Hi Helix community!
I have a very strange (to me) issue with what sounds exactly like ground hum (and probably is). I have isolated the problem as best as I can but I still don't know how to properly solve it.
Here's the most basic setup where I can reproduce the issue:
Electric installation: mains socket -> power strip (where speakers are powered from) -> power strip (where the Helix is plugged). I live in the UK and everything is grounded, and in theory to the same ground since they are all connected to the same mains socket.
Only thing plugger to the Helix are 2 KRK Rokit 5 speakers via XLR.
As soon as I switch the Helix on, I get the ground hum coming out of the speakers. It happens even with volume turned to 0, and doesn't ever increase in volume regardless of output level. If I disconnect the XLR cables from the Helix, it goes away, so it's definitely something between the Helix and the speakers.
At this point, with the hum going, I connect the USB cable to the Helix. Nothing changes. As soon as I connect the other end of the USB cable to my old 13" Macbook Pro, the hum goes away completely. It makes no difference whether the Macbook charger is connected or not.
Adding another layer to the weirdness. I have a newer USB-C Macbook Pro, and to use the Helix with it I connect it to a USB-C hub (https://www.amazon.co.uk/HyperDrive-Type-C-Adapter-50Gbps-MacBook-DUO-7-2-GREY/dp/B07DFG6TCG/ref=sr_1_1_sspa?keywords=hyperdrive&qid=1557237156&s=gateway&sr=8-1-spons&psc=1). Now, if I plug the Helix's cable to the hub the hum also stops even if the hub is not connected to anything, not even the computer. So just a floating USB hub connected to the Helix makes the hum stop.
Another one: I have some small USB-B to USB-C adaptors that I use to plug stuff to the Mac. If I plug one of those to the cable (again, even when disconnected from the computer) the hum amplifies.
I don't know what's happening or how to solve it. Right now I'm just keeping the Helix plugged to one of these miraculous hum-cutting pieces of equipment (USB hub or older Mac) but I'd love to resolve the issue independently of the USB cable being plugged to the Helix.
Any ideas or possible explanations?