum. Of course, the Denon had no ground prong on its plug! Maybe their solution to the hum problem??

Things I've tried:
- disconnected all cable, phone and network connections to the system. didn't fix it. I take it when you disconnected your cable system it went away, Chi-town? Doesn't seem like my hum is coming from the cable system if its still present even when disconnected from cable.

- disconnected RCA signal inputs jacks to the amp and the hum went away (this was the only zero hum configuration with the amp powered on)

- disconnected all other devices from the power outlet except the amp but left the RCA jacks connected between the preamp and amp. The hum was there!!! How are these other devices causing a ground loop without being connected to ground!?!

Note that this is a dedicated 20A circuit hence there is NOTHING else on this circuit except the amp. And I still get hum when the RCA jacks are connected.

Another interesting observation is that the amp itself hums when the speakers hum. Its like the whole thing is resonating at the same frequency as the speaker hum.

Also, when I put a 3-to-2 prong power adapter plug (thereby disconnecting the ground) the hum is gone. Is this actually unsafe? Is this a bad long term fix? If so, why? The Denon doesn't even have a ground pin.

I'm so sad and worried I'll have to return this beautiful beast. And I fear having to lift it back into the box!

Help. Please. Anyone.

Thanks
-Dan