Originally Posted by Bradp
I had a quick question about ground loops. I am going to be wiring a PMA 8000 series audio panel to an old KX-170B Nav/Com. The KX-170 used unshielded wiring and lacks pinouts for "audio low". I wanted to know which would be the proper method to wire the com output to a modern audio panel.
The first option would have the audio low outputs for nav and com brought together to the current airframe ground at back of the KX-170B. The shield terminations would be grounded at the audio panel back shell with everything else.
The second option would have the low audio outputs for nav and com brought back to ground with the shield terminations at the back of the audio panel (all grounds shared at the back of the audio panel).
Seems like the second option would be the preferred method for minimizing ground loops, but I wanted to check with the group. I can't seem to find much in the way of printed guidance on these old - new interconnects.
Thanks and take care,
Ideally, option 1 with the shield grounded at either end (to the audio ground) would be the best. Your noise source will be the difference in ground potential between the comm and the audio panel, so they should each be connected to the same power ground.
By connecting both audio high and audio low this way, noise caused by the ground differences should be rejected by the audio panel. I don't have the audio panel schematic handy, but I it should have a fully differential input structure for this purpose.
Of course, once the engine is running, the noise won't matter....