Electret microphone instead of dynamic and vice versa

Some transceivers have a microphone input adapted to the electret microphone and some to a dynamic one. It is not possible to replace them due to design differences. The electret microphone contains a FET transistor in the capsule, which must be supplied with DC voltage in the same wire as the audio signal.

The FT-991 transcieiver (like many other Yaesu) has a dynamic microphone input. In order to be able to use a typical PC headset unit, you need an adapter with the diagram shown below. After adding the input for the PTT footswitch, the implementation looks like in photograph 1. Be sure to adjust the gain of the microphone path in the FT-991. For the dynamic microphone MH-31, use the value 33, and for the excellent Koss SB/45 headset, the gain must be set to 10.





In turn, the IC-765 transceiver (like many other Icom) has an electret microphone input. To connect the top Heil Sound Pro 7 set with the dynamic microphone to the FT-991, a simple mechanical adapter is enough. However, in the case of IC-765 it is not enough. The signal from the microphone is too weak and needs to be amplified. The appropriate diagram is in the figure above. Physical implementation with the use of SMD components is cumbersome, but it allows you to fit the amplifier inside a heat-shrinkable sleeve, as shown in the photograph 2.

Photo 1. Adapter for FT-991
Photo 2. Adapter for IC-765

Number of Comments: 5

  • foloren torium

    Perfect piece of work you have done, this website is really cool with excellent info .

  • DR OMs I just solve the same problem with Kenwood TS 590SG. I bought headset Gulf Coast Avionics 4T having excelent 24 dB noise reduction, but also electret mike. Hence I have deep petition to ms who posted this article. Would be possible to adopt connection to Kenwood TS 590SG?
    Thanks a lot in advance.
    73 MIKA OK1MPU

    • Hi Mika
      I haven’t big experience with Kenwood, but I will try to help.
      I understand that you need to connect elecret mike to dynamic mike input of TS590SG.
      I found the microphone connector drawing in TS590SG manual:
      Basing on my schematic for Yaesu you should connect to this socket:
      – +5V to pin 5 (it does not matter that in Kenwood is 8 V)
      – MIC_GND to pin 7
      – MIC to pin 1
      If you need PTT as well, you should connect:
      – PTT to pin 2
      – PTT_GND to pin 8
      Please let me know if it worked.
      Mirek SP5GNI

  • I would be very grateful if you could provide me with an impedance matching circuit which would allow me to convert a “2k2 ohm Electret microphone into a standard low impedance mic input (50mv – 2v at 100 ohm).
    Many thanks in anticipation..
    Kind regards

  • Hi Simon

    I case of audio signals impedance matching is not necessary like in RF circuits.

    Electret microphone capsule has not internal resistor for FET, it must be added like 10k in my schematic.

    The output impedance is in practice equal to this resistance. If you want lower impedance you can put 2,2k. But the gain depends on this value, if you use too low resistance the gain will be proportionally lower.

    The gain depends also on the microphone input resistance. I think that for dynamic microphone it is typically rather 600 oms than 100 ohm.

    Miro SP5GNI


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