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Ground loop hum

I recently purchased a VTL 2.5 pre-amp (with integral VTL phono stage) and a VTL-ST85 power amp. They sound gorgeous, except for the grounding hum which appears only when phono is selected on the pre-amp.

This hum is volume-dependent and not insignificant, and is present even with nothing connected to the phono input on the back of the pre-amp. It is not present when any other source is selected. I have tried lifting the phono ground wire, cheating the ground on all power cables (turntable, pre & power), powering each of the individual components from different circuits, and most recently, running a dedicated audio circuit using #8 stranded copper. None of these has helped.

Interestingly, the hum does not exist when my entire system (including interconnects and the AudioPrism power conditioner I use) is hooked up at the local audio shop. So the problem is definitely in my house. My panel is grounded via a water pipe. Any ideas or suggestions?
melodious_vibrationz

Showing 1 response by dan_in_san_jose

dan_in_san_jose
4 posts
 
I noticed an interesting statement you made:
This hum is volume-dependent and not insignificant, and is present even with nothing connected to the phono input on the back of the pre-amp.
This says that your turntable is not the problem (yet!), although the above advice is quite correct about ground loops in a phono stage and how *hard* they are to fix. You must be right about the hum in the house power. In my fog, I remember something about terminating the phono input with a suitable (50K? ohm) input load before testing for hum and noise. I don't know about your VTL 2.5 pre-amp. There are folks out there who build moving coil (MC) pre-amps for fun and they may have more details for you. Once the turntable is hooked up, the potential for new ground loops exists. Check out the post: impedance mismatched ?
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