I designed this simple low distortion audio circuit to replace the circuit in old D104 microphones, to replace with a 600 ohm diaphragm mike cartridge since the crystal mike cartridges are hard to find now. This will match the 600 ohm cartridge good since the input impedance is exactly 600 ohms. The gain is 9.3 dB and the circuit is designed to look into the 1k potentiometer that most CB radios use for the mike gain control. A 9V battery is required and the gain will be adjustable on the CB mike control. I have used a 600 ohm cartridge mike in a D104 so I know that they fit. Anyways this will insure that new life is added to this old classic mike that is no longer in production. Furthermore for AM and SSB DX use I designed the circuit to have a bandpass suited for voice.
