There are a variety of adapters available to adapt a dynamic microphone to fit an 8 pin ICOM radio microphone socket. There are also hams that use the auxiliary DIN jack on the back of the radio to connect a microphone via an external mixing panel. The problem with the connecting a dynamic microphone via the front jack is the output of a dynamic microphone is about 1.8 mV whereas an electret condenser mike can output up to 56 mVpp. The prescribed fix is to increase the gain on the radio to try to compensate for the low output.

The following is a solution.

Note.  The above preamp uses the phantom power provided by the radio's microphone input.  Like most devices that use electret condenser mikes. ICOM radios have a DC bias voltage up to 10 volts applied on the input line. 

My solution is to replicate the output of a phantom low powered electret condenser microphone. I found an old schematic from the web about 10 years ago of which I derived the above.


150 – 600 Ohms Matching Transformer

This tiny matching transformer is one I had left over from an audio project. I found a bunch of these on eBay. For ham radio applications, you really don’t need a nickel core transformer.  It just has to be a 150 ohm to 600 ohm.

Note.  The transformer is susceptible to AC magnetic fields such as being too close to a power supply. 

After testing, I was able to achieve the same output from this box as the ICOM handheld microphone that came with the radio.

The following is the resulting control.


Red TX - Yellow Power

The rocker switch has three positions: Left – PTT button, Middle – off and Right – TX on. The cable between the radio and the box has one shielded conductor and three other conductors. I bought the 8-pin screw-on socket and two plugs from eBay.

Final Note.  You have to sit at about 8 or so inches from the microphone.  If you get up close, the rig will attenuate the higher frequencies and your voice will have too much bass. 

73s K8BSX