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Headset microphone level reducer

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etienne.phillips(at)gmail
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Posted: Mon Jan 02, 2012 12:10 pm Post subject: Headset microphone level reducer

Hi All



I have two dissimilar headsets that have very different microphone output levels. The one headset is an oldish basic Avcomm model, and the other is a new child-sized one. My MGL radio/intercom doesn't have the ability to reduce the volume of the new headset adequately.



I've never worked with electret microphones before, so am not sure how to build a signal reducer. I have noticed that they are polarity sensitive, which leads me to expect some phantom power is required...?



I need to reduce the volume by about 15dB (according to the MGL radio) to match the Avcomm. Ideally, I'd like to be able to build a passive circuit that I can squeeze into the headset itself, rather than an interface box or off-the-shelf device. This will allow me to use the new headset in other aircraft as the Avcomm's seem to play nicely with other major brands.



If anyone has some insight, I'd be most appreciative!



Thanks

Etienne

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nuckolls.bob(at)aeroelect
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Posted: Mon Jan 02, 2012 3:36 pm Post subject: Headset microphone level reducer

At 12:29 PM 1/2/2012, you wrote:


<etienne.phillips(at)gmail.com>



Hi All



I have two dissimilar headsets that have very different microphone 

output levels. The one headset is an oldish basic Avcomm model, and 

the other is a new child-sized one. My MGL radio/intercom doesn't 

have the ability to reduce the volume of the new headset adequately.



I've never worked with electret microphones before, so am not sure 

how to build a signal reducer. I have noticed that they are polarity 

sensitive, which leads me to expect some phantom power is required...?



   Are both microphones intended for interface to an aviation

   transmitter/intercom inputs? 15 db is a BIG difference.



I need to reduce the volume by about 15dB (according to the MGL 

radio) to match the Avcomm. Ideally, I'd like to be able to build a 

passive circuit that I can squeeze into the headset itself, rather 

than an interface box or off-the-shelf device. This will allow me to 

use the new headset in other aircraft as the Avcomm's seem to play 

nicely with other major brands.



   An aviation microphone, is powered by a bias current

   applied to a combination of ELECTRONICS as an amplifier

   and the electret microphone element.



If anyone has some insight, I'd be most appreciative!



   The goal is to reduce magnitude of signal voltage

   without materially upsetting the DC power that is

   supplied to the microphone's electronics. I would

   suggest a resisteor in series with say a 100uF/10v

   capacitor tied right across the microphone's signal/

   power wires. We know that the microphone's AC output

   impedance is some finite value. Let's assume 1000 ohms.

   Okay, you're wanting to attenuate the AC signal by

   about 2/3. So the resistor in series with the capacitor

   will be 500 ohms or less. This is just an guess/example

   and but it's in the ballpark. You can fiddle with the

   value of the fixed resistor until the two microphones

   demonstrate near equal performance.



   Having a capacitor in series with the resistor will

   keep the AC load resistor from also affecting the

   DC power that comes out of the radio or intercom to

   power the microphone.

   Bob . . .

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etienne.phillips(at)gmail
Guest

Posted: Tue Jan 03, 2012 10:19 pm Post subject: Headset microphone level reducer

Thanks Bob!



That's exactly the kind of circuit I was looking for. I'll get back to you on it's effectiveness.



The new child headset is supposedly an aviation one, and the destination is an aviation MGL radio, so one would have hoped that there wouldn't be such a large discrepancy in levels! 



Thanks again

Etienne

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