Piezo Buffer – OPAIC – opamp impedance converter

Updated 08.23.23

I have built a few of the OPA Alice Mini based Hydrophones and I got to thinking that the P48 circuit (48-volt power supplied by a mixer or digital recorder) might be useful in a Sound Urchin or some of the other piezo-based instruments I build. The original design by Jules Ryckebush is great, but it is also SMD (surface mount), and after building a few of the hydrophones with PCBway boards I figured I should give vero a shot.

Note: I used a 10M input resistor, I started with 1M like the original schematic and found it worked but was still pretty lackluster with the standard piezo disc. The ones I am getting from Tayda measure around 10nf (10,000pF) I remembered reading someplace about using 5M input resistors for cigar box guitars, so I dove into Google.

I gleaned some great info from Ross Elliot’s webpage, “One thing you can be sure of is that the effective capacitance may be no more than 12nF (12,000pF), with many being a great deal less.  Some piezo accelerometers can have as little as 200pF capacitance, meaning that the preamp input impedance must be at least 20MΩ to get a -3dB frequency of 40Hz.  If you aim for a higher impedance, the circuit is more flexible and can handle a wide range of different pickups. ”

I settled on 10M to best capture the most freq. with the most varied types of piezo. Be sure to read Ross’s entire article for a better understanding of how impedance buffers and piezos work in these types of applications.

I found some tiny 50 volt, 0.1uF electrolytic caps but you can use anything above 25volt, or film, mylar, ceramic, etc.

P48 Sound Urchin Mini

Now I have a P48 Sound Urchin Mini to take with me on sound excursions. The ability to power the Sound Urchin with my field recorder is great. I get more wild looks and the interactions with curious passers-by is always fun.

Tested with a TL072 and the OPA2134, the TL072 has a higher noise floor but isn’t a big deal with proper gain staging. The OPA2134 is pretty much dead quiet.

Should I make these available as an option on the Sound Urchin? Let me know in the comments.

Let me know what you build!