[AMRadio] Microphone recomendation

Paul Christensen w9ac at arrl.net
Mon Sep 21 22:42:36 EDT 2009

> pair of high-mu triodes in a push-pull mic preamp.  That way, each tube 
> can
> have its own 10 meg grid leak to allow for a 20 megohm  load on the xtal.

It's possible to duplicate a similar circuit using an FET differential 
amplifier with dual FETs in a single package (e.g., 2SJ109), or a JFET-input 
instrumentation amp (e.g., LT1102).  Both would perform best when using a 
bi-polar supply to economize on parts count and avoid having to use 
half-rail biasing technique.  In each case, input Z is 2x the single-ended 
amp for a total of 22 meg-ohm.  The circuit could be installed in the D-104 
mic head, with shielded twisted-pair running to a balanced-input speech amp.

Seems like there are other factors that limit the practical input Z to ~10 
megohm for a crystal cartridge.  The source Z and slight shunt resistance 
internal to the crystal cartridge may be the limiting factor.   I've used a 
series input R of 10-meg in front of my single JFET's gate and the response 
is pretty much limited to about 50 Hz with or without the added R -- with 
the only noticeable difference being lower output level and increased noise.

With the first fundamental of the deepest male voice being greater than 70 
Hz, it's kinda' hard to justify a response that extends any lower for any 
form of voice communication, including AM and ESSB.  Anything lower than 70 
Hz unnecessarily increases power duty cycle and produces other unwanted 
artifacts like room rumble and deep breathing sounds.

Paul, W9AC 

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