A few questions about Ground Loop hum..

D

Derek McCarthy

Active Member
Hello, wondering if I could get some input on something. I've been running two guitar amps in stereo - a Mesa Mark V and a Marshall DSL100, and I also send some pedals to a Bass amp to get those lower frequencies. As I'm in a 2 piece band - (just me and a drummer) I like to have a big guitar sound.

The Marshall is connected on Output A, and the Mark V on Output B. I've been lucky that the Marshall did not have any issues with ground loop hum. The Bass amp did have hum so I used an old ABY pedal with ground lift on it as a passthrough to resolve the problem.

I just replaced the Marshall with a Mesa Triple Crown and now have ground loop hum on this amp. I'm curious.. why would one amp generate ground loop hum and not another when all other variables remain the same?

I'm also hoping someone can help me understand the difference between an isolation transformer and a ground lift and why one would be better than the other? Or is ground lift something that requires an isolation transformer? I noticed isolation transformer units have a 'lift' button on them. I'd love to understand this a little better...

Lastly, since I now have ground loop hum on both the Triple Crown and my Bass Amp (but not the Mark V), would something like this Palmer unit make sense to eliminate the hum on both of these amps so that I only need one device? I'm trying to clean up and minimize the number of devices on my pedalboard as much as possible...

http://www.palmer-germany.com/pro/en/BALUN-Line-Isolation-Box-2-Channel-PLI05.htm
 
Ground lifts are simply disconnecting the ground at one point in the circuit. It assumes that you have a ground connection somewhere else in your system, otherwise you'll get tons of noise or no signal at all. In other words, it'll fix a ground loop, but if you didn't have a ground loop to start with, it will make the situation worse.

An isolation transformer converts electric current to a magnetic field and then back to electric current again. The conversion creates two isolated circuits. The plus side is that it will work regardless of whether you had a ground loop or not. The downside is that transformers color your tone a bit (and are more expensive).

Did you try turning on the isolation transformer in the PBC? (The Iso B switch) That might be all that's necessary. Otherwise, an external transformer box like you link to would be a good idea. The Hum-X by Ebtech is another solution, if you're in North America (120V power). It's an isolation transformer that goes between the wall outlet and the amp's power cord. It typically will fix a ground loop between two amps and has little or no impact on tone.

Different amps have different designs. The Marshall is grounded differently inside, or perhaps even has some protection against ground loops.
 
Thanks very much, I really appreciate the detailed explanation.

I was originally thinking I'd prefer something mounted on the pedalboard itself but as I don't want to affect tone I will try out the Hum-X for the bass amp.

Most importantly - I always had the Iso B switch engaged in the past so I assumed it wasn't part of the problem. But I just checked again and I must have knocked it when I was moving wiring around. With it engaged again, problem solved - hum is gone on the other guitar amp. :)

All the best,

Derek
 
You must log in or register to reply here.
Back
Top