|[AMRadio] Microphone assistance|
John E. Coleman (ARS WA5BXO)
wa5bxo2006 at pctechref.com
Thu Aug 31 16:54:40 EDT 2006
The trouble with low z microphones is that some times the output of
the mic is too low and you don't have enough gain to get the modulation up
high enough. If you do have enough gain then they work just fine in the
hi-z input circuit. Beware that some input circuits do not have, or use, a
coupling capacitor and the grid sets high above ground at 2.2 Meg or more,
and develops contact bias. (no cathode bias resistor) The circuit works
well with a ceramic or crystal microphone which has no continuity to ground
but put a microphone which has continuity to ground such as a dynamic, even
if it has a matching transformer, and it will kill the bias on the preamp
causing the stage to operate in a distorted manner. If you don't want to go
into the rig and modify the input circuit with a coupling cap then put one
in the microphone to block the bias drain.
The dynamic microphone will have a responds curve which extends much
lower that the Xtal or ceramic so to balance it out you may want to do some
extending of the high frequency range as well. You may also reduce the size
of the coupling capacitor that you use at the input to limit the low
frequency to a more expectable response for the rig.
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