To mneary:
Your comments leads me to some questions, which I hope is okay. They are:
1. On the output. I see your point. I made a very rough prototype on a breadboard and connected my Sennheiser head phones, which did result in audible sound. But maybe I could simply build a small common collector circuit just before the jack. Do you think that would work?
2. With regards to the bias resistors for the op amps, I'm not quite sure how to calculate the needed sizes. How can I do that?
3. R4, R9, R14 and R19 are there because I snatched this circuit construction from a tutorial on the net. I don't remember what the explanation for that resistor was, but when I experimented with removing them, it had no audible effect, so I guess they're superfluous...
4. The Sallen-Key filter was calculated using TIs "How to build filters in 30 seconds", which estimated that the Cin for that circuit should be 100-1000 times greater than C13, which I guess is to minimise its influence on the frequency characteristics of the circuit. Wouldn't a 330n be too small?
5. With regards to the mystery Cs... That's a bit embarassing... The reference designators should have been R2, 4, 6, 8, etc... Thanks for spotting that
These are a lot of questions, and as you can probably tell from them, I'm quite new to filters and analogue design in general, so this is also a great opportunity for me to learn new things. I hope you'll feel like taking the time to answer my questions
Also: This listening device is supposed to pick up rather faint and distant sounds and amplify them to an audible level, and the MCE100 is the only mic I have. Do you think it will do, or should I look for an alternative.
And finally: Would you happen to have the data sheet for the mics? I couldn't find them anywhere.