Hi Z Opamp Piezo Buffer

After building multiple hydrophones, (see this) it occurred to me that the same circuit would make an excellent Piezo pickup buffer circuit. It is phantom powered, has a high input impedance and excellent audio specs. My son plays the cello and he was interested in making his pickup sound better. I already had the circuit board, I just needed a way to easily put it in a case with an XLR jack and a ¼” input jack. That should be easy enough so I started searching. My design goal: I didn't want to cut XLR holes into a chassis. That's the hard part of DIY for me!

Here are the parts you will need and links to them

Case: https://prospeakerparts.com/collections/xlr-punches-19he/products/steel-project-box-4-1-2-x-1-7-8-x-1-5-8-pre-punched-for-2-d-series-xlrs

Input ¼”: This one has a connector lock, and uses an XLR cutout hole to mount.

https://www.redco.com/Redco-PM-TRS.html

Output XLR Neutrik NC3MD-LX-B

https://www.redco.com/Neutrik-NC3MD-LX-B.html

Printed Circuit Board (Same one as the Hydrophone)

https://www.jlielectronics.com/diy-accessories/p48-hi-z-piezo-buffer/

Miscellaneous:

(4) 4-40 XLR mounting screws and nuts 

https://www.redco.com/Redco-4C50MXFCUC-4-40-1-2-long-Countersink-Screws.html

https://www.redco.com/Redco-4CNKECTZ.html

Solder

Wire to connect the PCB and Connectors I really like this stuff: (24 gauge Silicone Insulated)

https://www.amazon.com/22AWG-Silicone-OD-Stranded-Insulation/dp/B07T4SYVYG/?th=1

Piezo transducers, like the one I used in my hydrophone, produce a voltage when force is applied to them. Their challenge is that they don't produce a lot of current. That means we need a high impedance circuit to cleanly capture the signal from one. There are a lot of piezo discs out there and they make good pickups for acoustic instruments. The key is to remember, they are not microphones. They do not pick up sound from the air, they pick up the vibrations of what they are attached to. Most commercial ones tell you where to mount them on your instrument for the best results. For DIY ones, experimentation helps.

Our Piezo buffer circuit is built around the OPA1642 Dual Operational Amplifier (Op Amp). Along with all the prerequisite excellent audio specifications, it also features low quiescent current. Which means we can power it from phantom power. No more batteries for your Hi Z buffer! OK, let's look at the schematic “venn diagram” and see what is going on. Looking at the Pink zone, we have phantom power coming in via Pin2 and Pin3 of the XLR. Pin1 is ground. This is coupled to our internal power supply circuit in the Green zone. Here it is turned into 12VDC via a Zener diode and associated resistors and capacitors. The Op Amp needs a split rail power supply so we create a virtual ground in the Blue zone via two resistors and a filter capacitor. That goes to both Op Amp stages to ensure they are biased correctly. The actual buffering and balanced drive occur in the Yellow zones. Our first stage connects to the input that is also tied to the virtual ground via a 1Meg input resistor. The actual input impedance of the opamp is so large that the 1Meg resistor is in essence, the impedance the piezo pickup sees. From the first Op Amp stage we go to Pin 2 of the XLR via a coupling capacitor and to the next Op Amp, an inverting buffer. This gives us our balanced output by supplying an inverted signal to Pin3 of the XLR via a coupling capacitor. Both stages also have a 47 Ohm resistor in series with the output to stabilize the opamp.

Construction is very simple with five wires to solder and a couple connectors to mount. 

I put together an assembly video which you can see below:

This works great with commercial Piezo pickups but also really well with DIY piezo disks. You can glue them to acoustic instruments or anything else you want to capture sound from. I use a bit of E6000 glue, it can be removed. I use thin shielded wire to solder to the disc such as this https://www.redco.com/Mogami-W2368.html It is very easy to work with. Connect the input to the pickup via a 1/4" cable. Then connect the output to a microphone input on a mixer or recorder. Ensure you have Phantom Power on (48V or 24V) and set level appropriately. Start recording.

I can also see this method working for piezo based Geophones. Hmmm… More to follow on that topic!

Enjoy!