Feedback in Pre-amp Compressor

[Merverus]

Hi!

I'm having trouble with my Joe Meek VC3 pre-amp/compressor. When I turn the input gain knob, at a certain point (with the out volume knob turned to 0) the on-board LED VU meter starts lighting up and receding in a throbbing pattern, gradually becoming louder. I realize this is some kind of feedback pattern or a problem with phantom power. Any suggestions would be very helpful.

Thanks,
Idan

[Merverus]

Since posting, I have upgraded the XLR cable from Hosa to Duroflex; I noticed that now I can crank the input gain on the pre-amp to a higher level before this feedback cycle begins. So it seems I'm picking up some interference from somewhere?

Thanks again!