[AMRadio] Mike Cable Capacitance

[Donald Chester]

>A 15 foot cable with 30 pF per foot would present a 450 pF capacitance to
>ground, and this would work with the piezo microphone's effective output
>capacitance to act as a capacitive voltage divider, which would cut the 
>mike
>level about in half (-6dB), or about 100% to about 50% modulation, which is
>about what was observed.  The effect of this capacitive voltage division
>should be uniform across the audio frequency range, but the load resistance
>causes another effect at the low frequency end..  The total effective 
>series
>capacitive reactance would be equal to the effective microphone output
>capacitance and the cable capacitance in parallel, or about 950 pF, so in
>this example the presence of the 15 foot cable would reduce the mike level
>by about 6 dB, and the relative low frequency response with any given load
>resistance would be extended down by about an octave.  That can be a
>dramatic response difference if the low frequency cutoff is in the 300Hz
>range.
>

I seem to recall reading, maybe something from Astatic, to the effect that a 
reasonable  length of shielded cable will have little effect on the 
frequency response and output level of a crystal mike as opposed to other 
high impedance sources such as resistive networks and high impedance 
transformer secondaries.

An interesting alternative with the D-104 is shown in the paperwork they 
used to include with the microphone, using a pushpull mic preamp.  The cable 
is shown to be a shielded pair of wires.  Each terminal of the mic element 
is connected to one of the wires, and at the amplifier end, each goes to one 
grid of a pushpull stage, each of which has its own grid leak resistor of 5 
megohms or so to ground.  The pair of grid resistors acts as a voltage 
divider with the common connection grounded.  This puts 180 degree 
out-of-phase audio on each of the two grids.  The pushpull preamp stage can 
be followed by a coupling transformer or R-C coupled to another pushpull 
stage.  Astatic suggests a 100% pushpull amplifier, from mic preamp to final 
audio stage.

This arrangement should be possible with any crystal mic element that has 
two ungrounded terminals like the D-104, as opposed to elements with one 
side grounded to a shielded metal enclosure.

-k4kyv

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