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I have been playing with FastLED Painter and it is an excellent addition to the FastLED library. Thanks so much for your contribution!
One issue I am having is that code that works just fine on ESP8266 does not work on ESP32. Specifically, if I make a sketch with 75 pixels it works fine but if I try 76 or more, everything is messed up. The animation runs super slow and, instead of the first 76 pixels lighting up, the entire strip of 300 lights up. It is as if the pixel array(s) are being expanded to many times the requested size.
I can run examples from the basic FastLED library on ESP32 that work fine so it is something in the brush creation code that is going astray.
Any thoughts as to what might be going wrong?

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Thanks for the directions to look in, most helpful. You are very generous with your time, and I appreciate it.

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There are thousands of examples available online and the values you need depend on what you want to do. A good starting point is to search for an ‘application note’ realeased by most manufacturers (analog devices or texas instruments have tons of them about almost any amplifier circuit, including part lists).
Building a piezo amp even though one of the simplest amplifier circuits has its pitfalls. You can use the default circuit called ‘charge amplifier’ (just google it). There is no such thing as a zero-gain piezo amplifier, as the piezo outputs a charge (i.e. a number of electrons) and the amplifier outputs a voltage. You need to select the proper C_f (feedback capacitor) for your sensor’s output. You can start with a 1nF capacitor. Just make sure it is a high quality capacitor, like a ceramic with the NP0 dielectric or a foil capacitor. Selectring the wrong capacitor will lead to signal distortion. Add a high value resistor in parallel with the capacitor, like 1M or 10M (that is megaohm). Also you need a dual-voltage supply as the charge amplifier inverts the signal (causing a 180° phase shift).
R_in = 1k (input resistor is just to protect the circtuit)
C_f = 1nF (feedback capacitor sets the amplification)
R_f = 10M (feedback resistor removes DC offset, solder parallel to C_f)
and that is all you need. Apart from the amplifier with really high input impedance. The LMC602 is a very good choice as it is one of the highest impedance amplifiers in existance and as a bonus is really low noise.
Extra Tip: only use shielded cables (i.e. coaxial cables) to connect the piezo sensor (connect the cable shield to GND), the tiny charges emitted by the sensor combined with the high impedance input will pick up any electromagnetic noise.

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It sure works with four coils, it would even work with just one at the expense of efficiency.

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