Voodoo Lab

Hi there,

I’ve recently dusted off my guitar rig thats been in a flight case for a while with the intention of hooking it all up and playing again. I dont have a band, havent played live for some time (and dont really intend to in the near future), so my goal was to simplify the rig and transfer the essentials to some studio rack furniture that looks more presentable. I love how the GCX/GCP system works, therefore I decided to keep this combination as the basis of controlling my gear.

I have been conducting some thorough tests on my gear to make sure its all in working order, however I have come across a stumbling block with using the GCX loops. Its probably easier to share some diagrams rather than explain my signal path as I’ve tried a few different things to try and help narrow things down.

For the record, there is literally nothing else powered on (power cables completely unplugged from units and mains), no other audio cables plugged into units/loops that arent in use, humfrees attached to all rack units when mounted, and all gear that is powered on shares the same power strip that is nowhere near the rack/amp/guitar to cause any electrical interference. Removing the GCX from the rack makes no difference and even completely unplugging the guitar from the GCX (via the front Guitar In) does nothing to eliminate the issue I am experiencing. I have tried cables of various lengths ranging from smaller patch cables to longer cables, newly purchased cables and even cables I’ve had for some years. All have been confirmed as working so I seriously doubt a bad cable is at fault here.

One other thing I should mention is that, whilst it is obvious on higher gain channels, you can hear the noise faintly on lower gain/clean channels too (increasing the amp/channel volume makes it easier to distinguish the noise, so its not ultimately exclusive to one channel). I have tried on three different amps (Laney acoustic amp, Revv Generator, and Engl Invader) and the noise is present with all of them, therefore it is not exclusive to just one amp.

Diagram 1
The following setup produces no unwanted noise/interference (removing the GCX from the signal altogether and plugging directly into the pedal produces the same effect):



Diagram 2
The following setup produces a distinctive buzz when the loop is engaged and the pedal is on with its volume set above zero. The buzzing still occurs even if the power supply is removed from the Keeley compressor whilst still being engaged, but disengaging the pedal itself (not via GCX) eliminates the buzz even if the GCX loop is stil engaged:



Diagram 3
At this point, I think its obvious to assume that placing the pedal in the GCX loop is causing the additional noise. To rule out problems with the pedal, I tried the following configuration:



I used a cable as a simple jumper in the place of the pedal and, whilst the noise is much weaker, it definitely was still present when the GCX loop 1 was engaged. I could also completely unplug the guitar cable from the front of the GCX at this point and it would make zero difference, so the guitar cannot be blamed for the additional noise I’m hearing. Powering the pedal using a 9v battery as opposed to a Boss branded power supply yielded no difference either.

I started to assume that maybe the problem was happening specifically with the GCX loop 1 so I tried the same tests with the other loops and all produced the same buzzing sound when engaged.

Diagram 4
As I’m at a loss as to what could be causing the issue, I decided to try out the following configuration by combining loops as I initially suspected that one or more pieces of gear was interfearing with the signal in some way prior to this test:



So what happens now is that I can engage either GCX loop 1 or 2 and it will produce the same noise as in diagram 3. Engaging just loop 3 with the pedal connected will produce the noise but louder as in diagram 2 (it actually scales off the volume knob on the compressor when the pedal volume is set above zero). This proves that the noise is not exclusive to one loop, however I assume the compressor is simply amplifying a signal that is present somewhere within the GCX loops.

If you engage loops 1 and 2 simultaneously, the noise is amplified even more. Even if I removed the compressor from loop 3 and replaced it with another jumper cable, the noise would become even louder albeit not as loud as with the compressor in the loop. The sound is still present if I swap the loops around and put the pedal in either loop 1 or 2 instead for example.

I know you wouldnt use this type of setup in a typical situation, but considering I have completely stripped my rig down to the barebones, I’m genuinely at a loss for what else to try at this point. As its been some time since I used this gear, I honestly dont know when I first started to experience this issue or if it even existed the last time I properly played with this rig a few years ago.

If this is not a regular thing to happen and could indicate a fault, could the problem I outlined in my other thread have anything to do with this too?

Any help is greatly appreciated.

Kind regards,
Matt

I have been trying some additional tests on my GCX and will post these findings below. I’m still using the stripped down rig as per diagram 2 and nothing has changed:



a) Firstly, I’ve been trying to determine if the sound is due to a potential ground loop somewhere in the chain. I did some research into past threads and there was another person who had a similar issue albeit a bit more complex (I think he was using a dual amp setup or something like that). The advice he was given was to connect a wire from the shields of the output and return of a loop to determine if the ground is bridged and in turn noise is relinquished (as I understand it, the ground is actually lifted on a loop return). I tried this and sadly it made no difference to the sound I was hearing (I dont think the other person resolved their original issue either). In fact, it seems as if the return is already grounded once the pedal was introduced into the loop anyway.

The next thing I tried was to lift the internal ground lug. The GCX is mounted with Humfrees anyway, so I doubt this would make any difference. Sure enough, lifting the internal ground made no difference to the noise coming through on the signal. Unless I have missed something completely obvious then I honestly dont think a ground loop is to blame here.

b) Another thing I noticed was a weird way that the GCX loops seemed to be interfering with each other. Say we go off the exact chain in the diagram above where the compressor pedal is in loop 1, it has already been confirmed that this will produce noise in the signal. The strangest thing is that if I enabled just the loop directly underneath loop 1 on the GCX (which is loop 5), I would get the noise in the signal. How is this even possible when nothing is even plugged into loop 5 (not even the In/Out)?

If I engaged just loops 2-4 and 6-8, the signal would remain silent (as it should considering they arent even connected).

If I plugged the signal and pedal through loop 2 instead (leaving loop 1 completely free this time), enabling loop 2 would introduce the noise. If just loop 6 is engaged on its own, I will also get noise in the signal. Enabling all other loops besides 2 and 6 produces no noise.

This pattern can be repeated with loops 3 & 7 and 4 & 8 in each pair sequentially, therefore this must rule out having one potentially faulty loop.

To prove it wasnt the pedal causing additional noise, I completely removed it from the chain as follows:



Enabling loop 1 produces no noise (or signal), however disabling loop 1 and enabling loop 5 adds noise to the signal.

I would be grateful if anyone could help me to resolve this issue as I am completely out of ideas on what to do next besides trying to compare with a brand new GCX.

Kind regards,
Matt