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Microphone levels

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fvalarm(at)rapidnet.net
Guest

Posted: Mon Jul 05, 2010 12:07 pm Post subject: Microphone levels

Bob,



Would you expect that the microphone output level from a standard Aviation

headset to be similar/ or even the same as a standard handheld audio

microphone as used with a professional sound board?  I don't own an audio

meter but just need to be close for a non-aircraft related project.

Bevan

Rv7A Implementing Z13/8

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nuckolls.bob(at)aeroelect
Guest

Posted: Mon Jul 05, 2010 2:30 pm Post subject: Microphone levels

At 03:04 PM 7/5/2010, you wrote:

 	  Quote: 	
		  



  Bob,



Would you expect that the microphone output level from a standard Aviation

headset to be similar/ or even the same as a standard handheld audio

microphone as used with a professional sound board?


	


    No. Aircraft microphones are victim to legacy compatibility

    standards that emulate the original carbon granule microphones

    (like those in telephones through the 70s).



http://en.wikipedia.org/wiki/Carbon_microphone



    The audio output is not only very high (as much as 1 vrms) it is

    generated with a power supply delivered by the transmitter.

    Hence, most aircraft microphones have active electronics so

    that modern TINY voltage microphones can dress up like

    a carbon mic.



 	  Quote: 	
		     I don't own an audio

meter but just need to be close for a non-aircraft related project.


	


    Sound boards are generally set up to accept 100 mV pk-pk

    (30 mV rms) output from dynamic mics delivered on twisted pairs.



    Bob . . .

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fvalarm(at)rapidnet.net
Guest

Posted: Mon Jul 05, 2010 8:46 pm Post subject: Microphone levels

Thanks Bob, exactly what I'm looking for.  



Bevan



--

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fvalarm(at)rapidnet.net
Guest

Posted: Tue Jul 06, 2010 12:07 pm Post subject: Microphone levels

Bob,



My interface circuit which brings a line level output through an impedance

matching transformer to the mic input of the aircraft radio may need to go

through a capacitor to block the DC bias voltage coming from the radio from

getting to the output of the matching transformer.  Do you agree with this

method and can you recommend a value for this cap?  



Bevan   



--

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nuckolls.bob(at)aeroelect
Guest

Posted: Tue Jul 06, 2010 1:34 pm Post subject: Microphone levels

At 02:58 PM 7/6/2010, you wrote:

 	  Quote: 	
		  



Bob,



My interface circuit which brings a line level output through an impedance

matching transformer to the mic input of the aircraft radio may need to go

through a capacitor to block the DC bias voltage coming from the radio from

getting to the output of the matching transformer.  Do you agree with this

method and can you recommend a value for this cap?


	


    Some experimentation is probably called for. You

    need to find a resistor value that becomes a

    'dummy microphone'. The resistor probably wants

    to have 2-3 volts dropped across it while the

    transmitter is 'talking'.



    Then make sure the effective output impedance and

    signal level of your transmit audio source can

    drive the necessary signal into this load . . .

    3 v pk-pk is probably a good upper bound to

    strive for. Then pick a capacitor who's reactance

    at say 100Hz is about 1/10 that of the sum of

    dummy load resistor + source output impedance.

   Bob . . .

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rjquillin(at)cox.net
Guest

Posted: Tue Jul 06, 2010 1:48 pm Post subject: Microphone levels

At 14:36 7/6/2010, you wrote:

  	  Quote: 	
		  signal level of your transmit audio source can

    drive the necessary signal into this load . . .

    3 v pk-pk is probably a good upper bound to

    strive for. 	


 Without grabbing my scope, 3Vpp --sounds-- a bit hot for a mic input.



 Is that value a typo?



 Ron Q.

     [quote][b]

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nuckolls.bob(at)aeroelect
Guest

Posted: Tue Jul 06, 2010 3:35 pm Post subject: Microphone levels

At 04:46 PM 7/6/2010, you wrote:

 	  Quote: 	
		  At 14:36 7/6/2010, you wrote:

>signal level of your transmit audio source can

>    drive the necessary signal into this load . . .

>    3 v pk-pk is probably a good upper bound to

>    strive for.



Without grabbing my scope, 3Vpp --sounds-- a bit hot for a mic input.


	


    Nope. That's what can come out of an aircraft microphone

    the emulates the legacy carbon mic. Of course, REAL

    carbon microphones are exceedingly rare in service now.



    Just for grins, put your scope on the mic line of

    a headset and see what kinds of signals can be

    seen there. They may have migrated down over the

    years . . .

   Bob . . .

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nuckolls.bob(at)aeroelect
Guest

Posted: Tue Jul 06, 2010 3:52 pm Post subject: Microphone levels

At 04:46 PM 7/6/2010, you wrote:

  	  Quote: 	
		  At 14:36 7/6/2010, you wrote:

  	  Quote: 	
		  signal level of your transmit audio source can

    drive the necessary signal into this load . . .

    3 v pk-pk is probably a good upper bound to

    strive for. 	


 Without grabbing my scope, 3Vpp --sounds-- a bit hot for a mic input. 	


    Nope. That's what can come out of an aircraft microphone

    the emulates the legacy carbon mic. Of course, REAL

    carbon microphones are exceedingly rare in service now.



    Just for grins, put your scope on the mic line of

    a headset and see what kinds of signals can be

    seen there. They may have migrated down over the

    years . . .



    P.s. Just checked some links as follows:



  http://www.capitalavionics.com/tip_0909.asp



  http://tinyurl.com/24g4wjf 



    and a number of others that 'specd' aircraft

    mics for sensitivity, current draw, bias voltage

    but failed to call out the output voltage

    present when those specs are being satisfied.

    I think it was DO170 that talks industry

    specifics. 



    I did see an anecdotal reference to 100-500 mv

    which would translate to 300-1500 mv pk-pk.

    Same ball park. We used to design for 3v pk-pk

    to have zero risk of clipping . . . at least in

    THAT part of the circuit!



    Bob . . .

     [quote][b]

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