JFET Piezo Pickup Buffer

The Problem

My Japanese Fender electric/acoustic fretless bass is a fine instrument, but the piezo pickup lacks low-end punch. Some inspection of the internal electronics reveals the reason: the piezo directly drives the pot that mixes the two pickups together; the output of the pot then goes to the active electronics. This means that the piezo pickup is seeing a best-case impedance of about 250k, and a worst-case (when the balance pot is in the middle position, effectively shorting the two pickups together in parallel) impedance of about 10k. It should be seeing at least 1M to be really happy. Here's what the factory active electronics look like. By the way, notice the tone-shaping network in the first stage of that circuit? Here's what its AC response looks like, as calculated by LTSpice.

The Solution

So that the piezo pickup sees the impedance it needs in order to have its full bass response, I built a simple preamp circuit. I present the circuit here, for the pleasure of any hobbyists who wish to build it. For those of you who just want the end results, it is also available for sale as a complete unit, requiring only a small amount of soldering to install. Contact me for details on purchasing.

Building It Yourself

I prototyped it on a small piece of prototying board, and it's a simple enough circuit that I'd encourage anyone who wants to give it a try. The JFETs could be just about anything with Idss of 1 to 5mA and cutoff voltage of 1 to 3V; I used 2N5457 because I had some lying around. (The schematic reads MMBF5457, which is the surface-mount version of 2N5457.)

On inspection of the circuit you'll notice that R6 and R7 form a voltage divider, so this preamp actually has attenuation rather than gain; that's necessary because, with the piezo properly buffered, it's got so much signal that it would otherwise be louder than the magnetic pickups. If you are building this preamp for a different bass and don't need to blend with a magnetic pickup, you might want to use a higher value for R7 (such as 100k), in order to get the maximum possible signal.

Those same resistors also affect the interaction between the pickups. In the middle blend position, each pickup is loaded down by the impedance of the other one. This is good, because otherwise the midpoint would be twice as loud as the extremes, and also because it helps provide tonal range. If you want, you can reduce the effect, by increasing the values of the resistors.

The pictures and schematic here describe an earlier version of the PZP-1, and are mainly of historical interest. The latest version of this circuit has adjustable output level and is built with surface-mount components. If you're considering building one, I'd recommend that you use the latest version of the circuit (whether or not you use surface mount, of course, is up to you).

Preamp Schematic The schematic

Prototype Preamp Photo A photo of a prototype

Finished Preamp Photo And here it is as a finished unit. The knob is to show scale; the board measures 1.2" x 0.8" (30mm x 20mm), and with components it is 0.4" (10mm) thick.

Technical Specs

Power supply consumption (with a 9V supply) is 250uA.
Input impedance is approximately 10 megohms in parallel with 100pF.
Driven with a 500mV signal, THD+N < 0.03% from 10Hz to 20kHz; distortion is mainly 2nd harmonic.
Running on a partly drained 9V battery (actual voltage 8.5V), onset of soft, asymmetric clipping is at about 1.5Vrms.
Frequency response is within 1dB from below 10Hz to well above 20kHz.

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