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Re: Help with Ground Loop Problem

4/26/2000 3:11 PM
Stephen Conner
Re: Help with Ground Loop Problem
Hi R.G.  
I got the idea by looking at the schematic of a Peavey PA amp. They had a 10 ohm resistor and a 100nF ceramic disc in parallel with the diodes. But since when did Peavey make anything that works ;) Most hi-fi amps have the same circuit minus the diodes.  
Mark: Before you start lifting grounds you really need to understand exactly what is causing your ground loop and why it is happening. Then you will be able to cure it with the minimum number of tweaks while keeping yourself safe from electrocution. Essentially, each of your pieces of audio equipment should be grounded at one point only, unless it has internal ground lifting already, in which case it should have one audio and one protective ground.  
Old-style amps with 2-prong leads and grounding switches will inject crap from the neutral line into your audio ground system and should be upgraded to real grounding, perhaps adding the 2 diodes/cap/resistor ground lift arrangement while you're at it.  
The 'OEO' practice RG recommends (attaching signal cable shields at One End Only) can be quite effective, but it causes RF pickup on long cable runs.  
Steve C.
And now, a word from our sponsors:

5/1/2000 10:50 PM
Stephen Conner

Hey R.G.  
"I've typed answers to this question so often that I'm going to put it into the Tube Amp FAQ."
I just remembered that I wrote a web page on this subject. It's not specifically aimed at guitars and guitar amps but it might save you the hassle.  
Steve C.
5/2/2000 2:01 PM

I did not hear a definitive answer here.  
I have the same problem and use the Kendrick as well. What I ended up doing is taking a power bar, opening it up, and cutting off the ground for all but the first position (nearest the source, of course). Then I plug one amp into that position, and the other amps, the DC power and the Kendrick into the other positions. That has eliminated the ground loop problem, but I continue to be quite terrified of forgetting to make sure everything is connected, before turning any power on, including the power bar itself. I only use this bar when I use the ABC box.  
I don't understand the dirty fix of opening the shielded wire at one end. Which cable to you leave the shielded wire connected to? From the grounded amp to the ABC box? from the guitar to the ABC box?  

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