Phantom power causes hum

I use my MR816CSX with different mike types : SM58, C3000 (AKG), Perception 120 (AKG) and Behringer pipes.
Since I built a ultra quiet vocal booth for vocals en accoustic recordings it’s obvious I get a 50Hz hum while using the AKG C3000 or Behringer pipes as soon as I switch the phantom power to ON for thát/those mike(s).

The perception 120’s (also 48V powered) do an excellent job without hum just like the Shure SM58 (dynamic mics)
The C3000’s and Behringers work fine on the Yamaha AW16G and AW1600 portable recorders so there is nothing wrong with the mikes.
I almost never use the Behringers, but I hate the humming C3000’s preamps.
Has anybody noticed this ‘feature’ also? …
Who knows the solution? (already contacted Steinberg Support, but no news so far)

Extra info:
It cannot be caused by a ground loop because it also occurs with de MR totally isolated from all other equipment. (only the MR connected to the AC power supply and mike thru a short cable)
There are no magnetic fields of power supplies or other equipment nearby.

Update 2-5-2012:
Problem found/solved
The cause was a (unmeant) phantom-ON at inputs 7&8 where unbalanced jacks were inserted. …ppfffff