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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

Showing 3 responses by jea48

jea48
4,255 posts
 
Sbank,

http://www.hamuniverse.com/rfi.html
Read past the part, "don't blame me".
There might be something in the article that may be of some help.
jea48
4,255 posts
 
It is possible that the ground loop is there already but not audible with the two amps while when using the 845 with higher sensitivity on the inputs, the hum becomes more pronounce.
Abe,
I have really enjoyed reading your responses. Especially your last one about the sensitivity of the inputs of the Amps.

I am confused though why you use the term ground loop. If the battery preamp is just hanging out there in mid air with no connection to the AC grounded system of Sbank's home, (providing at some point in Sbank's testing he had the charger unplugged from the AC power), I am having trouble finding and following the current flow.

I understand when you talked about each power supply of each component, I understand that. And I understand even with the ground lifted, on equipment connected to an AC grounded system, a ground loop hum can still exist due to leakage of the power transformers of the equipment. The equipment still shares the common AC mains.

But if the battery powered preamp is just hanging out there in mid air only connected to one Amp how can a difference of potential exist between the two units?

Thanks,
Jim
jea48
4,255 posts
 
11-17-08: Atmasphere
There could also be a preamp issue that has not manifested before. If you can get your hands on a DVM, check and see if there is any continuity (less than 1 ohm) between the connectors and the chassis.

" Sometimes if the chassis is only grounded through the wall and the circuit isn't grounded to it you can run into trouble. That's not so bad in an amplifier but in a preamp it can result in hum, even if it worked fine with other amps!"


11-13-08: Sbank
Abe, according to the preamp designer, "All of the rca's on the preamp are isolated from the chassis and connected vis the 8 ga. buss. The chassis is grounded thru the charger jack only."

Atmasphere,
Is it possible the preamp is acting like a big antenna?

I still think Amandarae is on to something about the SET inputs being more sensitive than the other Amps Sbank was using.

Sbank, did you ever connect a wire from the chassis of the preamp to one of the amps chassis?

http://www.audioasylum.com/audio/general/messages/52/525608.html
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