Use aviation headsets with phone

[Holton181]

So, as the title suggests, I like to use my aviation headset with my Android (or other type) phone. Not only to listening, but also talk on the phone.
But I'm having some issues I like to sort out before I go serious with this.

First, what I have and have done so far:
The aviation headset (hereon AHS) is a new Quiet Technologies Halo with a audio input jack. On the PCB of the volume control box, for each channel and in series from the audio jack to the earphone traces, there are one smd 100ohm resistor and a 1210 ceramic capacitor of unknown value. No possibility to switch the 3-pol audio jack to a 4-pol phone jack, so I plan to use the same values of these components for my own connection point (between headset and volume box). I'll find a way to figure out the cap value.

For the electret mic on the AHS:
From David Clark:
"Electrical Specifications–Microphone Model M-7A (P/N 09168P-33)
DC supply voltage and source resistance: 8 to 16 volts, 220 to 2200 ohms-not polarity sensitive. "
But electret mics can often be biased by lower voltages than that
Android headset specifications **broken link removed**

I have an old Android headset where I opened up the little mic-n-button-box and removed the mic from the small PCB, attached two wires at its place and connected them to the AHS (not actually on the AHS, but in a genav-to-helicopter adapter). The 2.9V from my phone seams to be sufficient to bias the mic. I measured the dB with an app on the phone and compared to an unmodified headset (not calibrated scientific values, but about the same). Also made some phone calls with good sound quality of my voice.

Now to my issues:
1) Bias isolation
I like to be able to talk in the phone while either connected to my aircraft or not, and be able to disconnect/reconnect to my aircraft while doing so. I need some automatic isolation between the com-radio and android bias.
Would a cap in parallel with a diode between mic high on AHS and the positive mic wire from the Android be sufficient?
When connected to aircraft the signal pass through the cap while DC is blocked.
When disconnected bias supplied through the diode (hopefully the 0.7v drop won't cause issues).

2 & 3) Mic Low VS GND and Bias polarity
So, on the AHS, the mic has two connections, Mic High and Mic Low. Mic Low is not connected to headphone GND as it is on the android counterpart. So if I connect the AHS mic to the android headset as described above, Mic Low will be short-circuit to GND when connecting android headphone.
What problems might this lead to?
I found an interesting wiring diagram:
**broken link removed**
Here one can see that headphones are connected to GND, and mics to "Point A" i.e. "Intercom Central Ground Point". Not the same?
But looking at the David Clark Specifications again: "...not polarity sensitive."
This imply the polarity of the mic bias might be switched, Mic Low get the 8-16V and Mic High are grounded!
This might complicate things.
Any ideas?

Any circuit related suggestions are welcome.
But please avoid "go spend money on this or that instead" kind of suggestions, I know there are commercial solutions available, but whats fun with that?
And if there are commercial products available, someone need to know how to build one...

BR
/H

Edit:
Realized the second issue was actually two.

 

[Holton181]

Realized while laying sleepless in my bed, that issue nr 3 Bias polarity is a non-issue. Remembered that there is a rectifying bridge mounted on the underside of the PCB making the higher potential go to Mic High.
And since I plan to add my phone connection between the volume box and headset I don't need to bother about polarity.
Yay, one down, two to go!

 

[Holton181]

Still awake...
My suspicion about issue nr 2 have been confirmed, different ground for shielding mic from potential noise sources.

I guess that shield will be broken somewhat when connecting as described. But will it be the end of the world?!

 

[dr pepper]

Do you have a schematic for the headset, your description is tricky to follow.
If the relavent levels are totally different then you wont be able to do this without some electronics.

 

[spec]

Hi Holton,

Welcome to ETO. Which part of Sweden are you from? Care to put it next to 'Location' on your user page so that it shows in the box at the left of your posts.

What plane do you have? Any pictures?

spec

 

[vne147]

I'm afraid I won't be much help to you here, but I did a similar project once. I made a box to connect a David Clark H10-30 headset to my cell phone. I use it to call for IFR clearance from uncontrolled airports prior to takeoff so I can make the call without having to shut down my engine. With my solution though, I had to unplug my headset from the airplane and then plug it into the box to make the call. I can't make a call and still listen to the COMM radio at the same time. If I remember, my mic was an older M1 and I biased it with a 9V battery.

With respect to your question about the GND vs point A, my guess is (and it is just a guess), that point A is specified instead of the generic ground to avoid ground loops. That's an electrical ground loop, not this kind of ground loop.

Anyway, I'm very eager to see what solution you come up with and would be grateful if you'd share it with me when you figure it out.

Thanks.

