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DIY ANR Headset

M

mlavigne

I'm New Here
Hi Everyone,

Long time lurker, first time poster. I'm saving up for an RV10 kit, but in the meantime I am in a flying club with a few Archers. Most of the club members are strict pilots and couldn't answer my technical questions- I figure the members of this forum would probably know the ins-and-outs of avionics off the top of their heads.

TLDR:
I'm looking for some incite into how the mic input works on a standard audio panel- namely, if something is wrong with my microphone circuit are they short tolerant or could that destroy the panel?

Details:
I have (for non-aviation use) a set of cowin (or insert other cheap imported) noise canceling headphones that are dirt cheap ($40) and work well in cockpit. I am looking to modifying them into an aviation headset - primarily for passengers as i cant rationalize owning 2-3 additional headsets at $500 (min) each. My goal is to be able to add a ~$75 attachment (adds a mic and matches the impedance on the audio) to my $40 ANR headset and have a nice option for my passengers.

The speaker part is easy- just a 300ohm to 8ohm audio amp which works, the mic is more interesting. I built an electret condenser to carbon mic conversion circuit to take advantage of the mechanically noise canceling mic's that are available for fairly cheap. I put my normal headset on a 10V power supply and spoke into it while recording the signal on an oscilloscope, and did the same with my test circuit and everything looks good. My major concern is that, because the audio panel is a 'black box' that i don't fully understand, that it is possible to damage the panel if the circuit is not correct. It is an un-powered circuit (the audio panel provides the power) so my main concern would be a dead short.

Is there anything I should be concern with? Am I overthinking this?

Thanks,
Michael.
 
Well, never say never, but I think it should be okay. Looking into an audio panel, the mike will see: (1) a bias circuit and (2) input to an audio amp. (1) is usually a 470 ohm or so resistor to 10-12 volts. The resistor limits the current in the event of a dead short (as well as defining in the input impedance) so it?s pretty fool-proof. (2) generally starts out with a dc blocking capacitor, so, again, it?s pretty hard to hurt it. Standard aircraft mikes will put out about 0.1 v (rms) when you speak loudly into them, so that?s roughly the signal level. Note any mike feeding a transmitter, you?ll want to hit the aircraft specs as accurately as possible.
 
Thanks Bob,

That was my assumption, but better safe than sorry.

I tuned the condenser mic circuit to match my QT Halo's on the scope with the same mic hood and DC bias voltage, so it should be close.

I'll report back on how it works and post schematics if everything is good.

Michael.
 
Welcome to VAF

mlavigne said:
Hi Everyone,

Long time lurker, first time poster.

Thanks,
Michael.
Click to expand...

Michael, welcome aboard the good ship VAF:D

Headsets Inc has some goodies you might take a look at. Here is a link to their microphone setup. https://www.headsetsinc.com/product-category/accessories/page/4/

I used their ANR conversion kits with older David Clark headsets, and love them. Also did a couple of FlightCom headsets. Good products.

They also sell onboard power setups, so no need for a battery case----and no batteries going south at the wrong time.
 
I think that this is somewhat related to your question.

I fly with a headset that I built myself last year. It is basically a spliced together collection of the following ACS parts

  • SIGTRONICS HEADSET SHIRT CLIP
  • .206" COMMERCIAL MIC PLUG
  • TWO PIN MICROPHONE PLUG
  • DAVID CLARK ELECTRET MIC M-7A
  • DAVID CLARK MIC MUFF M-7A MIC
  • SHIELDED 24GA 2 CONDUCTOR WHITE WIRE M27500-24TG2T14
and Amazon

  • Shure SE215-K-UNI Sound Isolating Earphones, Black with Universal Cable
  • Shure EAADPT-KIT Adapter Kit (Combines 1/4 inch Adapter, Airline Adapter, Attachable Volume Control)
and finally a length of 8GA solid core wire that I used for a frame.

It isn't pretty, but it works well and wasn't difficult to assemble. I took my direction from this web site. I don't recall how I found this site, it may have been from a post on this forum.

All told this cost me about $200 Cdn, most of the cost is in the headphones and the microphone. It took me a couple/three hours to build. The end result works well, but it doesn't block as much noise as I'd like and can get uncomfortable (as any in-ear monitor can). If you use a similar approach to adding a microphone to headphones, I think you could make a pretty good headset.
 
One thing to watch out for, when the batteries in the headset die, does the headset become a passive headset? ie. will it still pass audio to your ears? If not, such a headset could be a liability in flight. When the batteries die half-way to your destination, you'll have to pull out your spare headset.
 
Yea, the ones i have die with the battery. There are slightly more expensive ones ($100) that i have found that will become passive. My primary focus is for passengers at this point (not safety critical).
There are a few people making these devices (uflymike and nflightmic, and a few in Europe i think), but at $250+ each and you still need the $40-350 ANR headphones, they are not cost effective at all.
 
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