Piezo Buffer – OPAIC – opamp impedance converter

Updated 01.03.24

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. Watch that diode polarity, the first version of this image was incorrect.

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!