When I connect my Behringer X Air 18 to the host port of the MOD Duo X, I instantly hear a high pitched noise coming from my speakers. There are no other connections at the Duo X, only power and the XR 18. I’m don’t know much about electronics but my guess is that there’s some problem with the USB shielding.
Does anyone else experience this behavior?
This issue could very well be a ground loop.
We have some info about this on our wiki troubleshooting page
The best way to test this is to disconnect one of the 2 devices from ground and see if the problem still persists.
My response may not help much with your current issue but if the idea is to control the XR18 through MIDI, using the DIN connector would be a much better choice since it is opto-isolated.
That also frees the USB on the XR18 for connection to a computer for DAW recording.
Duh sorry… of course it could be a ground loop. I don’t know what I was thinking. It will try some USB isolator. But it’s the first time I ran into this issue since three years with this mixer.
Yeah, unfortunately I need the audio because the Duo is supposed to be my master effect 
I this pretty expensive thingy solved my problem: https://www.amazon.de/iFi-iDefender-Externer-USB-Audio-Erdungsschleifen-Eliminator-Typ/dp/B0849J33T9
If I found a cheaper solution, I’ll update here.
That sounds similar to my issue:
https://forum.moddevices.com/t/usb-isolator/
So did the ifi idefender fix it for you?
I’ve had digital ground loop noise when controlling a Zoom multistomp pedal and Source Audio C4, and solved the problem using a cheap galvanic usb isolators from eBay £15 and £10 each respectively.
I tried this one first https://www.ebay.co.uk/itm/175049788837 and then this cheaper one https://www.ebay.co.uk/itm/373670645387 both killed the noise and allow midi by usb without issue.