An audio pre-amp for wired headset use with the Motorola DM4000 series. by ‘Mot Orious’

This is a very useful article penned by Mot Orious  here at GB7MB – i only posted it  !

The circuit presented below is a simple op-amp based pre-amp to provide the necessary gain to enable a two terminal electret microphone to be used with the rear microphone input on the Motorola DM4000 series transceivers.
The circuit can easily be constructed on a small piece of 0.1″ Veroboard or similar.
For vehicle use the 12V supply is ideally obtained from the switched auxiliary output, maybe tapped off the cigar lighter socket etc. Screened cable should be used between the headset and the input to the pre-amp. The output is taken via screened cable to the rear accessory connector on the DM4000 series transceiver. PTT is also routed to the same accessory connector.


The pin-out of the DM4000 series accessory connector is shown below. This is the view looking towards the back of the set, pin 2 is top left, pin 25 bottom right.




by ‘Mot Orious

 All work, text and images © GB7MB

5 Comments so far:

  1. I have used this preamp design in the car with a hands free microphone on a DM3600 quite successfully – thanks for posting this.


  2. Thanks for sharing the circuit. The 12V supply can be taken from the ‘SW B+’ pin on the back of the radio which will switch off power to the pre amp when the radio is off.

  3. Hi,
    I’ve built a similar set up, however I am getting tremendous interference (the digital pulses from the radio) into the transmission leg. So when I TX the “thumping” clicks of the digitial signal are heavily present.
    Any ideas on how to eliminate it please?

    • Put a 470pF capacitor a cross the microphone terminals – this problem also occurs with cheap handheld microphones.

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