2. amps-preamps
  3. 1226436662

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
 
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.
sbank's avatar
sbank OP
2,038 posts
 
"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."

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."

Sorry, if I don't understand, but does that make sense relating to your comment above? Does this further point to need for ISOMAX? Thanks,
Spencer
amandarae
512 posts
 
Hello Spencer,

I see. On normal usage, is the charger connected directly to the preamp all the time or you have to disconnect after charging?

If it is the latter, then it should be quiet because it uses the main amp ground, through the negative pins of the output RCA's as the ground point. This is akin to having a star ground connection that goes back to the ground of the main amp power cord ground. If it is the former, then I would like to ask you if the charger have a three prong or a two prong ac plug? Have you tried with or without the charger, if possible?

If after trying all those plus the ones you already tried, the hum still persist, the only think you need to try is to have a ground loop isolator between the amp and the preamp before resulting to troubleshooting the connections for hum loops inside the preamp. I believe the preamp is your problem.

To understand how the isolator works, you can read it here http://www.jensen-transformers.com/an/an004.pdf

I hope this helps.
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