Most of the current QRP designs for ‘phone transceivers utilize electret microphones. They are cheap, reliable, and have excellent performance. Often they are not provided as part of the kit and it is the builder’s responsibility to provide it.
Electret elements have very flat response and high output. They employ an internal FET pre-amp within the compact package and require a power source and blocking capacitor from the transceiver. Kits like the BITX, Epiphyte, Survivor, and SlobBucket designed for these and are scaled for the electret signal levels and impedance range.
Many operators, especially those back-packing, need small and light equipment. These often choose to place the element in a fountain pen or other small tube and then add a push-button switch for the Push-To-Talk. Others, especially contesters, prefer the boom microphones on a headset. Those are common and are frequently found in the “Dollar Store” or “Pound Shop” emporiums and work very well.
For my own use, since I mostly use CW and appreciate a clear desk top, I prefer the hand-held microphone since I can hang it up away from the desk top until the time comes to use it. Since I like to purchase junque from the swap meets that I visit, old CB microphones are often available in bulk by the box and these are comfortable for me. The Push-To-Talk (PTT) switch is built in, there is a hanger button on the back so that I can place it in a suitable holder (like a magnetic one that clamps to the side of a radio cabinet or, in my case, a power supply) and a nice coiled cord that retracts itself out of the way.
Keeping it simple, I have often chosen to just replace the dynamic microphone element with an electret element. The little electret element fits nicely in a rubber grommet. That helps isolate it from the noises of the user handling the microphone and the noise of the PTT lever. The flexibility of the grommet holds it in position without glue.
After removing the dynamic element from the microphone shell I insert a rubber plug with a hole drilled in the center for the grommet. I can usually find one that just jams into the sleeve that held the old element. This blocks the hole and backs up the windscreen, maintaining the original acoustic configuration as much as possible.
It is almost as easy to keep the original dynamic element and add a pre-amplifier. I came up with a design that uses just three parts. That qualifies as “simple” doesn’t it? Just a few pennies in cost but I would assume that even the most meager of junk boxes already have these parts. No critical values.
The pre-amp is a simple common-base configuration. The microphone element connects to the emitter of a general purpose NPN transistor (2N3904, 2N2222, 2N4401, etc.) The other side of the microphone element connects to the common return (and probably the shield of the cord). The collector of that transistor connects to the wire that was where the emitter is now connected and is the output. Also connected to the output (and collector) is a biasing resistor that connects to the base. Since no significant power is dissipated it can be any size and the resistance value is not very critical, either. The base also connects through a 10 uF capacitor to the common return. The working voltage of the capacitor should be greater than the DC voltage applied to the microphone (usually 8 volts) and can be most any dielectric. Values between 5uF and 15uF will work just fine. If it is polarized (like an electrolytic or tantalum) then observe the polarity.
A common-base amplifier has a low input impedance and a high output impedance. This circuit adjusts to both quite efficiently and produces a very pleasant audio without adjustment.With just 3 parts, a printed circuit board is hardly necessary unless you are using SMD components. Most of the parts simply solder to the PTT switch. For those that want to use the SMD parts, a hand-carved board takes little room and makes for a clean installation.
I use both the electret and dynamic conversions interchangeably without gain adjustment on several BITX40s, my BITX60, a uBITX, an Epiphyte 2, an Epiphyte 3, and two of KD1JV’s SlopBucket II rigs. The latter has been my primary SSB transceiver for more than 6 months now and the audio reports have been sterling. The dynamic microphones always get the best comments, especially from those infamous audio critics. What better confirmation?
