From: jackson@stc.nato.int (Andy Jackson)
Subject: Was: Aircraft Mike Plugs. Now: Biasing Microphone
Organization: SHAPE Technical Centre, NL
Date: Mon, 31 Aug 1992 08:38:16 GMT

In <VBREAULT.92Aug28110305@rinhp750.gmr.com> vbreault@rinhp750.gmr.com (Val Breault) writes:

[stuff deleted]

>I wonder if Scott, Andy or someone else can tell me what the bias
>current is for those things.  I picked up a headset/microphone 
>set for a great price ($0) but it has NO identifying markings.
>Through some small amount of investigating I've discovered that
>it has an active microphone element and will work okay if I 
>provide a small amount of bias current.  What I want to know is
>just how much current is the "right" amount.  Also... I presume 
>that I can use a "suitable" dc blocking capacitor and run the 
>signal directly into my transciever without any compensation
>or matching network.  If this is false then please set me straight.

>-val-


Val,

		This is a fairly typical bias circuit for an electret
microphone.  I have found that the exact values are not too critical but
they will have an impact on frequency response and possibly sensitivity.
As for whether you need any further pre- or de- emphasis, that will
probably depend on your intended application.  In my own recent
experiments, I found that this setup sounded acceptable.  (I can't remember
typical output levels but in my Mark I intercom, I followed the mike
by an op-amp with a x5 voltage gain and had enough output to drive an
LM380 audio amp.) (BTW, my Mark II will use a TDA2822 because this
operates down to less than 3 volts supply and I'll get a lot more life
out of my battery.)



                           ___________\    +1.5 to 9 volts 
                          |           /
                          |_
                         | |
                         | |  About 1k ohms
                         | |
                         |_|
                          |          _
       ___________________|_________| | |___________\  To Mic amp, Zin >= 1kohm
      |                             |_| |           /
      |                         about 5 micro-farads
      |
     |O  Electret Microphone
      |
      |____________________________________\
                                           /  0volts
	 


If you are going to use an op-amp after the mike, anyway, you can use
the op-amp to bias the mike:


                                           4k7ohm
                                 ________-------__________
                                |        _______          |
                                |  | \                    |
                         1kohm  |  |   \------ +9V        |
          ______________------__|__| -   \ _______________|_____  Output
         |       22k    ------     |      /
         |   +9V_-----___ ____ ____| +   /
         |       -----  _|_   |    |    / 
  Mike  |O      50micro ---  | |   |  /|     1/4 LM324
         |              ---  |_|   |/  |
         |_______________|____|________|______________________ 0 volts

                              22k

(If you try to get too much gain from the mike - as I did at first - the
bias current taken by the mike pulls the op-amp bias off and distortion
ensues!)

This is not intended to be _THE_ circuit, it just works for me (:-)



Good Luck and if you ever find any more $0 headsets, let us know!!

A big THANK YOU to all who replied to my original query about the plugs
'n sockets.  At least I now know what they are officially called and I
have some mail order addresses to try.  I'll post a summary in a few
days - thanks, again, for all the help.

 - Andy.


PS - Isn't it a pain drawing  - and reading - ASCII circuits! (:-)

-- 
Andy Jackson                                   jackson@stc.nato.int  OR 
SHAPE Technical Centre,                        ...!sun4nl!shapetc!jackson
PO Box 174,
2501 CD  The Hague,
Newsgroups: rec.radio.amateur.misc