D-104 Information supplied by Doc W2IB

Here is info on using the D-104 Microphone with Kenwood.  See also the article on the Icom.
This applies to the original type d-104 without the amplifier.

Many hams love their Astatic D-104 microphones, both for their audio qualities and for their traditional
appearance.  The problem is that they do not work well with most of the newer rigs which mostly
require a low (500-600 ohm) impedance, while the D-104 has a high (50K ohm) impedance.
There is one late model rig the D-104 does work well with, and that is the Ten-Tec Omni VI.
Possibly it works with other Ten-Tecs, although I have not tried it with them.

While cleaning out the shack a while ago, I came across the well documented article by Steven Fraasch,
K0SF, in the August 1999 issue of QST, page 34.  This article refers to adapting the D-104 for use
with rigs requiring a low impedance microphone.

For a few years I ran a D-104 with a Kenwood TS-850 transceiver.  later used the same mike with a TS-570.
The modification I used was described by David Fentem, KW4M, in the thirteenth edition of Hints
and Kinks compendium, page 1-4.   He used it with an Icom rig.

Many hams have probably never seen this article.  The circuit is much simpler than the one K0SF describes. It uses only one transistor, an FET, in a source follower configuration.  Almost any FET will work. The circuit provides proper impedance matching and enough output to easily drive standard low impedance speech amplifiers.

After some initial testing in my TS-850, I found that the audio response resulted in tinny sounding audio. I then changed the gate resistance to a higher value to increase the low frequency response and used a larger coupling capacitor for the same reason.  I also used a 500 ohm source resistor to more closely match the input impedance of the TS-850.  These changes resulted in a more full mid range and low end response.

I mounted the components on a small piece of perf board.  There is plenty of room for the board in the base of the mike stand.  An existing screw hold the board in place.

The results are excellent.  The sensitivity is about the same as with the Kenwood microphone, and I get many good reports on the audio. Although I had trouble with another microphone on one band, there were no problems with RF getting into the D-104.

I suggest that hams wanting to use their old reliable D-104 microphones try this simple circuit.