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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 subaruguru

subaruguru
1,362 posts
 
I would've guessed a bad cap in the phono preamp's power supply that somehow got jiggled into function when you plopped the rig down at the dealer's....
If you're skilled I'd open her up and gently tap the caps and tubes with a chopstick (NOT A SCREWDRIVER OR ANYTHING METALLIC), and notice if you can elicit (or remove) the hum. I'm willing to bet it's not in your wiring, as your experimental groundig schemes should have eliminated it.
Good luck...and please don't yell at me, guys, for suggesting he go poking around in there. Chopsticks are ok.
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