Microphone pre-amp

The circuit above, is a simple low current audio pre-amp for most ICOM HF rigs.

I built one, and it give lots of gain, and no RF feed-back. The circuit should be small enough to fit on a postage stamp and all parts can be purchased at Radio shack. Just "bread-board" it on a Radio Shack "perf-board".

The capacitors that have no voltage rating, are disk ceramic type 25v or more. The caps with voltage ratings are electrolytic type.

I have built a pre-amp like it, and it works very well with the HEIL Gold-Line mic, and drives the ICOM rig the same as a ICOM SM-8 or SM-20 which are pre-amped mics also. I found that the circuit works with nearly all dynamic mics.

The version that I built, is a W1FB Doug DeMaw version that was written up in QST August,1989, titled, "Build a low cost, booster, mic." That version had a Op-amp in the circuit which gave a small amount of EQ function, but I did not include it in the above circuit, for simplicity sake.

It would be prudent to place the circuit inside a metal box or enclosure of some kind to reduce the possibility of RFI. I took the added step of adding a ferrite bead on each end of the audio leads inside the box. I have had no RFI problems with the circuit.

I know that HEIL claims that a pre-amp is not needed with ICOM rigs, but I have a low volume voice, and a pre-amp really helps me drive the rig without having to raise my voice to uncomfortable levels, to get proper drive.

I don't like to use Compression/mic prossesing to reach proper drive levels, this pre-amp allows me to use little or no compression.

Give it a try. Home brewing starts with small projects like this, and it's really cheap too.

Copyright © 2005 M.A. Erickson, KK5DR.  All rights reserved.