loud hum using voodoo labs switching through Randall RH50T

[SoulFetish]

I have a customer who is running a Voodoo Labs GCX audio switching system to patch his effects through the input and effects send/return of stereo Randall RH50T heads. He is getting excessive hum when going through his effects loop. We've eliminated power supply noise in his effects as the cause. The noise sounds to be in the range of 120Hz, but when plugging straight into his Randalls, the HV supply is quiet and does not seem to indicate bad filtering. The same setup when substituting a Fender Super instead of the Randall head seems to work fine (Although, the Randall is a much higher gain amplifier). At this point I'm thinking there is grounding noise and/or a potential problem in the solid state amplifier driving the loop.
I'd appreciate any thoughts on something I'm missing, or if anyone has any experience with this head or switching system. I've attached some clips, and schematics:
Randall RH50T • RG50TC.pdf

gcx_switcher_manual.pdf
Hum Example w-Randall.mp4.zip

[Mick Bailey]

What does the amp sound like with just a patch cable linking the FX send/return? Have you tried setting up the switcher and substituting the pedal loops with a patch cable?

[SoulFetish]

What does the amp sound like with just a patch cable linking the FX send/return? Have you tried setting up the switcher and substituting the pedal loops with a patch cable?

Mick, with the effects loop jumpered the noise was loud and clear. Dead quiet with it pulled. What are you thinking it might be? I don’t have it on the bench with me yet, but im leaning towards failing LV filtering.

[Mick Bailey]

If the LV filtering was failing the amp would most likely hum all the time - especially as the reverb driver/recovery is opamp based. Check if there's any noise if you output the 'send' to your bench amp. If not then that eliminates everything before there, including the loop buffer Check if the amp hums with a guitar plugged straight into the return. My first though is a broken or bad tip contact, or dirty switching contacts in the return socket. Make sure the loop switch operates correctly.

[nickb]

Check the continuity between the Randall return jack sleeve and the safety earth pin on the power plug. It should be connected. Do the same for out of the output of the switcher and its plug. if it i also grounded then there is your problem - a ground loop.

[Mick Bailey]

He says it still makes the noise with just jumpering the send/return - no switcher.

[nickb]

He says it still makes the noise with just jumpering the send/return - no switcher.

Ah! So he did. I misread it.

[SoulFetish]

Ah! So he did. I misread it.

My apologies to both of you.
We got the two Randalls in the shop on Friday. Before that I was Looking at phone recorded video evidence. I now know that when he pulled the effects jumper in which the noise disappeared, was actually a full defeat in the signal jack muting amp noise. So, here is what is really going on:
It is a ground loop. Check out this clip with the two heads powered on, into a speaker load. With no input into either amp, the noise you hear in the video is my boss connecting an alligator clip from one chassis to the other. Observe:
IMG_3450.mp4.zip
This was confirmed by lifting the earth on one of the amps to see if this broke the loop (which it did).
This brings me to seek some advice on the proper, safest way to solve this. I, personally, really do not want to lift the "earth" on either amplifier. Technically, there is still the earth bond via the other amplifier. But any fault current would have to pass through a signal cable to the bonded amplifier. Most instrument cables cannot handle that kind of current, although I could build him some with 18AWG shield and center conductor. However, I'm still uncomfortable with this approach.

The second would be to use audio transformers to decouple the input and effects return jack from the other amplifier using something like this:
http://www.ebtechaudio.com/hedes.html
This would probably be the cheaper option and the one I'm favoring at this point.

The third would be to create a switchable lift doing something like the "loop breaker" found here:
Earthing (Grounding) Your Hi-Fi - Tricks and Techniques

What do you guys think?

[Mick Bailey]

I'd use a commercial ISO box. I just did a job for a guy who had a dirt cheap Behringer and it works superbly. I think the newer model is the HD400 but you'd need to check. The one I had was just a couple of audio transformers inside a metal case and I was pretty impressed with the frequency response. I also use Palmers but they're more expensive. For me anything to do with the mains earth is too much of a liability.

[SoulFetish]

I'd use a commercial ISO box. I just did a job for a guy who had a dirt cheap Behringer and it works superbly. I think the newer model is the HD400 but you'd need to check. The one I had was just a couple of audio transformers inside a metal case and I was pretty impressed with the frequency response. I also use Palmers but they're more expensive. For me anything to do with the mains earth is too much of a liability.

Yeah, i agree about the mains. They audio transformers are what i was leaning towards as well.
Thanks Gentlemen.