JFET Design Spotlight: Phantom-Powered Piezo Preamplifier Design

Another glimpse into the creative mind of analog designer Kirkwood Rough.

"‘No batteries allowed on this instrument,’ was the statement from the luthier. So, a phantom-powered preamp needed to be configured using a standard guitar cable. 'Nothing special.'

First, the piezo loading had to be minimal so its charge capacity would not run out and sound 'buzzy' on the low end. Second, the piezo needed to be sensed differentially and therefore shielded. Third, no batteries could be attached to the instrument, so an active preamp had to be powered through a single- or two-wire guitar cable, preferably a single cable so that any standard guitar cable could be used. Fourth, the frequency response could not be affected by the length or loading of the guitar cable.

Piezo loading is critical where charge capacity is limited, and too low a resistance can deplete this charge at low frequencies. A distorted, chirpy sound can evolve from the E and A strings, as well as from transient loading during power chords. This particular piezo required a 20 MΩ or greater load. Therefore, a low-noise JFET front end was powered by a voltage from a low fixed-load impedance to minimize cable effects.

One JFET drain is AC-decoupled so that the other can current-modulate the cabled voltage source. This current modulates the Q1 emitter's Z < 50 Ω, and its drain current then flows through R2. That voltage, buffered by Q2, becomes a low-impedance signal output. Overall, not much to it.

In the end, the gain needed to be reduced another 10 dB with an improved sensor through a couple of resistor changes in the resolver circuit. As a matter of habit, Linear Systems parts proved to be an effective drop-in solution requiring very little board space in surface-mount fabrication. The -3 dB bandwidth was nearly 70 kHz.

Note: High-value resistors perform best when they are no smaller than a 1206 package and micro-pen formed for the lowest excess noise.

As usual...

Kirkwood"

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