McIntosh MC205 hum

I have new MC205 and I noticed humm in channel 2,4,5. The humm was present only when unbalanced RCA cables was connected to AV receiver MX121. I disconnected cables one by one and the humm disappeared from the channel when the corresponding cable was disconnected. I sent the Amp to RMA. They said there was cold joint on ground and repaired it. Now the humm is present only in right channel (no. 5). It is ground loop. But why only 5th channel is humming? I call technicians from company that sold me the Amp to my home. They bring another MC205, the humm was present only in channel 5 too, but a little lower. So I take a ohm meter and measure resistance from Mains PE(protective earth) to ground of RCA inputs. All channels have about 3ohms, only channel 5 has less than 0.5ohms. So I think there is design problem, and the channel 5 is grounded directly to Amp ground, not through the inverting input of amplifier. So I will use Balanced cables and will hope it will solve my problem. Can somebody measure resistance on the amp? Or have somebody disassembled the amp to check this problem?

I can confirm, my 3yr old MC205 emits this hum as well. I just recently noticed it as I got a newer amp and moved the MC205 in a cabinet. The new placement excaberated the hum. I don't mind it as it's in a closed cabinet and not really audible on any type of decent listening anyways.

Seems like this is inherent in the design of these amps.

My MC-126 used to hum though the speakers also. It appears the non-autoformer models are all prone to some sort of hum.

Well, the hum is definitely not thru the speakers. Its in the amp. I kinda figured the hum would be explained away by the manufacturer that it is due to the enormous power supply. (Thanks Theo) Don't you think for $8000 it should be quiet too? (I paid $6K for mine before the price went up.)

I have no cable TV. Too cheap. I mainly use my system for listening to music, DVDs and vinyl.

I was shopping at my dealer with my buddy yesterday (he wants a system too) so I sneaked away to the room where they keep their McIntosh amps. Lo and behold, their MC205 did it too. A salesperson was wondering why I had my ear pressed against their amp.

So Theo, what is a sorbethane footing and where do I get one? Aren't there air vents underneath the amp and would it interfere with venting and cooling?

Thanks all.

I just bought a MC205 as well and replaced an MC7205 which had the same issue. My hum was in the speakers and presented itself as a buzz more than a hum. I do have a slight hum at the amp and that according to McIntosh that hum at the amp is the enormous power supply. OK OK so why does my MC402 not have that hum? The buzz at the speakers I solved with a PC Audio power cord and removed the ground plug. Someday when I have time and patience I want to track down the source of the hum as it may be an interaction with phone, cd or something. I think I can eliminate the hum at the amp (power supply hum) by adding a sorbethane footing under that amp I just haven't done that yet. I will add the 7205 did not hum at the amp except at turn on which all my Mc amps have doen once they reach full warm up (about 1-2 seconds) it goes away.

If the sound is from the amp itself and not through the speakers, its likely a transformer vibration which I call buzz. Try telling your dealer its a transformer buzz and ask them to tighten the hold down bolts. These are sometimes difficult to prevent, but if you bought it from a retail dealer he should help you out.

If the sound is through the speakers it may well be a grounding issue, but since you say you have an HT processor, it may be your cable TV. Try disconnecting the cable co-ax and see if it goes away. If it does, try a ground isolation transformer which can be purchased at radio shack.