2. amps-preamps
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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
sbank
jea48
2,944 posts
 
FWIW, I demoed the amps at a local dealer, and there was no problem in their system.
11-11-08: Sbank
==============================

11-12-08: Ig316b
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.

Not really..... For a true test he would have to pack up and take his entire system to the alternate test location.
As Sbank said, at the local dealer's the Amps worked fine.

In my last post I asked Sbank if the Amps hummed if they were only connected to the speakers. The inputs of the amps open, without shorting plugs installed.

If this is the case then it is possible the transmission tower/s could be causing the problem.

Or maybe some interaction between the Amps, speaker wire, or speakers.
===================

Another question,
If just one power amp is connected to the preamp does the amp have the problem hum?
Jea48

Sbank I forgot to add do not have anything connected to the inputs of the preamp. What you need to do is break down, or isolate, the problem.


sbank's avatar
sbank OP
2,038 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
jea48
2,944 posts
 
With no interconnects or shorting plugs, there is no problem.
You have eliminated that segment of the system as being the problem.
===================

When I have just one amp(either one) connected to the pre, it still hums.
Did you have all the source equipment disconnected, ics, from the inputs of the preamp?

If yes, you have ruled out a ground loop hum problem......
===========================

If you want to try something else.
How long are the ics that connect the preamp to the Amps?
From your pics they appear long.

For a test move the preamp over to the Amps location. Use a shorter ic to connect to just one Amp at a time. Again no sources connected to the preamp inputs.
Still hum? Try another preamp.....

If no hum, connect both amps to the preamp.
Still no hum? Connect a source to the input of the preamp.
Still no hum?
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
jb0194
476 posts
 
Nope, the cheapo stuff on Ebay, etc. has nothing in common with the Isomax. Again, no guarantees, but the Jensen's a very high quality item.
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