Tsulzbacher, I had same issue with my 7701. In fact, I went through three brand new units and calling tech support as well talking to my retailer. The tech support was not helpful, and said basically they had no idea. I replaced the 7701 with the 8801 and no hum, call support and asked them why, they responded with, the 8801 is a better unit. I never got a clear cut answer, but the 8801 worked fine, and tried the Yamaha CXA5000, and no issues, so stuck with that. My buddy has the 7701 mated with a Marantz amp, no issues. I'm thinking its compatibility issue with the 7701. My amp is a Mac MC205.
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I'm using the 8801 with W4S and Bel Canto power amps.
I don't encounter a hum.
One of the main causes of the hum is the ground loop problem. Do you have a cable box attached to it? If you do, try disconnecting the cable box (power/HDMI/coax from cable etc) and see if the hum persists.
If it is still there, try unplugging all other inputs on the AVR to see if you can isolate the problematic source.
I've had the same issue with my 7701, i actually went through 3 brand new units all with same hum. I replaced it with the 8801 with no issues. Called tech support asked them why, and they just said the 8801 is a better unit. My buddy has the 7701 mated with the MM7055 and no issues at all. I believe its a compatibility issue with some of these. I checked all for ground loop and everything came back to the 7701. Called support for all my components and they believe it was the 7701. I went with the Yamaha CXA5000 at the end, cheaper than the 8801 and didnt lose sound quality. For power i have the Mac MC205.
Here are a couple of choices for the ground loop isolator you need.
They usually cost $25 and up, so the one at MCM is definitely cost effective, and the reviews are good.
I am running my AV 7701 with a Parasound 1206 and experiencing the same Hum/Buzz that others are talking about. My friend bought a 7701 and connected it to an older Adcom amp and also experiencing terrible hum worse than mine. I bought a Theta Intrepid and am going to test it with that Amp and will report back. I am wondering if all 7701's have this issue if you do not use the matching Marantz amp? This is bad news.
I contacted Marantz USA Support today and explained the hum situation. I informed the rep about the various online forums where numerous consumers are experiencing this same issue. He casually said that "many pre-pros have hum". He recommended that I buy a device called a "Hum- X". I said "so basically if consumers don't buy your matching amps, they will experience hum?". This seems to be the case. "So now I have to spend money to buy an ancillary 3rd party filter to fix what seems to be a Marantz design flaw?"
Well, this also seems to be the case. I ordered a hum-x via Amazon Prime for $59.00. What I like is that it installs inline with my Power Cord instead of being in the signal path via XLR's or RCA's. I will let everyone know in 3 days if the Hum-X cures the hum. This is disappointing and Marantz should fix the issue but if a $60 item will make the issue go away, this is still currently one of the best pre-pros for the money for sure.