TOPIC:

Getting some noise injected from USB GND, masking the pin removes significantly reduces the noise, the device still connects fine but is it safe long-term?
If it's not safe, would something like this hifimediy.com/product/usb-isolator/ work or not due to no multi-channel support?
2x4HD sits between an AVR and power amp and is connected to a PC at the other side of the room using a 5m active port.
Plugging them all into the same mains outlet isn't really an option.

Edit: So masking the GND pin reduces the noise significantly but its not 100% removed like it is when disconnecting the USB altogether.
There's also some slight pop when connecting/disconnecting the USB, does that sound normal?
I've been done some reading about USB and it seems devices shouldn't be able to connect without GND, what's going on here?

What is your setup exactly? Hard to get a handle on what the problem might be without knowing the detail...

 

What other kind of info do you need?
The 2x4HD is integrating a pair of subs using mso (which it does very well).
It does not introduce noise into the audio system until I connect it to the PC via USB, at which point it pops on first connecting and then creates some hiss and whine (I can hear my mouse moving).

I think you're missing the point.

With ground loops, you need to look at all the components that are electrically connected together, and their power supplies, and if/how their grounds are connected, if you expect to reach any kind of conclusion as to what is causing the problem.

A couple of years back I was playing around with linear PSUs for my Squeezebox Touch streamer in an attempt to see whether they made any difference. I very soon concluded that when I was using a Toslink (fibre) connection to the DSP, there was no audible difference, but when I used analogue inputs to the DSP I got hum with one of the linear supplies but not the other. Tracking it down was interesting. The Squeezebox, the laptop I was using, and the miniDSP all use stock switch-mode power supplies that are not referenced to the mains protective ground, so they are "floating". One of the linear supplies I tried (the one that generated the hum) had its -ve line connected to protective earth. However, to create a ground loop, you need a second connection to ground...where was that coming from? It turned out that removing the HDMI cable from my laptop (I use a second screen when at my desk) killed the hum - the loop was crreated via the HDMI cable and the second screen's earth connection. It was only a problem when using the analogue inputs to the miniDSP, because of course the fibre Toslink connection broke the loop.

By the way, my conclusion was that there was nothing sonically to choose between the stock SMPS and a linear supply, but that is another discussion!

I thought the main noise from ground loops is a 50/60Hz hum?
The PC is already connected to the AVR with HDMI (a 'noisy' gaming GPU even) so there's already a potential for ground loop, if it were to buzz surely it would with just this connected?
Yet as I stated, not until the USB is connected to the 2x4HD do I get this more high frequency noise through the mains (sens 96dB@1m) all speaker channels, it's possibly getting to the sub amp too but out of range to be heard.

In relation to Tony_J's story and to further elaborate, I've tried running mains power over to the PC from the AVR/AMP outlet, disconnecting every HDMI from the AVR so the only things connected are AVR and AMP with the 2x4HD in between, AVR chassis is grounded to AMP chassis. AVR to 2x4HD is a .75m RCA-RCA and 2x4HD to AMP is .75m shielded twin cable with the ground floating at the 2x4HD side.
Still it's the USB connection to PC which adds noise. If I plug the USB into the AVR, beyond the initial pop there's no noise, but I obviously can't program it through the AVR.
My guess it's just picking up junk from my PC which is sailing along the GND and bleeding into the signal? Breaking that reduces the noise a lot but it's still getting in a bit, perhaps along the shields?

My cheap fix for now is a short USB extension cable so I don't wear the PC USB port out with constant unplugging and re-plugging. Looking at something like this www.olimex.com/Products/USB-Modules/USB-ISO/ as a potential solution.

Yep similar story here... My laptop ppwer supply died some time back and until a new one arrived I cobbled together a replacement from an old transformer I had in the garage... Charged the laptop no problem but when the laptop was connected to minidsp (I think via headphone jack to analogue in) I remember hearing a horrible hum/buzz through the speakers. Disconnecting the power supply from the laptop made it disappear. Can't remember if I heard it via USB connection too.

Last night I tried connecting via a shield-only RCA cable, the spare input - on the 2x4HD to AVR chassis ground, this reduced noise by about the same amount as masking the GND pin alone. I then tried connecting the same input - to Pin 1/chassis of the AMP - as that is already grounded to the AVR chassis, this made the noise worse (but still no hum). I took this out and then pulled the chassis ground from AVR to AMP and... silence!
So in the end connections are pretty standard requiring no additional ground wiring, no GND pin masking and no isolator .

Fascinating isn't it!

Good to get a result.

It's bizarre that's for sure. I needed the AVR and AMP ground connected previously to kill a fairly loud hum. Didn't think to remove it as I thought it was solving an old issue not causing a new one!