[TenTec] Computer Headset Secured -- Now What?

[Richards]

I recommend including a small L attenuator pad to reduce the very HOT 
electret condenser mic element signal so the resulting signal strength 
is about the same as that of a dynamic microphone.

Otherwise, your mic input is too hot and the AGC circuit has little room 
to work in.  If the electret mic capsule is as hot as many are, you will 
find you have to turn the MIC GAIN down to about 2-3% to
avoid having the AGC light on all the time, instead of popping on upon 
voice peaks.  In effect, the mic comes on at about 2-3% and the AGC 
comes on full blast about 3-4% and there is little room for adjustment 
for the AGC circuit to sort of "flex" in - if you try to reduce MIC 
GAIN, you quickly run out of room and it is off below 1%.  (on my Omni VII)

  If you include the attenuator, you can set the MIC GAIN at something 
much higher, more like 35-40% and that gives you more room to adjust it 
up or down without either being on or off, with little room for 
adjustment in between on and peak input, so that you can set it where 
the AGC only comes on when there are voice peaks.

You can buy a 10 dB attenuator adapter from Andrea Electronics, famous 
for making active noise canceling mic circuits for the Air Force, or you 
can build your own with a pair of resistors in an L-pad arrangement.  I 
went further and included a voltage divider to reduce the 10v bias 
voltage to closer to 5v, because my experience and research with these 
little electret mic capsules tells me they sound a little fuller that 
way.   I think Jim Brown said he likes to put a little capacitor on them 
to shape the audio more for ham radio, which I believe thins it up a 
bit, giving it a tad more punch  (HEIL used to sell the HC4 cartridge 
for a similar, probably more pronounced, effect)  -  and I suspect he 
gets good results that way.   I prefer to tailor the audio with the on 
board input EQ setting or just run stock, as these little computer 
microphones tend to run thin anyway.  It may depend on what your voice 
is like as to whether or not you want to add that or not.  You certainly 
want a capacitor Bob mentions to keep the DC bias voltage from the AC 
MIC Input line.  Radios usually have one on the input circuit, but I put 
on on for good measure.


That is MY take, anyway.

----------------------  K8JHR ----------------------------


On 3/28/2013 8:40 AM, Bob McGraw - K4TAX wrote:

> I used a 1/4" to 1/8" adapter from Radio Shack for the headphone part
> and made my own 1/8" to 8 pin DIN for the mike audio. I also included
> the resistor and capacitor inside the DIN connector to provide
> polizaration voltage for the mike element using 1/8 watt resistor 6.8K
> and a capacitor of 0.1 uF at 15VDC.  It is a tight fit but works.
>
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