Melos triode and computer as source: BUZZ BUZZ BUZZ

[zigo3]

Hello
I installed a Melos triode 200 plus stereo power amp in my system and I'm using, for the moment, a NAD 3020i as preamp. No problem if the source is tuner, phono, cdp integrated or dac (mhdtlab Constantine usb plus), but when I connect the DAC to the PC, via USB or spdif is the same, begin a BUZZ. And the buzz remain if I select different sources from the pre. It stops only if I disconnetc the cable to the pc.
And this problem born now with Melos, before, using different SS amps, in different classes, never had problems. And the buzz remain changing different passive loudspeakers.
Please what could be the problem? What can I do to resolve it?

Thankyou very much in advance

 

[dsavitsk]

You have a ground loop. Easiest solutions are a) use a digital transformer on the spdif connection or b) use toslink. Both of these will isolate the computer's ground.

 

[zigo3]

Thankyou very much dsavitsk, I will try it.
I made some test and the BUZZ continue even if I switch off the computer: maybe could be a cable interference too (there is a lot of cable very close)? I will try to isolate each cable too.

 

[dsavitsk]

The computer is still grounded, and probably partially powered, even when off. Try unplugging it.

 

[zigo3]

The computer is still grounded, and probably partially powered, even when off. Try unplugging it.

I unplugged it but the buzz is still there. Maybe because there are other devices self alimentated plugged to it. When I unplug everything the buzz stop. But sometimes it stops when I unplug one device, sometimes when I unplug another: seems that there is a kind of casuality. For sure the buzz is there when all are plugged.

The best result I obtained, for the moment, is using a shielded loudspeaker cable like this http://www.tnt-audio.com/clinica/star.html: I measured the buzz before with the RS meter and was 67db, with this cable it was no more a buzz, but a kind of "breath" and is 56 db. Do you know why? What kind of interference can generate this?