Astatic D-104 Silver Eagle Mic Mods

Astatic D-104 Silver Eagle Mic Mods

Modifications for even more basses and warmer sound

The D-104 has been around since the 30’s and is still popular today for communications. The current ones are still pretty much the same except that in the early 70’s a two- transistor pre amp was added to the base. The D-104 is considered by many to be the coolest looking mike around.

The early elements used a Rochelle salts crystal but later a ceramic crystal was used to make the element more rugged. These elements are unique in that the diaphragm is convex instead of concave. Although these diaphragms were made of aluminum foil, Astatic used an ingenious method of placing batting in contact with the element. This eliminated the “tin” sound of these elements and they are capable of a surprisingly good sound. The peaky sound that we sometimes hear from these mikes is due to improper loading, not the diaphragm material.

Without the pre amp, these microphones sound very good with tube equipment that provided a very high impedance load to the element.

With solid state equipment these elements do not sound as good by themselves. Even with a high impedance load such as 50k ohms, they are often high pitched and peaky sounding. With some applications this high pitched sound is preferred. But sometimes this results in a thin or nasal, very middy, unnatural sound. These elements exhibit very low impedance at very high frequencies and very, very high impedance at low frequencies. Even at 50K ohms, output voltage is swamped at low frequencies making the sound high-pitched.

If we bridge the element with a very, very high impedance load, several Meg ohms, it will sound much smoother, be more natural and have low end while still retaining presence.
In the 70’s Astatic, made a base available with a two-transistor pre amp. TUG8 followed by the TUG9. It is commonly thought that this was done to make it a CB Power Mike. However the actual reason was to interface the element to modern solid state equipment. This amplifier provided a 1 Meg ohm load on the element and greatly improved the sound with solid state equipment. Even with this pre amp this microphone still did not sound as natural as it did with the element feeding a very high impedance tube grid.

Since that time a device called a Field Effect Transistor (FET) has become popular. One type of FET is the junction FET (JFET). These are frequently used for receiver RF amplifiers and mixers. They can also be used for audio applications and have the advantage of having a very high input impedance of up to 100 Meg ohm. A FET amplifier can be used to bridge the D-104 element at very high impedance and achieve a remarkably good sound from these attractive classic microphones.


Andre VA3AGV
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