PC to amp problem

I just tried to throw together a tempory system. It's a pc as source (and pre amp) to a quad 303 to kef drivers in homemade cabinets. When I touch the amp input cable to the case of the pc there's a high pitch humming from the speakers (and when I plug it into the socket on the pc this continues and there is no music coming through).
But when I connect another source to the amp (straight to an ipod) the sound is fine. And when I connect earphones to the pc all is fine.
So what could the problem be. Could it be an earth is needed, in which case how do i go about it?
I've also just tried putting it through another integrated amp. There is no noise with this, just sweet sound. But when I run the pre out from this amp into the quad the noise comes back and I get no other sound. What's going on?
Thanks for any help

 

sound like a nasty ground loop.....these are notoriously difficult and without beinging there its hard to offer much advise ...try feeding everything fron the same mains spur. Ah and your feeding straight to the power amp from the pc....spoted that on the second read have you the right cable ?

 

I used an extension lead with my PC (temporarily) that didn't have an earth wire, and you tell just by touching the case with a sensitive area of skin (no, not there, like the inside of your wrist). I would have thought an earth is essential.

 

Yeah both the PC, the hub (which they are both in the same one of) and the amp are all earthed. Someone has suggested Quad have funny earthing techniques and it'll never work. SHould I give up?

 

I had similar issues when using my quad 33 - 405 from the PC - I wasnt game enough to connect it straight. The fix was to connect a wire from my pc case to the case of my break out box (monitor audio delta 1010) then on to the quad 405 case and then onto the 33 case. This solved 99% of the noise.

 

right well I tried that. It does eliminate a lot of the noise, but not all. So I reckon I'll just find another amp, it's not worth much time and money. Thanks dreftar

 

I do believe that by using an isolating transformer on the interconect you should get rid of the noise but I havent tried that and like you and now use another amplifier (Pre Amp Tag McLaren PA10) and no issues with noise - my quad amps are in the second system in the bedroom.