The resistances need not be changed, that way the mid range level stays
constant. Only the series and shunt capacitors need to change to modify
the low and high roll off frequencies. With the lows and highs reduced,
then you increase the mid range by increasing the microphone gain.
I can't suggest capacitor sizes because I don't know the impedance of
the Scout audio input. The capacitors and switches could be located in a
box external to the scout. I believe the scout supplies DC for the
microphone element also which may complicate adding series capacitors
unless its a separate wire. The scout manual specifications or schematic
should show the audio input impedance. From that you can choose a range
of capacitors, probably with ratios between them of about the square
root of 2, for both the series and the shunt capacitors and then can
experiment ad nauseam with listeners. Remember that many hams have tin
ears made tinnier by their radios with infinitesimal speakers. Many
Tentec built in speakers are not out of that range. You will find
totally opposite opinions from any change you make.
There are more complex circuits possible using some classic tone control
circuits so that the amount of bass roll off can be adjusted with a pot,
and the treble with a separate pot.
When working at microphone level you need to be sure to keep the
circuits shielded or RF will get in and cause feedback and weird radio
sounds while transmitting.
A couple switches and a gaggle of capacitors is cheaper and more
versatile than a single Heil microphone.
Using op amps with RC circuits gets more rapid roll off, and sometimes
much more roll off per RC element at the cost of adding power and noise.
I don't think you need that much roll off to make the improvements you
desire. There are makers of microphone equalizers for ham use that will
be glad to separate you from your pocket change (up to twice the price
of the Scout) for fancy microphones and adjustable equalizers.
73, Jerry, K0CQ
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Entire content copyright Dr. Gerald N. Johnson, electrical engineer.
Reproduction by permission only.
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