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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