I found this by the grace of FB who reminded me about it (10 years).
A customer came with a specific request: to increase the volume of the headset microphone for his AM Air band transceiver.
If I remember well, it was a ICOM AM transceiver.
The problem was that the space was very small inside that helmet and the second, and bigger, problem was that the microphone had around 8V DC phantom power.
An intervention in the radio itself was excluded because, you know, "life support device" and going inside it was a no-no...
The solution I found was a small preamplifier for both of his headsets (two pilots there).
The schematic:
The PCB was draw by hand. Pretty ugly but it fitted perfectly in the helmet and by using SMD components, the result was very solid:
