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Dual com with GTR205XR and GTN750XI

Y

yjchai

Member
Hello, experts,

I am not an expert but learning in progress, and encountered this dilemma. I started with an idea of single com in my RV 8, and bought a GTR205xR first. Then, I became greedy and thought why not have a second com with GPS/NAV and bought a GTN750XI, not knowing what it takes to build an audio panel, or need to buy one.

I have two com antennas mounted and wired. GTR205xR works as intercom and so far works fine. It can have upto 4 Aux channels. It does not have the capability for audio panel, which I found out after I started tinkering. GMA35c costs 3k, but I'm trying to see if I need an audio panel for my mission, which probably is mostly day VFR, some formation, rare IFR.

I started to draw out plan for second PTT button on my throttle (first PTT to GTR205xR is an stick) linked only to GTN750XI with pilot MIC only, used for formation or emergency use. I would mostly use GTN750XI to listen to weather or monitoring, but GTR205xR has that function as well.

My question is, GTR205xR has com audio out(pin 16,37) and GTN750XI has com audio in (5,20). Would that work to transmit pilot MIC? Second PTT would be wired to GTN750XI pin 11. Alternate would be wiring pilot MIC to both devices, but I'm not sure whether that's the best method or I'm overthinking it.

In terms ot connecting GTN750XI audio, my plan is to connect as following. Aux1 - PFD, Aux 2 - GTN750xi main, Aux 3 - Com, Aux 4 - Nav.

If this makes sense and anyone can offer some advice before I damage anything, I would greatly appreciate it. Thanks!
 
If you hadn’t mentioned formation flying-where many pilots prefer to never remove their hands from the stick and throttle-this would be an easy answer. That said, here’s what I would do:
Get a ‘mixer circuit’. Buy for $60 or build yourself. Route all audio outputs (com1, com2, VOR, EFIS alerts) through it. Control individual volumes with their volume controls.
Install a double pole, double throw switch on the panel, marked ‘transmit 1’ and ‘xmit 2’, which switches the mike and PTT signals between the com radios.
Install a control line from your stick grip to a remote ‘active-standby flip’ input on your #1 com.
In flight, set com1 active to ATC frequency, standby to formation frequency. Panel switch to com 1. Now a single grip switch will flip between atc and formation.
Short answer: you don’t have 2 VORs, Marker beacon audio, adf audio. You don’t need a full up audio panel. Depending on how easy or hard it is to adapt the com 1 intercom to a 2 com set up, an independent intercom (with unswitched ‘mixer’ inputs may be needed.
 
Thank you for your response. I read several posts regarding mixer and poor man's audio panel etc and I think that would be most cost effective way. But if I have four aux channels to GTR205xR, and I don't have any other audio input, would I need to build a mixer?

My initial thougbt was to have both PTT button active, primary PTT to GTR205XR on the stick and secondary PTT to GTN750xi on the throttle, thereby remaining hands on during occasional times I want to fly formation. Does one PTT button need to be deactivated to properly send MIC to transmit?
 
yjchai said:
Thank you for your response. I read several posts regarding mixer and poor man's audio panel etc and I think that would be most cost effective way. But if I have four aux channels to GTR205xR, and I don't have any other audio input, would I need to build a mixer?

My initial thougbt was to have both PTT button active, primary PTT to GTR205XR on the stick and secondary PTT to GTN750xi on the throttle, thereby remaining hands on during occasional times I want to fly formation. Does one PTT button need to be deactivated to properly send MIC to transmit?
Click to expand...
You’ll have to read the 250 manual carefully to see if its internal intercom can function properly when a second radio is being used. I don’t know.
As to two PTT buttons. Yes, this will enable you to select which radio transmits. But, in a small airplane it’s highly likely that the signal from #1 will ‘bleed over’ (force its way) into radio #2 (you just can’t get the antennas far enough apart), so unless you have a way to mute #2 when transmitting on #1 (and vice versa) you may get horrible noises from #2 in your headset when transmitting on #1.
 
Assuming you have a G3X why not install a GMA245R and enjoy all the benefits of an audio panel?
 
