Weird ground loop when using RM4 with a GCX guitar switcher

[Steven Latch]

Hi,

I was on my way to rebuilding my rack but have been struggling with a "buzz" issue for hours!

Here is the issue.

I use a GCX guitar switcher to midify some pedals and switch between preamps.

Whenever I insert the RM4 in one of the loops, it adds some annoying buzz to the signal ONLY when the RM4 is selected.

With the following signal, no buzz: Guitar in GCX - GCX loop to RM4, RM4 in power amp

With the following signal, buzz!: Guitar in GCX - loop to RM4 - RM4 out to GCX return - GCX out to power amp.

An even weirder thing is that I use two preamps. If I select the RM4 only, I get the buzz. But if I add ANOTHER preamp to the signal chain, the buzz gets away!

I tried switching the RM4 ground loop but it made no difference. I also switched the RM4 to another GCX loop but no difference either. I have also changed all the cables that linked the GCX to the RM4.

Anybody has an idea how I can get rid of this?

Cheers

 

[JKD]

Doesn't the RM4 have a ground lift feature ?

 

[Steven Latch]

It does and I have checked both positions.

With the ground lift, it gets worse. On top of the resident ground loop, I get some extra noise on the modules.

I have checked the documentation of the GCX and it says the ground is lifted on the return of the loop. As the problem occurs when the signal returns to the GCX could it come from this and what can I do?

 

[davesalad]

Check out the site HUGE RACKS Inc.. - great guitar rack info..
Make sure that each unit is isolated - ie not touching each other...
I would pull everything out of the rack ..
Hook it all up - see if there are any more problems.
Many times alot of preamps don't like to be near - other wall warts , transformers.

Alot of guys use thin plastic sheets between each unit for isolation.
Plastic washers may help.
Bad cables .
Do make your own?
I would cut all ac to lenght.
I could go on and on...
I would first pull everything out of the rack and go form there..
Here's a pic o my rack wired up..z

 

[Steven Latch]

All sensible advice.

I still find it hard to find plastic screws that would allow better isolation from the rack chassis.

On the other parts, I am pretty much sorted. George L cables cut to lenght, furman power conditionner...

I stripped down the rack to the furman, the GCX and the randall and got the same issue. As soon as the RM4 out hits the GCX return.... BUZZZ!

Then I noticed something odd... When my ADA MP1 is rigged is the second loop and I activate BOTH preamps at the same time... the buzz goes away. As soon as I add the mp1 to the signal chain, it's sorted.

Could it be that the earthing problem the RM4 has is sorted if the signal goes in another properly earthed machine?

I then added the G-Major to the signal after the main out of the GCX. No problem anymore.... Sorted for then now but I still don't get it.

 

[Steven Latch]

Cool legacy you have by the way.

I have it too and am considering getting it modded so that I have two switchable levels of gain on the lead channel. This and a couple of boosts make it a much more versatile amp.

 

[MJ82]

Have you tried removing the ground on one side of one cable?

 

[seeker]

Davesalad- Dude, you need more heads in that room. You're missing a couple. :shock:


H

 

[davesalad]

There's more....

Sometimes when I am off the road or no local stuff going on ,
I bring some heads home..
I ve been using a vetta lately.
z

 

[lucidness]

I had the same problem with my set up. I had an RM100 with an RM4. I'd switch between the RM100 preamp and the RM4 with the GCX. I only had buzzing when I selected the RM4 preamp. It's a ground loop hum between teh preamps. Its kinda confusing. I took everything otu of my rack except the preamp. I even had the GCX suspended in the rack so it wasn't touching anything. I still had the hum. My advice to you is to try an EBtech Hum Eliminator. I was gonna try one but I never got around to it. But people say they work good. You just insert it between the two devices that hum.

cody