I love using piezo contact mics - they're small and cheap, and can do a whole bunch of different things.
Unfortunately, they can sound pretty rubbish. In particular, they don't like long cables - all of the bass disappears, and they get quite "muddy".
I came across [http://www.zachpoff.com/ Zach Poff]'s writeup of Alex Rice's piezo preamp [http://www.zachpoff.com/diy-resources/alex-rice-piezo-preamplifier/ here], which is a great solution - phantom powered, low noise, pretty simple and a little bit of gain.
With a lot of help from [http://www.mattvenn.net/ Matt Venn], I've redesigned the board layout with a view to putting it inside bottle-cap hydrophones: as small as possible, and approaching circular.
The current version is always on GitHub: https://github.com/mo-seph/PiezoPreamp
(Creative Commons, as per the originally shared versions)
I've had a batch of 100 boards made - if you want to buy one, email me. They'll be unpopulated (i.e. you have to solder components on it), but cheap (£1-2, probably). If there's enough interest, I'll put together some kits with components in. A Farnell parts list is attached.
== Project Blog ==
[http://mattvenn.net Matt] put this picture of the next round of Piezo Preamp boards up on his [http://www.flickr.com/photos/matthewvenn/7142271029/in/photostream/ Flickr Stream]. I've removed the piezo -ve input, and tied it to ground, which might loose 3dB of gain, but makes figuring out ground planes a lot easier. I couldn't hear the difference when I shorted piezo -ve to ground, and it makes the layout much smaller and easier. We're down to about 50% of the original size now.
Got the first set of boards from [http://mattvenn.net Matt Venn] for prototyping piezo preamps, and they're great! I've soldered both boards, after using the JFET matching described [http://www.nrgrecording.de/html/fetmachting.html here] (NOTE: link seems to be dead - I'll copy the circuit diagram from my notebook).