These are simple designs for high impedance preamplifiers. They are intended for piezo pickups, and in particular for piezo bridge pickups for (nylon-stringed) guitars. There is a single-channel and a dual-channel design.
The designs are in KiCad. PDFs with the schematics, and the Gerber files for the PCBs are also provided.
This circuit was built for use with a Shadow SH097 bridge pickup. The pickup has a capacitance of 680pF. Yet, input impedance of this preamplifier is only 1MΩ, because the basses were already quite pronounced.
It is a pretty classic opamp circuit. Amplification is fixed at 1× —the circuit is only to adapt the impedance (piezo bridge pickups are pretty loud already). The main feature of the circuit is that it is compact, and especially narrow. The idea is to fit this preamplifier into a slot immediately below the pickup.
The PDF with the schematic also shows the wiring for a TRS socket and battery; these are not on the PCB, and technically not part of the circuit). The main idea of the wiring is that the battery is connected only when the jack plug is inserted. When the plug is pulled out of the instrument, the battery is disconnected.
This circuit was built for the ARTEC PG-333 bridge pickup. The pickup connects two piezo bridge pickups in parallel, where each pickup covers only three strings. For use with this preamplifier, the two piezo segments have to be separated, and each connected to a separate input of the preamplifer.
I measured one of the piezo segments to have a capacitance of 580pF and the other as 970pF —to my surprise. This being the case, you will want to position the 580pF segment at the treble side, and the 970pF segment at the bass side of the bridge.
The dual-channel circuit has two independent impedance converters, and a third preamplifier to mix the outputs of the first stages. The balance of the "treble" side relative to the "bass" side, can be adjusted with a trimmer pot.
The preamplifier has a "bass boost" filter that boosts the frequencies below roughly 250 Hz by roughly 5 dB. The resonance chamber of a classical guitar has a main resonant frequency at around 200 to 250 Hz, which is what the bass boost emulates.
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