Ground loop with a amp and a preamp in a GCX

[Steven Latch]

Hi,

I have spent a few hours trying to solve a ground loop issue and failed...

The rack is quite simple.

I have a marshall amp in a loop, and a randall RM4 in another loop. There are a few pedals, a multi effect and a decimator in the rack.

Even tough all is racked with humfrees and fed by a furman there is one HUGE ground loop I can't get rid of.

When I activate the loop that feeds the marshall preamp, all is fine.

When I activate the loop with the randall, there is a lot of background noise. I experimented and found out that as soon as I unplugged the jack that went in the marshall guitar input, it went away.

So, I cut the shield of the jack going into the marshall... and it reversed the issue! The noise dissapeared from the randall loop to appear on the Marshall loop.

I then went through many trials, unsing unshielded jack with one or both preamps and found the following:

I can get one preamp completely "clean" by messing with the ground lifts of the decimator and the randall... but always at the cost of a huge ground loop on the other preamp.

I'm sure this is quite a common issue. What method could I use to get around this?

Many thanks!
When

[JohnClark]

How exactly have you connected things in order to switch between the two preamp sections?

The basic starting point should be to split the signal and actually drive the inputs of each preamp section at all times with a single GCX Loop used to A/B the outputs of the two preamp sections. Check out Example 1 in the A/B Switching section of the GCX manual for a diagram of switching between two preamp sections.

[Steven Latch]

Hi,

I did that initially and then went back to having one loop for each preamp. But I will have another go at it.

Thanks!

[Steven Latch]

Since I am using a couple of pedals that can be fed into each preamp, this is the way I am splitting the signal:

Loop 4 has "send" into the Randall" and "out" into the Marshall
Loop 5 has the marshall's return in "in" and the Randall's output in "return.

Is that the correct way to split/regroup?

[Steven Latch]

Well I tried the regular A/B switching and it didn't improve the issue.

However... when the jack that goes from the input of the Randall into the output of the GCX touches any of the jacks that go into the Decimator.... the humming dissapears altogether. What could that mean?

[JohnClark]

Perhaps we should first simplify this setup for testing.

1. Run your guitar to the Guitar In.
2. Run the Guitar Out and rear panel Feedthru to the inputs of both preamp sections.
3. Run the outputs of the preamp sections to the In and Return of GCX Loop 5.
4. Run the Out of GCX Loop 5 to the Effects Return of the head.

[Steven Latch]

Well I did jyst that. Removed everything from the path apart from the preamps and even changed the loops for the slip/return of the signal but the result is the same.

- Fine as the signal passes through in the Marshall preamp, back in the GCX through the FX loop out and back in the power amp in the FX loop in.

- Huge ground loop when the loops are activated to route the signal in the randall preamp. But as soon as I unplug the jack that goes in the marshall input, the hum dissappears altogether.

This is where I am stuck now...

[JohnClark]

Have there been any modifications to this GCX?

With this simplified setup, try lifting the audio ground on the cable that feeds the instrument input of the Marshall.

[Steven Latch]

Well no mods to the GCX to my knowledge but... it did come without the original power suppy. The previous owner patched up a US robobtics supply. Could this have an influence?

And I will try with a cable with the shield lifted.

[Steven Latch]

Do you think using the wrong power supply could have an influence?