Here is a very basic (and not very good) condenser microphone using aluminum foil, a 9 volt battery and a 300 kOhm resistor. Don't worry, I'm going to make a better one.
Just a few comments to get better performance. The resistor is way too small, it needs to be greater than 50 meg ohm. Also the voltage is way too low, it needs to be at least 60 to 80v. The output needs to go into a Jfet in grounded source config. Take the output from the drain through a 10uf cap. The jfet is fed from 9 or 12 volts and the drain resistor can be around 4.7k Following these recommendations will get you a lot more output. You have done very well to get this going at all as this can be a very tricky circuit to experiment with. I did get some success using the metalised film you see on some crisp (chip) packets instead of tin-foil. You will never get hi-fi but you should get it working so you can clearly hear voice on playback. Nearly forgot, the back plate should have lots of little holes drilled in it to avoid dampening the movement of the film / foil. Good luck with your experiments!
