Grounding/Background noise while H320 is charging

[melbo]

I'm using H320s for long-term recording of bat calls through the line in.  When the H320 is running solely off of its internal battery, the signal is perfectly clean and there isn’t any background noise.  But, when the charger is plugged in, I have to raise the trigger db so the background noise doesn’t set off the recorder.  The detectors, which ground themselves to the H320s via the patch cable, are running on 12 volts.  The H320s are being constantly charged at about 5 volts off the same battery with a modified cigarette lighter plug-in adapter.  I’ve tried various wiring/powering/adapter options but they all resulted in some level of noise, sometimes as loud as –35db on the recording meter.  The lowest noise levels are achieved when the detector ground wire is not hooked up to the battery and the detector is therefore grounded only through the patch cable.

I’m assuming this all has to do with the recorders and detectors being grounded to each other yet both working off of different voltages from the same battery and different resistances at the input/output jack.  Trouble is, I really don’t know much about that sort of thing.  Any suggestions on how I might correct this problem?

[pabouk]

The problem is caused by a ground loop. The solution is to minimize the loops in which current could be induced. You avoided one loop by disconnecting the ground of the detector. How to minimize the loops:
- use shortest possible cables
- lead the cables which make a loop as close to each other as possible
- break unneeded loops
  - for example you can disconnect ground at the one end of the signal cable if the grounds are already connected by a different conductor
  - or you can use different batteries for every device
When breaking the loops check whether there could not be a considerable current flowing through connections which are not designated for such a current. (for example powering current going through the signal path)

Please let us know about the results.