Voodoo Lab

I just took delivery of a new GCX. I wanted to use an A/B setup so that I can switch between two different preamps.

I have the preamps patched into the GCX as shown in "Example 1, Page 8" of the GCX manual.

With the loop deactivated I get signal from preamp 1 no problem.

When the loop is activated however, I get gawd awful humming (really loud)and no audible signal at all from preamp 2.

My first thought was I have a patch cable with no ground connection, so I tried a few more cables, same thing(really loud hum, no signal). The patch cables all metered out OK (no opens or shorts).

I tried all 8 loops, and swapped preamps around, the same thing occurs.

All the loops work fine when a single stompbox is connected.

Now here is the strange part that has me wondering if my GCX has a problem.

With the loop activated, if I lay a finger across the metal caps (ground) on the two patch cables in the "out" and "return" jacks the hum level drops dramatically and I begin to get signal.

This would be the equivalent of a resistive connection between the grounds on the two cables (huh??).

If I lick my finger (thus creating a lower resistance) and repeat the action, the hum level drops to "barely" audible and I get a good strong signal.

Am I doing something wrong or should I be taking this unit back to my dealer for an exchange?

PS: I can't afford to pay someone to stand around with a wet finger on the patch cables, nor can I find anyone willing to give up a digit to keep my rig nice and quiet.

Thanks in advance for any help, advice or suggestions.

Brian

How exactly are you feeding the input signal to these two Preamps? The Return jack is ground lifted in order to prevent ground loops, but this does mean that the Inputs to the preamps need to be fed from a commonly grounded source.

Thanks for getting back John. The fact that the grounds are lifted on the "returns" does shed some light on the matter. This would be why the hum level would decrease considerably when I gripped the metal caps of the patch cords going into the return and output jacks. The return seeing ground (all be it a high resistance connection) thru the skin of my fingers.

After posting my last message, I discovered that connecting the chassis ground screw of one of the input sources to the ground screw of another piece of equipment made the humming disappear entirely and everything seems to be working just fine.

The only problem now is I am probably going to want to buy a second GCX and my lovely wife may try to murder me as I sleep.

Thanks again

Brian

So how are you feeding the inputs to the two preamps? Are you using an isolation transformer or are they being fed from two separate guitars? Your method of establishing the common ground needed here should work but it is interesting that this common ground has somehow been previously eliminated.

Hi John

Yes I am using two separate guitars and switching back and forth (only one guitar can be active at any given time). One of the guitars is feeding a Roland VG-99, which connects into the returns of two loops (#5 & #6) (left & right stereo).

The other guitar is going straight into a preamp which connects to the inputs of the same loops (#5 & #6)the VG-99 is using (again left and right stereo).

Outputs of Loops 5 & 6 connect to inputs of Loops 7 & 8, sends and returns of 7 & 8 are connected to the ins & outs of a TC Electronic G-Major.

As I had mentioned previously, if I connect the chassis ground connection of the VG-99 to another chassis (in this case the G-Major), all is well.

If I remove this connection, the loud humming (and I mean loud) is back.

Brian

PS: Just for the heck of it, I clipped a jumper lead on the strings of one guitar and clipped on to the strings of the other guitar the humming again disappeared completely.

...and there you have it. Your setup is not exactly like the diagram because you are switching between two totally unique sources rather than a single feed split out to two preamps. The ground method you used should be fine for this particular application. Good luck!