97. Astatic D104 PreAmp Circuit. Mark ZS6MDG 27June 2025.

Using a preamp with the Astatic D-104 microphone is often essential when pairing it with modern transceivers—and here’s why:

🎙️ Why Use a Preamp with the Astatic D-104?

The Astatic D-104 is a high-impedance crystal microphone originally designed in the 1930s for use with vacuum tube radios, which had high-impedance mic inputs (typically around 100kΩ). Modern radios, however, use low-impedance inputs (around 600Ω), which creates a mismatch.

🔧 Key Reasons for Using a Preamp:

• Impedance Matching: A preamp acts as a buffer to match the D-104’s high output impedance to the low input impedance of modern radios. Without it, you can lose up to 40 dB of signal, resulting in very weak or muddy audio.

• Gain Boost: The preamp amplifies the mic signal to a usable level for modern transceivers.

• Preserve Audio Quality: Without a preamp, the mismatch can roll off high frequencies, making your audio sound dull or muffled.

• Power Compatibility: Some preamps can draw power from the mic line of the radio, avoiding the need for external power supplies.

🛠️ Types of Preamps Used

• Built-in transistor amps: Found in some D-104 bases (like the UG8 stand), these use a simple two-transistor circuit to boost gain.

• Op-amp buffer circuits: More modern and efficient, these can be built into the mic base and offer better fidelity and lower power draw.

⚠️ Without a Preamp?

If you connect a D-104 directly to a modern radio without a preamp:

• The audio will be extremely low or distorted

• You risk damaging the mic element if improperly wired

• You’ll likely need to crank up mic gain, which introduces noise

Simple schematic of the pre-amp.

3D diagram of the PCB.

Pre-Amp Build on VeroBoard.

The project PCB's will be available soon from Mark ZS6MDG or Ronnies Homebrew Kits.

Transistors fitted in opposite positions ..

All installed in the bottom of the microphone.

Completed and tested .