Modify an Astatic D104 Desk Microphone

[Chuck Gehring]

I have an old Astatic D104 Desk Microphone that I would like to modify to
use with a modern Transceiver.  Does anyone have some suggestions?

73, KI4DGH Chuck G.

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[Stuart Rohre]

Some rigs will take it fine, as is.  Is it the one with amplifier in base?
that type will surely work with all modern rigs and proper adjustment of the
amplifier.

On some rigs, with low Z mike inputs,  a high Z to low Z transformer can
match the high impedance crystal element of the D-104 to the mic input low
Z.  See Hammond transformer catalog for these.
72,

Stuart
K5KVH


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[Jack Brindle]

At 3:34 PM -0500 7/2/04, Chuck Gehring wrote:
>I have an old Astatic D104 Desk Microphone that I would like to modify to
>use with a modern Transceiver.  Does anyone have some suggestions?

The D104 works quite well with the K2. There are a couple of ways to
make the D104 match the input impedance of the K2. A 600 ohm - to-10K
impedance matching transformer can do the job. A better solution is
to use a small amplifier with high input impedance and low output
impedance. One such amplifier using a transistor is described on the
Elecraft Web page. I put together a circuit using an op amp based on
a circuit from the 2003 ARRL Handbook. Op amps are perfect for this
situation since they naturally have high input and low output
impedances.

I almost forgot - my D-104 has the original G stand without the
amplifier. If you have a stand with the amp built-in, then just hook
things up, everything you need is already in place...

-Jack Brindle, WA4FIB
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[Bob Nielsen]

On Fri, Jul 02, 2004 at 03:34:46PM -0500, Chuck Gehring wrote:
> I have an old Astatic D104 Desk Microphone that I would like to modify to
> use with a modern Transceiver.  Does anyone have some suggestions?

The Heil Sound web site <http://www.heilsound.com> shows modifications
to put an electret element into a D104 (which I intend to do as soon as
I locate my 50-year old Astatic.)

Bob, N7XY
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[RABRUNER]

   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
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