[ChiefPilot]

After searching and reading several threads on similar topics, I'm still confused and need to ask this at the risk of starting another grounding debate ( I hope not! )...

When routing audio signals between an the various pieces of avionics, what is the best practice when the source of the signals does not provide both a high and low signal? For example, my audio panel (KMA 24) does not have an input for COM1 audio (low), only COM1 audio (high). The wiring on my comm (KX 155) has both, with an installation note to ground the shield at the audio panel.

Given that, is my assumption that I tie COM1 audio (low) on the comm to the chassis ground of the audio panel (along with the shielding) correct?

 

[N131RV]

Audio ground

There is usually (not always) on the intercom unit, a pin set aside for common audio grounds. This is usually separate from chassis ground. There may also be channel specific grounds (such as pilot mic and copilot mic, etc).

In the installation manual for the KMA24, there should be some example diagrams.

In general, if there is no channel specific ground (low) pin, then tie your input ground to the common audio ground pin. If there is no common audio ground pin, then I have no suggestion, as I would not routinely tie audio ground to chassis ground without specific instructions in the manual.

YMMV.

 

[rvsxer]

Brad,

I have wired a number of audio panels and audio panel/intercoms (usually PS Engineering) at SteinAir. Some have all the audio returns tied to the shields, then to a common ground, while others use a separate pin for each audio return. There is nothing wrong with using a chassis ground for an audio return. It is important, however, to ground all the audio returns and shields to a common point. Even two adjacent, riveted together pieces of structure can have a small difference in potential and cause a noise producing impedence loop. The "body" of a mic or headset jack is for the return line. This used to provide that ground when mounted to metal structure. Now, the accepted method is to isolate the jacks from structure using phenolic washers and wiring the return back to the common ground. A few com radios and intercoms call for using the shield as the return but we always use a wired return with the shield grounded at one end. This is how my RV-6 is wired and there is no noise, even from the strobes.

 

[ChiefPilot]

Thanks for the information - it sounds like a common ground is definately the way to go.

-Brad