Hi all,

I'm working on a DIY project that requires the use of a radio handset and I'd like to use the Moto RJ45 handsets because they're cheap, rugged, and easy to find. I've been trying to figure out exactly what they need in terms of support circuitry to get the proper audio out of them, and the best I've found so far is the mic circuit from the MCS2000 service manual.

mcs2000-mic-bias.PNG

The manual also gives this explanation for the operation of that circuit but it's not exactly very informative.

R0222 and R0223 provide the 9.3 VDC bias and R0224provides input protection for the CMOS amplifier input. R0223 and C0219 provide a 560 ohm ACpath to ground that sets the input impedance for the microphone and determines the gain based onthe emitter resistor in the microphone’s amplifier circuit.
I'm trying to figure out how to adapt this to work on +12V as opposed to the original 9.3V in the Moto circuit. I'd rather not have to add another regulator to my design just to power a mic. I know that these mics have a decent amount of circuitry inside but I haven't been able to find a good diagram of what exactly is going on. My first instinct is to use the 2.5V at the mic pin and change the resistor values to get the same current/voltage from 12V as opposed to 9.3 but I'm not sure the answer is that simple.

Any suggestions or help is appreciated!