The easiest solution would be GMA35C with GTN750xi, but then I'm trying to make an excuse not to buy another avionic and doing experimental thought exercise and asking if anybody has tried this combination of dual com without an audio panel. If not, I guess I just have to wire them up, push some buttons and see what happens.
 
yjchai said:
The easiest solution would be GMA35C with GTN750xi, but then I'm trying to make an excuse not to buy another avionic and doing experimental thought exercise and asking if anybody has tried this combination of dual com without an audio panel. If not, I guess I just have to wire them up, push some buttons and see what happens.
Click to expand...
What did you wind up doing?
I’m planning a dual comm setup when upgrading my Rocket panel in September. My plan is similar to yours, with two PTT buttons.
After reading Bob Turner’s post above, I’m wondering whether I’ll need to mute the opposite radio when transmitting, and if so, how to do that?
 
GyroF-16 said:
What did you wind up doing?
I’m planning a dual comm setup when upgrading my Rocket panel in September. My plan is similar to yours, with two PTT buttons.
After reading Bob Turner’s post above, I’m wondering whether I’ll need to mute the opposite radio when transmitting, and if so, how to do that?
Click to expand...
If you have 2 PTT buttons, why do you think you need to mute the 2nd radio?
 
Pilot135pd said:
If you have 2 PTT buttons, why do you think you need to mute the 2nd radio?
Click to expand...
Well, as I mentioned in my first post, Bob Turner (in Post #4 above) wrote:

“As to two PTT buttons. Yes, this will enable you to select which radio transmits. But, in a small airplane it’s highly likely that the signal from #1 will ‘bleed over’ (force its way) into radio #2 (you just can’t get the antennas far enough apart), so unless you have a way to mute #2 when transmitting on #1 (and vice versa) you may get horrible noises from #2 in your headset when transmitting on #1.”

So I was wondering if that turned out to be an issue for the OP.
 
Walt said:
Referred to as "Interlock" (ITLK), both units provide an input for this function.
Click to expand...
Ah- that sounds promising.
I’ll be using a GTN 635Xi and a Dynon SV-COM-760 remote mounted radio.
I’d hope that both of those have that input as well.
Not planning a comm panel, just a Dynon intercom.
 
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I did use two ptt buttons, one on throttle and one on stick. On ground test, I didn't have any noticeable bleed over problem. I wired interlocks for the sake of completion and future proofing. I wired GTN PTT to GTR interlock in since it didn't have a specific pin for it.
 
GyroF-16 said:
Ah- that sounds promising.
I’ll be using a GTN 635Xi and a Dynon SV-COM-760 reomote mounted radio.
I’d hope that both of those have that input as well.
Not planning a comm panel, just a Dynon intercom.
Click to expand...
No need for the Garmin interlock if using the Dynon (Trig) radio. It has a solid receiver front end, providing very good selectivity. I run a GTN-650 and this radio and never had interference on the Dynon receive while transmitting on the GTN-650.

Carl
 
GyroF-16 said:
Well, as I mentioned in my first post, Bob Turner (in Post #4 above) wrote:

“As to two PTT buttons. Yes, this will enable you to select which radio transmits. But, in a small airplane it’s highly likely that the signal from #1 will ‘bleed over’ (force its way) into radio #2 (you just can’t get the antennas far enough apart), so unless you have a way to mute #2 when transmitting on #1 (and vice versa) you may get horrible noises from #2 in your headset when transmitting on #1.”

So I was wondering if that turned out to be an issue for the OP.
Click to expand...
Today's radios aren't the same as older radios. Shouldn't be a problem.
 
Carl Froehlich said:
No need for the Garmin interlock if using the Dynon (Trig) radio. It has a solid receiver front end, providing very good selectivity. I run a GTN-650 and this radio and never had interference on the Dynon receive while transmitting on the GTN-650.

Carl
Click to expand...
Thanks- that’s a valuable data point for me.
Is the opposite also true- that is, no interference on the GTN-650 when transmitting on the Dynon (Trig)?
I’m planning to use the Dynon as the primary radio.
 
Walt said:
Assuming you have a G3X why not install a GMA245R and enjoy all the benefits of an audio panel?
Click to expand...
I'm with Walt the GTN 750XI is a $ 18,500 box and a GMA 245R is a $ 1,700 box. At a minimum improves resale value and you can enjoy the benefits in the meantime. I do !

Also, without the audio panel no "Terrain System Test OK" from the GTN. (I think) And if the audible "click" when you enter data (one can mute this if desired) you won't get it. Personally, I like the "clicks".
 
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