Re: [Elecraft] Modify an Astatic D104 Desk Microphone

[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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Re: [Elecraft] Modify an Astatic D104 Desk Microphone

[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  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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Re: [Elecraft] Modify an Astatic D104 Desk Microphone

[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
===
MacDobs - helping to shift the paradigm for low-cost amateur astronomy.
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Re: [Elecraft] Modify an Astatic D104 Desk Microphone

[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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