Using an Old Style Astatic D-104 "Lollypop" Microphones with Modern Low-Z Mic Rigs
Fred M. Spinner, W0FMS, April 2001


I recently acquired a CCA Excellent condition KWM-380 from a local ham, but with two very well worn (CCA fair?) microphones, both with low output.  The rig didn't sound too good with these mics, and that was a shame.  The KWM-380 has the famous "Collins Audio" with a good mic.  I decided that probably the best mics I had lying around were a couple of old-style Astatic D-104 Microphones (and I think they look cool...).  The dilemma was how to use these cool mics with the cool rig, and get cool audio.  There was a little impedance mismatch problem to overcome first with the rig and the microphone...

Here is the solution I came up with:  a simple JFET Source Follower circuit that I use with a "garage sale" (literally- my mother bought it at a garage sale for me years ago) D-104 crystal element/"UG-8" stand.  This stand has a dual pole switch for PTT, so I use it to "turn-off" the 9V battery bias in the microphone when PTT isn't depressed.  I'll eventually put in a switch in the stand somewhere so I can turn the battery bias on (override?) for VOX and MOX operation, but that's a job for another weekend, and when I see a suitable switch turn up at surplus.   The 9V battery should last a very long time in this setup as when active the measure current is around 1.3 mA DC.  This microphone currently in use with my Rockwell Collins KWM-380, and sounds very good-- not "tinny" like D-104's "supposedly" sound like with modern Lo-Z rigs.  The circuit is built on a Radio Shack Phenolic "Anyboard" cut to fit in the base, screwed down in one of the terminal strip support screws.  The 9V fits fairly securly in the base when the bottom is screwed back on.
 


What this circuit does is provide around a 11 Megohm impedance for the crystal element in the D-104 to work into.  Remember, in the 1930's when this microphone was originally designed, and into the early 70's, most rigs used a tube for a microphone amp with a high imput impedance.  This type of microphone sounded great with the tube rigs, but lousy with the modern low-Z solid-state rigs of the 80's, 90's and 2000's.  The reason why is that the low-end -3db point of the microphone element suffers greatly at lower impedances, due to the excessively heavy loading of the element by the lower impedance in modern rigs.  What solid state device works like a tube input?  A FET, of course...

The JFET preamplifier provides a slight voltage loss, but a current/power gain for the microphone, resulting in a lower-Z output (selected by the "swamping" resistor marked Zrig above.)  This provides a low-Z impedance to the rig, very Hi-Z to the D-104 crystal element, and everyone is happy.  The device is very low noise at audio, and overall the microphone and circuit sound very good on the air.

This circuit can be adapted to other rigs as well.  Newer rigs like Kenwoods, Icoms and Yaesus often have 8-12V available on the mic plug.  If this is available, then that bias can be used instead of the 9V battery.


Fred M. Spinner, W0FMS
April 14, 2001