Question about Audio Panel inputs

[piperboy84]

I notice on the PS engineering PMA8000b and most other audio panels there are AUDIO inputs for connecting COM and NAV equipment, but there appears to be 2 options i.e." NAV1 Audio In" and "NAV1 Audio In LOW"
Not sure what the LOW is for

Anyone know what the difference is between the 2?

[wigglyamp]

The 'low' is the ground return. Most radios have audio outputs referenced to chassis ground, so although the install manual will show a twisted pair for the connection, in reality you only need the 'high' connected to make it work. However, sometimes a radio will have a transformer-isolated floating audio output (King Kr86 ADF being such an example if memory serves me correctly), in which case the low side needs to go to ground and gets this at the audio panel. There is a convention is larger aircraft audio systems that calls for all audio grounds to go to a single point in order to minimise noise pick-up from the airframe. That's why you'll find headset sockets with insulated washers to keep them above aircraft ground.

[piperboy84]

Thank you very much wiggly, your help is appreciated