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Aviation headset test amp

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ShawnR

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Hi
I have occasion to work on aviation headsets occasionally and rather than buy an aircraft intercom to just use on the workbench, I am thinking there might be an easy solution with parts on my bench. The mics are called amplified electret.

Spec sheet of a typical set is here http://www.davidclarkcompany.com/files/literature/10-13.4.pdf

I have an old set of computer speakers that I thought might be just the ticket but connecting them together (headset mic to speaker input) did nothing so I am thinking that I need a small preamp. I don't understand the DC voltage or how to incorporate it in my test amp, that is mentioned in the literature (Mic DC Supply V. )

Would an LM386 do the job? http://www.ti.com/lit/ds/symlink/lm386.pdf

Or maybe there is an easier way. Since the speakers are brought out on one plug and the mic on the other, I could probably connect, through an amp, the mic to the same headset speakers. The most common fault is broke wires but occasionally, a bad mic or speaker or some have volume controls. By having them as one complete system, I would be talking to myself while wearing them. In the past, I might hold one set to talk into the mic and wear the other set.

For straight audio, I was planning on simply connecting an audio source but the speaker impedance is 150 ohms and most audio outputs are designed for 8, no?

Any suggestions?
Thanks
Shawn
 

AnalogKid

Well-Known Member
Most Helpful Member
As indicated on the datasheet, you need to provide a DC power source for the microphone to operate. This comes down to the pinout of the mic connector. There probably is a standard within the aviation audio world. If you find it and post it, we can go from there. The 400 mV output should be enough to drive PC speakers that have a small power amp built in.

Separate from that, the LM386 is a small speaker amp chip. While there are modules on ebay that use it as a mic preamp, the chip is a very poor choice for that due to excessive noise (at microphone signal levels), distortion, etc. It is attractive because it has lotsa gain, needs very few external components, and biases itself internally for operation with a single power supply.

ak
 

audioguru

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Most Helpful Member
The spec's for the headset say to power the mic, use 470 ohms from 8VDC to 16VDC. If the amplified computer speaker does not have an input coupling capacitor then add one.
 

ShawnR

Member
The only schematic I have is this...

http://www.davidclarkcompany.com/files/partslists/pdf/PL19515P-39.pdf

I will try to find an intercom wiring schematic.

The mic uses a connector .203" I believe, smaller than the speaker plug which is a standard 1/4", and one mic wire goes to the shank and one goes to the ring. I have a portable radio that can be used with an adapter. I will see if I can take some voltage measurements. What you two are saying is that I should find a DC voltage between those two connections in the mic jack...?
 

AnalogKid

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If you are sure the mic plug has only two contacts (tip and sleeve), then the schematic on the site you linked is not quite what you need. It probably (confirm, anyone) is the case that the hot side of the mic goes to the tip and the case/ground/return side goes to the sleeve> At the connector, the sleeve goes to your system ground, the tip goes to *both* a resistor that goes to a DC power source and a coupling capacitor to the amplifier input. Below is an example. In your case, M1 is the jack for the mic cable, R1 goes to the DC power source, C1 goes to the input of your powered speakers, and everything else does not apply.

ak

 

audioguru

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Most Helpful Member
The David Clarke Headset says it has an amplified electret mic. It says you should feed it through a 470 ohm resistor from 8V to 16V. Its output level is 400mV when you scream at 114dB and will be about 40mV at 94dB which is also pretty loud. Its output impedance is 150 ohms and its level are like an old carbon mic.
An amplified computer speaker needs more input level so a preamp is needed. The preamp shown in post #6 needs its 2.2k resistor replaced with 470 ohms. Its gain of 100 is fine when your voice level is 80dB.
 

ShawnR

Member
Thanks both AK and guru
AK, the mic connector is a 3 conductor plug. The mic signals are carried on the ring and shank. To cause the radio to transmit, the procedure is to incorporate a switch so that the tip of the mic plug goes to ground. This switch can be a male female setup that goes between the headset and the radio or intercom connection ie https://www.aircraftspruce.ca/pages/av/ptt/asaPTTSwitch1.php or just a pushbutton switch mounted into the yoke that the technician wires in to complete the circuit to ground for transmit purposes. I am sure your other suggestions are correct, it is just the wiring of the actual connections that differs. In General aviation aircraft, we talk on the intercom all the time but only the pilot gets to transmit when he pushes the switch. Hence, the audio is always functioning. The tip is reserved for the transmit function.
 

ShawnR

Member
AK and Guru

I wanted to say Thanks! The circuit works great, exactly what I wanted! Although I built the circuit very close to what AnalogKid posted, I used an LM358 as I noticed the OPA344 had a max Vcc of 5.5 and the Electret mike, as audioguru pointed out was 8-12 volts. Plus, I had the 358's in stock. I should have built one years ago. Now to put it on a circuit board and into a box.

And, I learned something about Electret mics...

Thanks!
Cheers and have a nice Christmas
Shawn
 

audioguru

Well-Known Member
Most Helpful Member
Your LM358 dual opamp uses the same opamps in an LM324 quad opamp. They are awful for audio because they cause crossover distortion. It causes any sound to make a buzz.

But maybe aviation audio is severely distorted anyway? An hour ago I played a video of two US jet pilots talking about a UFO they were watching. Their speech could not be understood due to the extreme distortion and lack of sibilants and consonants of speech (high frequency cutoff) so that the video used subtitles of what they were saying and even the subtitles missed some words. "au i a!" (What is that!). "ee, i i urneen" (See, it is turning).

Here is what the crossover distortion looks like on a scope of a sinewave:
 

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AnalogKid

Well-Known Member
Most Helpful Member
But maybe aviation audio is severely distorted anyway? An hour ago I played a video of two US jet pilots talking about a UFO they were watching. Their speech could not be understood due to the extreme distortion and lack of sibilants and consonants of speech (high frequency cutoff)
Probably throat mics (which sound very poorly), then analog-compressed, then digitized, then digital-compressed, then sent on narrow-band FM. General aviation audio from things like the DC headsets is vastly better.

ak
 

audioguru

Well-Known Member
Most Helpful Member
I agree that throat mics and digitized garbage sound like a person speaking only in vowels, not words.
Ee, oo au.
 
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