Given the success of the ceramic capsules for SSB transmission, I decided to modernize a vintage microphone: the Astatic D104 in the “Silver Eagle” version.
Unfortunately the Astatic D104 Silver Eagle uses a non-transistor preamplifier. suitable for the impedance of the installed capsule, it translates into totally wrong tones and levels for modern radios.
They can be easily found on eBay in excellent condition, and it is a real pity not to be able to use them in transmission.
In this change, I relied on a classic Astatic scheme that has adopted it for decades to pre-amplify ceramic capsule microphones.
I made some improvements, of which:
- Pi filtering – Greek for power
- RF return protection
- Toni control possibility
- Gate JFet decoupling
- Possibility of use on Icom radio
- 100% PCB dimensions identical to the original circuit
100% compatible pcb to the original:
Preamplifier circuit completed and installed in the base of the Silver Eagle:
With the two trimmers we can vary the level and pitch (more or less low).
The trimmer of the level is placed at the same point of the original so that it can be operated even with the base closed.
I rewired the cable and the PTT switch using the convention:
- White wire – BF
- Stocking – GND BF  Black Wire – GND PTT
- Red Wire – PTT
- Yellow Wire – VCC
If connected on radios that do not have power on the BF microphone pin (Icom) the 6.8uF capacitor can be replaced with a bridge.