Recording audio with piezo contact microphones

[ErnstHot]

I am planning to do some field recordings with a piezo contact microphone. As I understand it I need a high impedance buffer to avoid high-pass filtering, and as the sound card (http://us.focusrite.com/usb-audio-interfaces/scarlett-2i4) I will be using has phantom power (+48V) it would be nice if I could use that, rather than having to use batteries.

Now, I found this circuit: http://www.jensentransformers.com/as/as004.pdf and was wondering if the part on the left of the balanced cable would work with the microphone input on the sound card?

Does the circuit need some clamping so dropping the piezo doesn't fry the input of the sound card, and if I build two of the circuit how well would they match for recording in stereo?

[fcb]

That Jensen circuit will do fine, you may want to remove the R1/C1 network though, and you probably won't need C2 either (although no harm in keeping it).

You could use a balancing transformer, although I would try it first without.  Just make sure you use good quality screened cable for the piezo, plenty of braided screen cover.

As for protection, then probably a couple of back-back 3v3 zeners on each line would be adequate - you may not need it depending on the design of the soundcard i/p.