Figure 1 In the original circuit, the first stage of the D-104 amplifier provides the load for the mike element. Originally it uses a common NPN bi-polar transistor.
The microphone element is connected through a .01uf capacitor to the base of the first transistor. A 1 Meg resistor is connected from 9v to the base to bias the transistor for linear operation.
The transistor emitter is grounded through a 150-k ohm resistor. A 0.1 uf capacitor also connects to the emitter. This couples the audio output into the next stage providing for DC inter-stage voltage isolation. This is a very common emitter follower configuration.
A 450 pf. (00047) uf capacitor is connected from the emitter to the base. The purpose of this “boot-strapping” capacitor is to create positive feedback from the emitter to the base. This artificially raises the base impedance of this input.
The collector connects to board foil that supplies 9v.
There is a second stage that feeds the mike cord. This is not needed with the FET modification as the FET amplifier provides plenty of output for most applications. I have found that the sound is better without this second stage so it is eliminated in the modification.
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