Mesurement electret mic problems

[Bazukaz]

Hi,
A few days ago i got a JBL measurement microphone from LSR4300 monitor set. The mic was designed to work with these speakers(the speakers are active and have mic input to equalise frequency responce automatically).There is +15V written on mic jacket on speakers.
I want to be able to connect this mic to a sound card.
I disassembled the microphone cover, and found a few components inside.The is one IC , JRC 4580(or 5616?) , and a few caps and potentiometer.
On the little PCB , there are three connections(Vcc , Gnd , A-Out).The VCC is 15 Volts , ground is ground , and A-Out i suspect to be audio out.The +15V is red on mic's minijack , white is A-Out , and ground is shield. The two other connections go to mic capsule.
So , this is mic amp.
I built a little regulated +15V power supply , and connected it to the minijack of microphone.It works, but i am having lots of 50 Hz Hum in SpectraPLUS ... Ground loop ? Touching the microphone increases this hum. I tried to replace the regulated PSU with 9 volt battery , but the things didn't change , so it is not supply's fault.I included the connection block diagram.

Regards,
Lukas.

[Bazukaz]

So, no ideas ?
I also observerved that touching the microphone's case together with PC case while holding peace of wire diminishes the hum almost completely.

Regards,
Lukas.