#13: D-104 Microphone amplifier / Equalizer for Ham Radio

#13: D-104 Microphone amplifier / Equalizer for Ham Radio

This video describes the microphone preamp and equalizer that I designed in 1999 to use with my Astatic D-104 microphone with my ham radio. The D-104 suffers from two issues when used with modern ham radios - impedance mismatch and non-ideal frequency response. The circuit addresses both of these issues. The video briefly describes the circuit, it's frequency response, and demonstrates the bass and treble adjustments using an oscilloscope and signal generator.

More information can be found on this webpage:
http://www.qsl.net/w2aew/myd104amp.html

Note that it is quite likely that you will have to employ strategically placed bypass capacitors, ferrite beads, etc. to keep RF from coupling into this circuit. Every application and installation is different, so you may have to "play" with these components to eliminate RF from your circuit. You can see some of the ferrite beads I used when the video shows the circuit board in the base of the microphone.

Wade's D104 hiFi mod

Here we take a standard D104 non amp and make it into a economy HiFi amped mic. Enjoy Stickman

Original Circuit (Amplified D-104)

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.

D-104 Mod unamplified stand

D-104 Mod Component side