[Elecraft] Astatic D104 mics

[Tim Groat]

The basic D-104 (without amplifier) is a crystal microphone, designed 
for very high impedance (>100K ohm), and delivering fairly high output 
voltage. It worked well with the vacuum tube amplifiers used when the 
D-104 was new. The K3 (and other modern rigs) have much lower input 
impedance and will load down the crystal mic element considerably.

There were several amplified versions, the "power mikes" of the CB 
craze. These have low output impedance (a few K ohms, varies with the 
output level pot setting) combined with high output voltage. Their 
output level can over-drive radios designed for dynamic microphones 
unless the gain setting is kept very low, or an attenuator is placed in 
the output.

Links:
Adapting D-104 to modern radios (using an op-amp buffer) 
<http://www.w6ier.org/images/9908034%5B1%5DAdapting%20the%20Astatic%20D104%20Mic.pdf>

Schematics for amplified Astatic mics 
<http://www.barovelli.com/projects/radio/radioastatic.htm>