a preamp for a piezo-disc microphone

Piezo discs are inexpensive electromechanical components that are commonly used in simple contact microphones for guitars and other acoustic instruments. They have a high impedance, and they require a suitable, dedicated preamp prior to a recorder or audio interface. A piezo signal without a preamp tends to suffer from attenuation or loss of signal in in low- and mid-frequency regions; a preamp dramatically improves the frequency range and quality of the sound; here’s a quick comparison:

How important is a preamp? Just ask a blown glass bowl.
with preamp

without preamp

My basic recording kit consists of a Scarlett 2i2 audio interface and an iPhone with the AudioShare app. An ideal preamp circuit would utilize the phantom power from the Scarlett, and has been detailed in a blog post by Zach Poff (via Alex Rice) and subsequently refined by SmudgerD of Stompville. The Stompville blog page has several posts related to preamps for piezo discs, and they sell inexpensive units in their web store. The Stompville preamps are particularly nice because they’re quite small and well designed (matched JFETs, frequency range, etc). The Phantom Piezo Preamp uses XLR connectors and runs on 48V phantom power.

From my basic and limited understanding, the circuit employs a closely matched pair of JFETs to transform the high-impedance output of a piezo disc into a balanced signal that is more suitable for a typical interface or recorder. Stompville also sells matched pairs and quads of various JFETs for those who want to build their own preamps, or tinker with various aspects of the circuit. By changing components in the circuit (resistors, capacitors), the low-frequency response can be attenuated or the amp’s impedance can be altered.

I agonized for quite a while over how to house them. An ideal container for field recording would be lightweight, durable, shielded, inexpensive, easy to acquire, replaceable, and easy to modify (drill, epoxy-friendly, etc). Metal tea tins were a serious consideration for a while, but they’re slightly too large and perhaps difficult to drill. Spices sometimes come in boxy metal containers, and while smaller they also seemed more flimsy and tenuous. Larger pill bottles seemed like they could work, but the lids didn’t secure without the inner cap. Many pill or spice containers have openings that are slightly too small for the preamp.


 
I finally settled on some small polystyrene (or perhaps polycarbonate) containers with HDPE (?) lids that I had in the spice cupboard. The lids screw securely onto the jars and they are light in weight. Polystyrene is quite brittle, but holes can be reinforced with epoxy and other materials (small metal washers). A 7/8″ hole is just about the right size for the Neturik XLR chassis connectors.

If handled inappropriately (smashing, dropping, etc), a piezo disc can potentially produce a significant spike in voltage. To help protect the preamp from this, I soldered a couple of 1N5221B Zenner diodesĀ (in series, opposing polarity) to pins 2 and 3 of the female connector. SmudgerD also has a great blog post about voltage protection. It shouldn’t really be a problem, but the diodes were cheap and easy to solder onto the XLR connector.

These small jars were at least good place to start; I’ll have to see how they handle in the field, but I’m optimistic about their durability. I have yet to apply the shielding, and will likely line the inside of the containers with copper foil and then solder a lead to it from pin 1 of one of the XLR connectors.

 

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