@Newsee @Gayron all you need to do is to ground your 3XR. Usually this is done via the USB cable if using a desktop with grounding, but in case of using a laptop without grounding then try something else (audio cable connected to a grounded amplifier).
You can also switch vice-versa the plug in the outlet (live with neutral) and see if hum goes away. Or check the outlets from another room.
DC will not cause this, not the internal "transformer", as C3X is using switching supply follwed by linear regulators inside.