The old reliable Astatic D-104 microphone always works well with vacuum tube boatanchors but when connected to the rigs with transistor input circuits, the audio sounds very tinny. The reason for this is that the D104 requires a microphone input impedance in the range of 500k to 4.7 meg-ohms. Vintage transmitters meet this requirement.

Newer ham gear with transistorized input circuits has input impedances in the range of 600 to 10,000 ohms. The effect of the lower input impedance is dramatic. Low frequencies drop off rapidly when connected to lower input load resistors:

100k input resistor     rolloff begins at 500 Hz

500k input resistor     rolloff begins at 250 Hz

1meg input resistor    rolloff begins at 100 Hz

4meg input resistor    rolloff begins at  30 Hz

Here is a circuit I have used to adapt a D-104 to a Ten-Tec Pegasus transceiver (figure 1). This circuit uses a MPF102 field effect transistor (FET) in a common source configuration. The D104 cartridge sees a 2.7 megohm load impedance. The 1K source resistor provides the proper load impedance to rig with transistor input stages. The 1mh and 100pf capacitor combinations are used to filter any RF that may be picked up by the D104 element and mike cords. Voltage to operate the MPF102 transistor is supplied at pin 4 of the mike connector. (Any voltage in the range of +4 to 15 volts is adequate.) A 9v battery may be used if voltage is not available from the transceiver.

I purchased this D104 in non-working condition at a hamfest  for $10 (picture 1). I completely dismantled the microphone. The ‘power-mike’ electronics were removed. An extra hole in the microphone base was plugged with a screw and nut. I removed loose paint on mike base with a wire brush and then painted the base with Rust-oleum Hammered 7213 silver finish.

After reassembling the microphone, I installed the FET circuit on the existing terminal strip (picture 2).

The rebuilt D104 now gets good audio reports with my Pegasus. There is plenty of low frequency response plus the rising audio characteristic from 1kHz to 3 kHz is preserved.