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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