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Connecting a broadcast headset to a GA aircraft

Discussion in 'General Electronics Discussion' started by Evan Palumbo, Jan 3, 2017.

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  1. Evan Palumbo

    Evan Palumbo

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    Jan 2, 2017
    Hi guys,
    After a multiple-year hiatus from electronics hobbying, I've decided to take up a project to support a newer hobby of mine, aviation. Most of my experience is in microcontroller programming, and I'm a little lost In the analog world. Ironically, I actually engineer large PA systems as part of my job, but all pro audio equipment either has the same specifications, or you can buy a nicely engineered adapter to convert signal for you :)
    I found a headset that I have experience with that I would like to adapt to use in a general aviation aircraft. I'll try to post what the requirements are and what I have so far; if I could get some insight on whether what I have makes sense and whether there are better options, I'd be most appreciative!

    General aviation radios are still circuited as if there are pre-WWII carbon microphones connected to them. The radio is expecting a 50 Ω (from what I read) carbon mic, and provides a 8-12v bias voltage on the signal lines. Most modern headsets use an electret or dynamic mic with a small preamp to convert the signal, using the radio's bias voltage to power the circuit and output the correct levels. For those of you who don't know, carbon mics are less microphones and more audio-controlled resistors: they don't actually create voltage from sound energy but instead modulate the bias voltage and send it back to the radio.
    I, of course, am wanting to use a headset that's not exactly designed for GA use: the Audio Technica BPHS1. The microphone is a 560 Ω dynamic mic with a balanced output.

    Here's what I found so far. This looks to me like it works in theory.
    [​IMG]
    I would insert a 600 Ω 1:1 transformer where the microphone is in this diagram to convert the balanced signal to unbalanced for the amp. I realize that I could just ground one of the conductors and use the other for signal, but I feel like the interference cancellation could be useful in a potentially RF-noisy cabin. I'm not sure where the headset's ground would tie into the circuit.
    I'd also like to install a pot to control the level out of the amp, but am not sure where this would get inserted or what impedance pot I would need.

    The rest of the project--putting a volume control on the audio and a PTT switch (which basically just shorts tip to sleeve on the plug) is relatively straightforward.

    Hoping this project can benefit someone else as well once I get it working! Thanks!
     
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