Garmin GTR-200 intercom

[Rhino]

This is a first draft. I tend to draft things, put them down, and come back later to recheck everything. I usually do that several times, so this could change more than once. Plus, others could point out mistakes or make suggestions.

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This configuration switches the radios, not the microphones. Both mics are hot on one radio when either pilot keys the PTT switch, but which radio is being used depends on where the switch is set. This is an audio distribution diagram only. Power, network and serial connections are not depicted.

EDIT: Second draft posted.

 

[Rhino]

One concern I have here is microphone bias. Since we're using a mixer, the bias voltage an intercom or radio normally sends to the mic may not be available. I'm pretty sure there's a way around that, but it complicates things a bit. I'm going to let Vern look at that.

 

[Jaque]

Wow, I haven’t even had a chance to get back to my desk to start gathering schematics. Thanks Rhino, I’ll see if I can catch up!

 

[Rhino]

Was only considering the first scenario. I’m used to just having the secondary radio set up for up coming frequencies en route anyway and as a back up of course.

Actually, you'll hear three frequencies at once if you use the monitor function for the standby frequency on the GTR 200.

 

[Rhino]

Wow, I haven’t even had a chance to get back to my desk to start gathering schematics. Thanks Rhino, I’ll see if I can catch up!

The wiring was easier to figure out than trying to make it fit nicely on the page was.

 

[Jaque]

I spent most of the day moving snow!

 

[Rhino]

Already posted draft 2. I forgot to put shielding on all the audio lines. All audio cables should be shielded.

Not much snow in Tennessee.

 

[Rhino]

Do you have a 12 volt system? If Vern says the AMX can accept a bias voltage on the input pins, we should be able to just tie system power to them. I might at least add a filtering capacitor though.

 

[Jaque]

12 volts. I took the shielding for granted.

 

[Rhino]

Added an example pinout for the switch.

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[Rhino]

I forgot to mention earlier, if you want to let the copilot listen to everything, but not have transmit capability, this becomes WAAAAY simpler.

 

[Rhino]

Just realized we'll need to jumper the mics to the GTR somehow or you won’t be able to talk on the intercom when the switch is in the Dynon position. I'll look at that later.

 

[Rhino]

Okay, I created my own problem by overcomplicating this. We don't need to switch audio now that it's mixed. We only need to switch PTT. That also solves the mic bias issue, and makes the wiring simpler. I'll redo it later.

 

[Rhino]

Remember what I said about drafting and then coming back to revisit and make changes? Yeah.

This does not fix the mic bias issue like I thought, but it does fix and simplify the other stuff. Still want input from Vern on the mic bias.

The standard pin 11 output we were using on the AMX-2A can't go to both the GTR 200 and the Dynon radio simultaneously because hooking up two devices in parallel divides our impedance down to 75 ohms, and we want 150. However, pin 12 of the AMX has double the standard output impedance, so we'll use that instead. That should still give us 150 ohms to each the GTR 200 and the Dynon radio.

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Stay tuned until later today or tomorrow, when I'll probably discover even more mistakes.

The AMX-2A has four adjustable inputs used to balance or adjust input audio levels. We aren't using them here because headsets shouldn't need balancing. Be aware though, that if you ever decide to use the variable inputs, you have to access the AMX wherever you installed it to make adjustments with a screwdriver. It isn't anything you want to attempt adjusting in flight.

As noted on the diagram, don't use the additional audio inputs on the AMX for anything else. Don't want to be broadcasting your gangsta rap to ATC. The GTR 200 has other audio inputs to use for that kind of stuff anyway, and you could always add another mixer if you needed more.

EDIT: Simplified the diagram and added notes.

 

[Rhino]

I will also note that it's common practice to ground all shielding at the audio panel or intercom to reduce the possibility of ground loops. You'll notice I haven't done that on this diagram. The GTR 200 doesn't have a dedicated pin for grounding audio. Using power ground has the potential to be problematic sometimes. Garmin asks you to ground the shields to the connector backshell, but this is also an imperfect solution that requires extra parts, and you may not have done that. Since the HDX has a dedicated pin for audio ground, I drew the diagram using that. If you have shielding grounded to the GTR backshell though, using that would be preferable to using the pin on the HDX.

The headset mics only go to the AMX, so their shielding is grounded there.

 

[Jaque]

Finally got some time to pull the Garmin and Dynon literature but looks like you’re way ahead of me. Thanks again!
Definitely want to keep it simple and no problem with 2 live mic’s, not the preferred scenario but use to it now with the GTR. Certainly no rush on the project as it’s dropping to -25C this week, probably working on in house projects for a bit. Oh what I’d give for heated hangar space.

 

[Rhino]

I will also note that it's common practice to ground all shielding at the audio panel or intercom to reduce the possibility of ground loops. You'll notice I haven't done that on this diagram. The GTR 200 doesn't have a dedicated pin for grounding audio. Using power ground has the potential to be problematic sometimes. Garmin asks you to ground the shields to the connector backshell, but this is also an imperfect solution that requires extra parts, and you may not have done that. Since the HDX has a dedicated pin for audio ground, I drew the diagram using that. If you have shielding grounded to the GTR backshell though, using that would be preferable to using the pin on the HDX.

The headset mics only go to the AMX, so their shielding is grounded there.

Browsing the Dynon manual and they say to ground both ends for alert audio. So if you have the backshell ground on the GTR 200, connect it to pin 30 on the HDX. If you don't have the backshell ground on the GTR, go ahead and test the system without it. You may not need it.

 

[Jaque]

The HDX audio ground pin 30 is connected to pin 31, Audio lo, on the Garmin. I used the R232 serial bus connections for the RX and TX inputs.
I do have a back shell with pin 20 and the shield ground connected to a common ground terminal. The radio works great in this configuration.

 

[Rhino]

Well, Vern confirmed we can put the mic inputs to the AMX, but he didn't say anything about the bias voltage. I'm leaning toward putting 12 volts system power into both mic lines, and inserting a capacitor in series before the line goes into the AMX. That should block the DC bias voltage from reaching the AMX, but it could also affect impedance, so I was hoping Vern would just say 12 volts into the AMX was okay.

Something like this:

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But this is preliminary. I'd want an engineer type, like Vern, to confirm this would work.

 

[Rhino]

New draft. Added the chassis block ground and corrected the mic jack wiring.