 

[Holton181]

dr pepper
Sorry, when you have your private project in front of you and almost everything sorted out in your head, its easy to forget others don't. I'll try to draw something up when I get the time (not often, being home taking care of a 16 month Duracell rabbit i.e. my daughter)

spec
Hi and thanks spec!
Location updated. I don't fly planes, to easy, wings to firmly attached to the fuselage. Helicopters adds some real excitement to your life
No pictures, but so far I only fly the Robinson R44, got my CPL(H) this august

vne147
Thanks for sharing. I first planed to bias with a 9V battery, too, as in this article (I plan to use the same caps for DC blocking though). But then I realized the power provided by the phone are sufficient, no need of a separate source (hopefully). The main thing now is to find how to automatically block the aircraft bias (8-16V) from feeding the phone (1.8-2.9V) jack and still have the signal going to both radio and phone. Maybe a single diode isn't sufficient after all if I like to avoid ground-loops (both of them)?
"Aircraft Wiring for Smart People" is a rather interesting document regarding this issue, I just found out. Unfortunately, since the android headset (and iPhone for that mater) has common GND for speakers and mic, conflicting to whats suggested in the document, it might be a tough challenge to get rid of ground loops.

 

[dr pepper]

Ok we'll all probably understand your situation better with a schem.
On ground loops, you can get car audio ground isolators cheaply, when pulled apart theres just a pair of audio isolation transformers inside, these are kinda usefull for this kind of problem.

 

[vne147]

I only fly the Robinson R44, got my CPL(H) this august

Congrats on earning your CPL. I got mine for single and multi-engine airplanes a few years ago. I always wanted to fly helicopters, but they're just too expensive for me. Right now I'm working on my instructor rating for airplanes.

 

[spec]

Don't let MikeMl see this thread.

spechttps://www.electro-tech-online.com/members/mikeml.125427/

 

[Holton181]

dr pepper
The mic is biased with DC on the same wire as the signal, either from the aircraft transceiver or my phone. I'm afraid the transformer solution would block the bias. It's not only my own headphones that might be affected by the potential ground loop, but I believe also the (very sensitive) mic will, and so it will be broadcast to everyone in the aircraft and to the ATC whenever I'll talk to them? Maybe I'm wrong?

vne147
Indeed expensive, but SOOO satisfying
Good luck with your IR!
Maybe something I would have taken too if I had any money left...

spec
It was actually reading some of his posts on other similar threads here that lead me to register and submit my issues...

 

[spec]

It was actually reading some of his posts on other similar threads here that lead me to register and submit my issues...

As you probably know Mike is a pilot, and has a plane.

I'm ex Royal Air Force, so I have clambered over many aircraft, but never flown one, except on the end of a control line. I did land an Anson once when the pilot let me have the stick but I made a complete hash of it and came down on one wheel.

spec

 

[Holton181]

I got that feeling MikeMl being a pilot knowing one or two about these things.

Sounds exciting being a Royal Air Force guy. Got to see all those aircrafts up close.
You can't really be blamed for landing hard on your first try, can you spec
Besides, I assume the pilot would have taken over the control if he didn't believe you would make it.

 

[spec]

I got that feeling MikeMl being a pilot knowing one or two about these things.

Sounds exciting being a Royal Air Force guy. Got to see all those aircrafts up close.
You can't really be blamed for landing hard on your first try, can you spec
Besides, I assume the pilot would have taken over the control if he didn't believe you would make it.

Yeah, I liked the aircraft and the technical side, but I wasn't too keen on the spit and polish and marching around the place.

The Anson pilot was an old hand, and was laughing his head off at my antics, but I could tell that he was ready to sort any serious problems.

I have met quite a few pilots, from the 'steely eye' fighter pilots as we called them, to the transport pilots, to test pilots. Also some WW2 pilots, then desk bound. The RAF pilots, any way, all seemed to have a gung-ho, can-do, fearless attitude, and always had plenty of good yarns. If the riggers told the skipper, "Sorry but there is a crack in the main spar.", he would just as likely reply, "Yeah, but can I fly tomorrow morning?"

And chopper jocks were plain crazy, especially the Army ones.

spec

 

[Holton181]

So dr pepper and others, I finally got the time to draw a schematic of the two headsets and my planed interface:

* The "Volume R/L" are just a 130ohm resistor and a 530ohm pot in a attenuator configuration not further investigated, not important for this project
* All four caps with unknown value are of the same value and type, ceramic (1210 SMD on the AHS)
* The 22uF are taken from this article

Does this make any sense?
Will the bias isolation be sufficient?
As you can see the Mic Low will be shorten to GND via the Mic-n-Button-box

 

[spec]

On your diagram, the signal from the mic does not appear to go anywhere.

spec

 

[Holton181]

I see what you mean. I didn't add the signal receiving end since I do not know how the transceiver is constructed internally, only the specifications for the bias.
The mic is an electret with an integrated amplifier, the signal uses the same wires (mic high, mic low) as the bias.
Bias from the transceiver are polarity independent.

 

[Holton181]

So, no one has any thoughts whether this could work or not without to much bad things happening?