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Help with Ground Loop Problem

4/19/2000 8:25 PM
Sean C.
Help with Ground Loop Problem
My current setup is guitar -> pedalboard -> Kendrick ABC box -> 3 amps. The pedalboard (powered by a Voodoo Labs Pedal Power) and the Kendrick ABC box are plugged into the same powerstrip. That strip is plugged into the powerstrip into which all 3 of the amps are plugged.  
Despite these attempts at eliminating ground loops, my amps still hum like crazy. They do not hum when I plug the pedalboard output directly into any of the amps (i.e., bypass the ABC box). The amps continue to hum even if I unplug the "B" and "C" outputs from the ABC box (so it is the only out). There is no hum if I plug the output from the ABC box into a CAE isolation transformer box, but there is some signal degredation, so I'd prefer to avoid that solution, if possible (plus the CAE box has only 2 channels).  
Any clues/ideas?  
4/19/2000 11:12 PM

for a quick and dirty test, put one of those 3-prong to 2-prong ac adapters on the plug to the ABC box (i'm assuming the kendrick box is powered/buffered, correct? if that does it you wil be ok, because the box will essentially ground thru the cable and yoru pedalboard or your amps. another fix would be to cut the ground wire from one end of the cable between the pedalboard and the kendick box, if the hum goes there ya have it... if not, then put a regular cord there and try the cut'ground cable between the kendrick box and one of the amps.. ALSO you never told us if you tried eliminating the pedalboard, going directly into the kendrick box then out to any one of your amps. Best is to find WHERE the ground loop is happening at before trying to fix it
4/24/2000 7:35 AM

"That strip is plugged into the powerstrip into which all 3 of the amps are plugged."  
Good God!!!! Is that safe? Kinda reminds me of people who use them 3 into one plug in adapters... Just not that safe. It sounds to me (I could be wrong) that you answered your own question right there. If you have all this equipment running to what is essentially 1 plug in, you've got more than 1 ground loop to deal with! first off, I'm assuming that all 3 of the amps have 3 prong cords, all going to these 2 power strips... GROUND LOOP!!! ABC box is buffered, right??? If not,... GROUND LOOP!!!! I'd start by doing something different for a power supply source. By the way you describe it, it's just an electrical (FIRE? SHORT? CIRCUIT OVERLOAD? DEAD BODY?) waiting to happen. I'll let someone else chime in on this one... -Jeremy
4/24/2000 4:40 PM

"By the way you describe it, it's just an electrical (FIRE? SHORT? CIRCUIT OVERLOAD? DEAD BODY?) waiting to happen. I'll let someone else chime in on this one"
ACTUALLY, if hte aggregatee current draw is less than 15A for everything total, the powerstrip woudl be fine. I've yet to see oen rate at less than 15A, and if you are plugging into a 15A house circuit, you'll still be ok. As for the ground loop, well that's a different thing altogether
4/24/2000 5:54 PM
You have a ground loop caused by the AC leakage(s) of one or more of the amps. Ordinary amps have NO common mode signal rejection, so any AC on the input, whether the desired signal, induced hum, or AC leakage, is amplified by the amp.  
You can do one of a couple of things.  
A quick and dirty fix is to open the shield braid wire at ONE end of all but one of the cables. This prevents induced hum from being picked up by the ground loop itself.  
You can use either a transformer isolator as you already know, or an active hum rejection circuit.  
You can also rewire the amps to provide a ground lift that separates the signal ground inside the amp from the AC safety ground and selectively lift all but on of the amps. This is DANGEROUS if not done properly, as one of the amps MUST be attached to safety ground to prevent possible electrocution (that's YOU we're talking about dying, by the way.)  
I've typed answers to this question so often that I'm going to put it into the Tube Amp FAQ.
4/25/2000 6:14 PM
Stan B
Could one seperate the circuit ground from the chassis/AC ground with back to back diodes? Or did I not interpet this correctly from an earlier post by Stephen Conner.  
4/25/2000 10:07 PM
One could, as long as the hum voltage is less than +/- 0.6V. Using a diode bridge rectifier will give you 1.4V, a little more headroom.  
The problem with this is that you really need to parallel this with a resistor to keep the signal and safety grounds DC tied. Low resistors cause more hum, and high resistors cause RF pickup problems. A compromise is a modest resistor - 10 to 100 ohms, and a big, high voltage cap, maybe 1uF/200V.  
It's worth a try, I guess.
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