[Elecraft] Modify an Astatic D104 Desk Microphone

[RABRUNER at aol.com]

   D-104 microphones come as either Crystal or Ceramic models, which are very 
high impedance elements.  They have an equivalent circuit of a very small 
capacitor and are intended to be loaded by a resistive impedance in the vicinity 
of 1 megohm.  The tradeoff is that the lower the load resistance, the more low 
frequencies will be rolled off in the audio.  10K might leave you with a 
response quite a bit on the bright side.  Some people recommend sticking the D-104 
into the 50K load common on older gear intended for Hi-Z dynamic microphones, 
to create a "peaky" intelligible sound, but this has always seemed 
excessively bright to my ear. Astatic has already built a "communications" sound into 
the microphone working into the nominal load.  You don't have to go to any extra 
trouble to achieve "intelligible" sound.
   The best universal answer to the problem of plugging a crystal microphone 
into a solid-state input is to use some sort of active buffer amplifier such 
as an op-amp, or an FET to provide a very high impedance load to the microphone 
and a very low impedance drive to the following transmitter.  This circuit 
would also be compatible with any legacy VT gear.  As has been suggested, if 
your D-104 has the Astatic preamp in the base already, these issues have already 
been dealt with and you don't need to worry about matching; it's Just a 
question of setting the gain of the amplifier so you are not driving the input to 
the radio too hard.  
   Someone was offering a buffer amplifier kit for general purpose 
interfacing between hi-Z microphones and Lo-Z radios a couple of years ago, but my saved 
files don't go back that far anymore.  If you have a "directbox" either 
active or passive, for matching high impedance musical instruments to lo-Z 
microphone inputs, that might be an effective way to go as well.  Behringer makes 
active direct boxes that sell through Internet music dealers for as little as 
$19.95, which is competitive with the cost of a good quality transformer.

Bob Bruner
W9TAJ