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Ground Loop Issue 427

After talking with the manufacturer's of both the amps & preamp, I still have a 60hz hum. Here's the story:

Just changed amps to 845 SET monos, and now I hear the hum whenever the amps are on and RCA interconnects are connected to the amp inputs. Didn't have the problem with two prior pairs of amps. The pre could be turned off, and I still hear the hum. With shorting plugs in the amps, no hum.

The pre is battery powered, with no ac cable, and the trouble persists whether or not any sources are connected to the pre.

I've tried multiple types of interconnect, including the heavily-sheilded cheapos from a vcr, but no change.

One friend questions if adding a "hum potentiometer" to the amps would make sense. Others have suggested the $600 Granite Ground Zero, which is unaffordable for me.

I've already tried cheater plugs any/everywhere. I've added a grounding wire between the monoblocks metal bottom plate and floating one power cable while leaving the other grounded, all per the amp manuf's suggestion. I've tried HighWire LiveWires, which might be good sonically for RFI, but aren't helping the hum issue. This is a music only rig, no cable tv anywhere in the room. I am in a heavy RFI area, 1000ft from a radio tower, if that matters...

Anybody got any suggestions other than moving elsewhere? Thanks,

Spencer
128x128sbank
ig316b
104 posts
 
SBank
If you can take the amps to a friends house that's nowhere near the radio tower and plug them in. This will eliminate the radio tower from the equation.
Post removed 
sbank's avatar
sbank OP
3,379 posts
 
Okay here is what I've tried:

Plugging the amps in next room via extension cords, didn't change anything.

I did try ground lifters on both amps, as well as on only one; no difference

When I have just one amp(either one) connected to the pre, it still hums. Strangely, with the right amp on, removing left interconnect from left amp increases the hum heard in the right speaker slightly. Conversely, with the left amp on, removing right interconnect from right amp decreases the hum heard in the left speaker. With no interconnects or shorting plugs, there is no problem.

Isomax is a possibility. Would the cheapo similar products(e.g. $12 ground loop eliminators on ebay) give me an idea if Isomax would be effective?

As far as trying amps & pre at a friends' house away from radio tower, that might be worthwhile. Borrowing a different preamp and trying that here will be worth trying too. Thanks to all! Cheers,
Spencer
Post removed 
amandarae
512 posts
 
Hello,

When you connect the shielded cable, is the cable shield connected on both ends? Have you tried shielded cables where the shield is only connected at the amp side? preamp side?

I think that what you are experiencing is common to SET amp where the input ground is directly connected to the chassis as soon as it enters the amp. If your preamp is not configured the same, there is always a loop everytime you connect the cable. In this case, the only solution, if you do not want to re-configure your preamp, is to have a Ground Loop Isolator because it seems that the ground plane of your "battery powered" preamp is of different potential as "seen" by the amp. You can also try connecting a wire to a ground point in the preamp chassis in series with a 10 ohm resistor (two watts rating or more)and see if by connecting it to the amp grounding point, the hum will be less or there is a change in magnitude. If that is the case, then you can confirm that the ground potential on both components are indeed not the same. In any case, you might want to try a ground loop Isolator.

Hope this makes sense.

regards,

Abe
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