Interesting thing just cropped up....I often make scratch recordings into my PC using my Digitech RP-350. This inputs via USB. Easy enough.

Ok, I wanted to be able to monitor with headphones, so I built a box with two LM386-based headphone amps inside. This has two stereo inputs. I take the L from the PC, and the L from the Digitech, and combine them passively (thru 1K resistors). Same for the R channels. Outputs to stereo headphones. Works great! I can use it alone and have independent L/R volumes, or input PC and the pedal, and set a good mix between them. This box is powered by a 9V wall wart (therefore the only 'ground' here is referenced to the wall wart). This and the Digitech PS wart are 2 prong, no ground connections.


Then, I connected the USB to input from Digitech to PC....and the headphone amp dies. Completely. Until you remove the USB from the Digitech, then it's fine again. It's acting like there is a direct short.

Ok, so there is some conflict....something doesn't know what ground is, or so on. Somehow, maybe these three things need to reference something in common, or maybe the signal grounds need to be isolated thru capacitors? When I measure from the USB ground (the metal, exposed part of the plug), I get DC voltage readings between it and the grounded jacks of my headphone box (7.5V)! ODD. Everything is plugged into one outlet, which tests fine with a 3-light tester (and DMM). The USB port/cable work fine, and has the proper 5V power pin INSIDE it.

Any suggestions, or has this happened to anyone around the studio, etc? I'm kinda stumped....everything gets power thru transformers, so they have no common ground....

Is there something 'mystical' about USB power that I should know about??

 

When I measure from the USB ground (the metal, exposed part of the plug), I get DC voltage readings between it and the grounded jacks of my headphone box (7.5V)! ODD.

It's not odd at all, when connecting power supplies, that you can get current to flow on the ground wire alone. The rest of the circuit is some path between the supplies. Somehow your supplies are not fully isolated from one another. It's a little hard to picture from your description - could you post a diagram or photo?

I don't think this has anything to do with USB, per se. By the way, I'm not so sure the metal case is at USB ground, it might be at chassis ground of the PC. Not sure.

 

Hey, thanks for taking the time, Wayne. Odd issue; I've never had a situation like this come up, altho it IS similar to the backwards-wired outlet, PA system & guitar = shock issue. I think it's funny that the PC and pedal, headphone outs connected together, are fine, but adding the USB to the mix causes this "short", for lack of a better word. Don't know what to do about it.

This is a diagram of what I have set up. PC out and Digitech out are stereo 1/8" cables to stereo jacks at the headphone box, and that all seems to be working fine. The speakers, Digitech and my phone box are all powered via separate 2-prong wall warts. PC is typical internal supply.

 

Your devices will be linked via 'common'. If you've made any of the cables, I'd suggest carefully reviewing your pin outs.

 

Check your PC too, look for a voltage between USB ground and audio ground, as both of those are showing up at the Digitech.

 

Not meaning to hijack or derail your thread, I checked into the Digitech RP350 and it seems like an amazing stomp box. I play guitar but have never used an FX box. My current amp is a Fender Superchamp XD.

I have been considering experimenting with building my own DSP FX but I know time is too short to take on another project like this.

I see ebay has used RP350 for about 1/3 the retail price.

What is your experience and recommendations on the RP350?

 

Looking at your specific ground problem, I would assume the following:

1) The RP350 power supply is not grounded.
2) The USB shield is chassis ground, not PC COMMON.
3) The USB +5V and GND is actually referenced to PC COMMON.

Next, we have to examine how the PC speakers are powered.

4) The speaker power supply is not grounded.
5) The speaker COMMON is in conflict the the PC COMMON.

Use a DMM voltmeter referenced to PC COMMON and look for conflicts in the above.
An oscilloscope would be even better.

Hope this helps.