[sdiy] Little Phatty ground issues ?

[Gil W.]

before I start let me say that I'm not quite sure that I don't have a defective machine under my hands, so either all I read about the LP is correct, or that there is some problem with my specific item which needs to be solved, in either case, I would like to ask for your help cuse this drives me nuts !

I just bought a used (but very slightly used) LP, which was purchased from local the authorized dealer earlier this year. I tested it using headphones for about a hour and saw no issues, payed and went to my studio. I didn't play at my studio and than took it to a rehearsal with my band. I played it for a few hours without any seen issues and it sounded great.

Yesterday I placed it in my studio and plugged the headphones - everything was right. But as I plugged a TS cable into the Audio Out of the LP and connected it to my Fireface card on the other side of the cable, something strange started to happen - the sound changed spontaneously ! Then I noticed that the actual rotating leds indicating the position of the rotary controlled parameters, are "running" left and right without having anyone touching the synth at all ! 

I unplugged the cable on the Fireface side and the synth stopped acting by itself and worked properly. Connected again - same problem ! I tried to connect it to an amplifier monitor instead of the Fireface - same problem.
I unplugged it from the monitor - problem stopped. I then only touched the metal cover of the TS plug, which was plugged into the LP Audio Out, and problem started again. Stop touching it - problem stops...

I then noticed that the 3 prong power cable I used has a bad ground pin making it deliver no active ground connection to the synth, so I replaced it with a proper, working 3 prong cable.

OK, now when I touch the metal cover of the plug, nothing happens (no problem), but if I connect the Audio Out to the monitor or Fireface in my studio (even the mixer...) the problem returns !!

I tried a few TS-to-TS cables, tried connecting it to different wall outlets in my apartment - same issue !

If I only plug in the headphones (to the headphone jack) and unplug the TS from the Audio Out - no problem :/

I called Moog and the representative suggested that I have a ground loop in my studio ?? I can't get how this can affect the synth... and I also can't get why would this happen at all as I have many synths and never saw and quirks of that kind !

The representative guy suggested that I test it the same way in another apartment, and so I did, and so no issues (I plugged a TS into the Audio Out and the other side of the cable to a mixer, which is connected to an amplified monitor).

Is this a known Little Phatty issue ??? and solution for this problem ?
I really like this synth but can't put it to serious work like that !

Has anyone seen this before ?

Thank you all !