Yaesu MH-31 Electret Condenser Mic Modification

When they built the FT-817, I think its such a shame they did not include a speech compressor. A good speech compressor makes such a huge difference to talk power and average RF power and is very helpful when using only 5w SSB. I did try a ‘One Big Punch’ speech compressor board that fits inside the MH-31 but I wasnt impressed with its performance, especially the noise gate. I found that with the MH-31 microphone, I have to have the mic gain set to 100 when using SSB and even then talk quite strongly within a few inches of the microphone. Even then I dont think the standard dynamic mic element sounds that good.

One great thing with the MH-31 is that there is an unused 5v supply connection. This allows an electret condenser microphone element to be used in place of the standard dynamic element. I decided to experiment with this, and after some testing I came up with the circuit below.

Condenser microphone schematic for the Yaesu MH-31

As you can see, I’m still using the tone selector switch as a high pass filter to provide some tone adjustment. I find that the 47nF setting works great for SSB by reducing the low frequency and putting more of the midrange into useful talk power. For FM I use the 1uF setting which provides more full range audio suited to FM ‘Rag chewing’. This setup also provides much more mic gain options. With the stronger gain of the electret mic element, I now use between 30 and 40 mic gain on the FT-817 on SSB as well as now being able to talk further away from the mic. I find talking ~6 inches away from the mic much improves voice clarity. Something I found I just couldnt do with the standard dynamic element.

Some photo’s are below. I have stuck the electret element onto the front of the mic with some hot glue, then padded it with some foam before placing the PCB back in.

If you seem to share the same issues with the MH-31 microphone, I suggest giving this a go. Please let me know how you think it performs!

Electret element glued in place

The completed modification

Tone switching arrangement


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  1. Hi
    I have a Yaesu FT-600 which uses this MH-31 microphone.
    Can I use this mod too?


    1. Hi Mehdi.
      As long as you have 5v on the mic pins, it should work fine. I’m not sure if the FT-600 has power to the mic.
      73, John.

  2. Good Mod – makes a world of difference to the transmitted audio

    FYI the circuit diagram you show is incorrect , the switch is on the other side of the capacitors and the common end of the the capacitors is the end that goes to the mic out



  3. Hi John

    I’m interested in your MH-31 electret mic mod and would like to know which module you used?



    1. Hi Phill.
      I used a Panasonic back electret. I cannot remember the model number.
      Good luck! john.

  4. Thank you for your website with many interesting projects, I plan to try your suggested modification for the MH-31 microphone this weekend.
    The schematic shows 1uF electrolytic capacitor and a 47nF capacitor connected together on the microphone side, with the positive side of the polarised capacitor going to the switch. Whereas the photo at the bottom shows the capacitors joined on the positive side of the electrolytic capacitor and the negative banded side connected to the switch. Which is correct? I’m guessing the photo?

    1. Well spotted Tony!
      I would put the negative side to the switch, due to the electret power, or use a non polarised capacitor 🙂
      Good luck, John.

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