GMA340 audio panel COM1 passthrough

[newt]

Yes, I'm aware that the GMA340 has been EOL'ed by Garmin.

I have a GTR 20 remote-mount VHF cabled into COM1 on a GMA340 audio panel.

The GMA 340 has a COM1 pass-through, where an internal relay connects the command pilot's headset, mic and PTT to COM1 when power is removed.

I've designed my panel to make use of this feature, with one switch which lights up the GTR 20 and the G3X Touch PFD so I can listen to ATIS or talk to clearance delivery with minimal power draw before engine start.

This mode of operation works perfectly when I plug in the cheapest nastiest oldest most falling-apart headset I own.

When I plug in any other headset (a David Clark, a Clarity Aloft, and the headset built in to my Bonehead Composites flight helmet), I get very low receive levels. Low enough that I can't hear the radio at all if another airplane is taxiing past.

It seems there's a subtle wiring fault, or maybe an impedance mismatch of some kind (??), but I can't think of one that'd cause these results.

(It's not the stereo/mono selection on the headset. The GMA340 manual notes that it's not supposed to work at all if that's wrong)

Any ideas?

- mark

 

[BobTurner]

Is this 340 an early generation? Garmin’s first in-house built audio panel? If it’s the unit I’m thinking of, it had an incredible design error. The ‘fail-safe’ connection from #1 com to the headphone jack went to the ‘ring’ terminal, not the tip - so it doesn’t work with mono phones!
Easy test: pull the phone plug out about 1/4” (so the tip will connect to the ring connection. If that works, you know the problem. Get a stereo to mono adaptor, plug it into the 340, headphones into the adaptor.

 

[Longez]

Is this 340 an early generation? Garmin’s first in-house built audio panel? If it’s the unit I’m thinking of, it had an incredible design error. The ‘fail-safe’ connection from #1 com to the headphone jack went to the ‘ring’ terminal, not the tip - so it doesn’t work with mono phones!

Bob - this is incorrect! No Garmin audio panel has ever connected the failsafe audio to only the ring terminal, including early GMA 340 audio panels.

Mark - the first thing to do is check the mod status sticker on your GMA 340 and see if Mod 4 has been installed. Knowing this will guide you on whether or not you need the mono/stereo switch for proper fail safe operation when using a mono headset as discussed in the GMA 340 installation manual.



This picture helps explain the challenges of using a mono headset plug in a stereo headset jack.



Steve

 

[newt]

Thanks Steve (and Bob!) that’s given me something to check out at the hangar this weekend.

My guess is that mod 4 isn’t installed. Is that something an avionics tech can do, or is it a factory thing that’ll be unobtainium now that the GMA340 is EOL?

- mark

 

[BobTurner]

Bob - this is incorrect! No Garmin audio panel has ever connected the failsafe audio to only the ring terminal, including early

Steve

Steve, I sent you a PM.

 

[newt]

What'd it say?

- mark

 

[Longez]

Thanks Steve (and Bob!) that’s given me something to check out at the hangar this weekend.

My guess is that mod 4 isn’t installed. Is that something an avionics tech can do, or is it a factory thing that’ll be unobtainium now that the GMA340 is EOL?

- mark

Hi Mark,

I checked with Garmin today, and the Mod 4 update (board change) is no longer available for the GMA 340.

This really is a "don't care" if you are like me and have your plane wired with stereo headset jacks and only use stereo headsets.

Is your plane wired for stereo and are all of your headsets stereo?

Steve

 

[newt]

Is your plane wired for stereo and are all of your headsets stereo?

Yes.

Which thickens the plot a bit I guess.

I'll do some multimeter investigation over the weekend. The fact that it works with a rubbish headset suggests that I'm inadvertently discovering some kind of corner case.

- mark

 

[Longez]

Yes.

Which thickens the plot a bit I guess.

I'll do some multimeter investigation over the weekend. The fact that it works with a rubbish headset suggests that I'm inadvertently discovering some kind of corner case.

- mark

Mark,

That is good information.

For the headset that does work and has strong audio, are you hearing the radio in the left ear only (as you should) when the audio panel is turned off?

Same question for the headsets that have weak audio. Are you only hearing audio on the left side when the audio panel is turned off?

Do all of the headsets work normally and have comparable volume when the audio panel is turned on?

Steve

 

[newt]

For the headset that does work and has strong audio, are you hearing the radio in the left ear only (as you should) when the audio panel is turned off?

No, I'm getting it in both. It was the workshop headset I used to say, "Phew, that works, job's finished!" when I did a big rewire and panel upgrade last year.

Same question for the headsets that have weak audio. Are you only hearing audio on the left side when the audio panel is turned off?

My recollection is both, but that's something I'll have to confirm at the hangar at the weekend.

Do all of the headsets work normally and have comparable volume when the audio panel is turned on?

Yep. Everything it perfect when the GMA 340 is powered up.

I think the audio panel has some kind of preamp and audio signal processing. If that's all turned off, I'm wondering if there's an impedance mismatch between the GTR 20 and some headsets that could affect performance when they're directly connected.

- mark

 

[Longez]

No, I'm getting it in both. It was the workshop headset I used to say, "Phew, that works, job's finished!" when I did a big rewire and panel upgrade last year.



My recollection is both, but that's something I'll have to confirm at the hangar at the weekend.



Yep. Everything it perfect when the GMA 340 is powered up.

I think the audio panel has some kind of preamp and audio signal processing. If that's all turned off, I'm wondering if there's an impedance mismatch between the GTR 20 and some headsets that could affect performance when they're directly connected.

- mark

Hi Mark,

More good information.

The failsafe audio is only provided in the left side, so if you are hearing it on both sides, then your headset is acting as if the Mono mode is selected and both headset speakers are tied together. Not good.

I just repeated your test with a GTR 200 radio and a GMA 240 audio panel. I used a Lightspeed Zulu stereo headset and a Telex Stratus 50D stereo headset, both in stereo mode. The volume controls were turned to max on both sides on both headsets which is how I always use them (with a relatively low COM and intercom volume).

When in failsafe mode (audio panel turned off), I only hear COM audio on the left side of the headset. When the audio panel is on, I hear audio on both sides. This is working correctly as designed.

With the Zulu headset, the speaker volume is pretty much the same on both sides and the same when the audio panel is turned on or off.

I never recognized this before, but the left speaker on the Telex is noticeably quieter than the right, so when the audio panel is turned off and I am only using the left speaker, the radio is noticeably quieter and I would need to turn up the COM radio volume to use it in this manner.

Hope this helps in some way,
Steve