I have tried 7 different condenser mics with it from large diaphragm cardioid, to multi-polar pattern, to pencil condenser. Each with leads that have been tested in a cable tester. The hum is there with all of them, regardless of whether the unit is grounded.
To conclude. I have literally just tested it, and the pencil condensers have the hum when powered by the units phantom power, but don’t have it at all when phantom powered by a single AA battery. Completely silent.
So anyone that has this unit and it is exhibiting this horrible hum. My conclusion after days of testing all of my gear is that it is definitely a defective UR22.
Side note: all of my gear works fine with my UR824. No hum anywhere.