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Thread: Boom Microphone

  1. #1

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    Boom Microphone

    I have aquired a M-33A/AIC microphone which I would like to use with my flight helmet. The M-33A is a dynamic microphone and I believe it has an impedance of 3 ohms. What do I need to do in order to use this microphone with a civilian radio? Perhaps it would be cheaper to purchase a different mic. I have tried using an impedance adapter for going from a military helmet to a civilian radio and this resulted in a squeal but no voice.

  2. #2
    rwanttaja's Avatar
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    Quote Originally Posted by exftrplt View Post
    I have aquired a M-33A/AIC microphone which I would like to use with my flight helmet. The M-33A is a dynamic microphone and I believe it has an impedance of 3 ohms. What do I need to do in order to use this microphone with a civilian radio? Perhaps it would be cheaper to purchase a different mic. I have tried using an impedance adapter for going from a military helmet to a civilian radio and this resulted in a squeal but no voice.
    A civilian mike element costs $30-$100. I make a slim-line headset to wear under a leather helmet, instructions are at:

    http://www.bowersflybaby.com/tech/h_headset.html

    Ron Wanttaja

  3. #3
    Derswede's Avatar
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    What does your transmitter call for? Depending on what the transmitter is looking for, you have several possibilities. A cheap Electret Mike element may work just fine. I have made mikes for radios using electret Mike elements out of old cell phones, misc radios, etc. An element is a couple of bucks. First see what your radio wants/needs for input, then look for a match on the element. Several good articles are on "eham.net". Or do a google search for microphone impedance eham and thy should pop right up.

    Derswede

  4. #4
    rwanttaja's Avatar
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    Quote Originally Posted by Derswede View Post
    What does your transmitter call for? Depending on what the transmitter is looking for, you have several possibilities. A cheap Electret Mike element may work just fine. I have made mikes for radios using electret Mike elements out of old cell phones, misc radios, etc. An element is a couple of bucks. First see what your radio wants/needs for input, then look for a match on the element. Several good articles are on "eham.net". Or do a google search for microphone impedance eham and thy should pop right up.
    If it's a traditional aircraft radio, then it wants a carbon mike.

    Seriously, that's the standard...when technology triggered improvements in aircraft radios, they required compatibility with the same 'ol carbon mikes planes had been using from the '20s (which were adapted from switchboard microphones).

    What this means is that even if an aircraft mike uses a dynamic or electret element, it still must include the circuitry to mimic the old carbon mike. THAT'S why trying to adapt a non-aircraft mike element to an aircraft mike is a crapshoot. A good designer might make the radio compatible with straight dynamic or electret types, but they're not required to.

    Ron Wanttaja

  5. #5
    Derswede's Avatar
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    Ron, most of the newer handhelds are going to use electret mikes, but the key point is what does the manual say for the radio and what impedance the radio needs. If the OP can give us that info, it makes it easier to give a proper guess. You are right, the older radios were all set for crystal mikes. And the circuitry to adapt one to the other is possible, but the radio type info is the most needed most.

    Derswede

  6. #6

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    Thanks for all your comments. I will be using the mic. with an ICOM hand held. I understand that all US aviation radios are designed for a carbon mic. I use the hand held for open cockpit airplanes. I thought the dynamic mic. would work better with the noise. I think I have found a way to adapt the mic. by using an impedance adapter and an amplifier.
    Thanks again

  7. #7
    Yes, you can buy an adapter from Pilot or Marv Golden that will change the mic from dynamic to carbon-equivalent, and it also changes the 8 ohm headphone drivers (if it's a military helmet) to 600 ohm civilian standard. The circuit is not complicated.

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