Odd ground loop and solution - anyone got an explanation?

C

Clarktronics

Member
Hi guys - I've been hunting down a ground loop in my board for the last two weeks which sounded like transformer hum bleeding into the cables - it would increase if my cables moved closer the the amp transformer or PSU. This would only happen when using the four cable method either through the PBC or an Eventide H90 (didn't test with any other pedal). Someone suggested putting a line isolator on the line that goes to the amp input - finally tried with a ground lift on a DI box as I don't have an isolator to hand, and the problem has been solved - no more transformer hum etc. I'm chuffed it's worked, but I'd quite like to know why! I tried the ISO B switch on the RJM from output 2, and lifting the ground on the send line with no result, it only works on the amp input line. Anyone got any ideas why this works? I'd like to know before I go out and buy a line isolator.
 
With the four cable method, you can have multiple ground loops (or one complicated one, depending on how you look at it). The amp input isn't the first place I would recommend isolating the connection, but sometimes it's the one that needs it the most. Any noise coming in the input is going to be amplified a lot, so you'll notice a noisy connection there more than anywhere else.
 
The input side assuming you mean preamp in has your gain stages, so if your injecting a signal into the preamp side, it's going to amplify the signal on those gain stages.
 
Thanks guys! I was just a bit confused - partially as I've seen that you don't always recommend an isolator on the amp line, and also because of the nature of the ground loop - instead of it being a constant hum, it was a hum/buzz that would increase the closer my cable loom got to the transformer in the amp. As long as I know there's an explanation and nothing is majorly wrong with my stuff I'm happy.

Thanks for the help!
 
A ground loop is a noise antenna that primarily picks up the magnetic field from AC power lines, transformers, etc. So, what you're seeing makes sense.
 
rjmmusic said:
A ground loop is a noise antenna that primarily picks up the magnetic field from AC power lines, transformers, etc. So, what you're seeing makes sense.
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Ah OK! Sorry, I don't know much about this stuff beyond the terminology, so thank you for explaining it. Looking forward to getting the rig back together now!
 
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