Microphone Amplifier

Awhile ago I purchased a used Icom IC-208H mobile VHF/UHF FM transceiver (now sold). It had a problem with low transmit audio; a little research revealed that there are two settings that must be checked: microphone sensitivity (low/high) and narrow transmission (on/off). Once set to high and off the rig worked fine with the supplied microphone.

Because I was driving a standard shift transmission car at the time, I wanted to use a headset microphone instead of the hand mic. I found one at a fleamarket and replaced the dynamic element with an electret element from an old casette recorder. However the transmit audio level was low again, with no settings left to boost it up. My solution was to build a simple op-amp audio amplifier with adjustable gain, powered from the rig's microphone connector.

mic amp schematic

I built the circuit on a small piece of perfboard and installed it in the mount I built for the radio head. (The mount lets me place the head in one of the cupholders on the console.) Adjust the gain trimpot until you get good audio reports.

The circuit uses an old 741 op-amp, because I had a few in my partsbin and its fine for this application. Almost any common opamp would work in this circuit. If your rig uses a different microphone jack, check your manual for the correct connections.

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