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 connecting a dynamic microphone via the front jack is the output of a dynamic mic is about 1.8 mV whereas an electret condenser mike can output up to 56 mV_pp. 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