|Electret mikes need 1.5V to run their internal pre-amp circuitry. This 1.5V DC voltage needs to be decoupled from the audio signal by means of 4.7 uF capacitors. More mikes can be connected parallel, to capture the sound over a wider range (we used 4 for our organ). With potentiometers it is possible to balance between high and low side-mikes, one end-potentiometer is added to adjust output volume. The battery power is switched on/off by means of the output jack’s switch contacts. That’s all. See the schematic for more details. The schematic is drawn with TinyCad. I have ordered the parts at Farnell, see the partlist. The parts have been mounted on stripboard.