[AMRadio] Microphone assistance

Bob Bruhns bbruhns at erols.com
Thu Aug 31 17:56:39 EDT 2006

I read that a low-Z mic has a low source Z, but it wants to see a
high load Z.  The trick with a preamp is to present a high Z to the
mike, but produce minmum noise with a low-Z source.

But that's for perfectionists.  A transformer will work fine.
Capacitive loading at the hi-Z side will affect highs, even a few
feet of coax is enough to be noticeable.  So the transformer should
be fairly close to the radio.  Since the transformer inductance and
the RF filrtering and cable capacitance will be shunting the mic,
some resistive loading (10K, more or less) on the high-Z side might
help the frequency response, at the expense of level.

About two feet of shielded cable would allow the audio transformer
to be a little bit removed from the power transformers, and might
help keep hum out of it.

I think Radio Shack carries audio transformers of this type.

  Bacon, WA3WDR

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