So my goal then is to use this buffer to increase the lows, and if it's possible, to increase the gain to match the magnetic pickup level. A problem for a later date probably is that there is bleed from the magnetic pickups in the blend pot (at 0% magnetic and 100% piezo. I do like the extended frequency range of the piezo, but the piezo by itself is thin sounding in the lows and is unbalanced in level compared to the magnetic pickups. I have modified my guitar (came stock with three magnetic pickups) to now include a piezo pickup (I installed a large piezo underneath the metal bridge piece).
though this is eventually what I am intending to do with this. I don't have an onboard active preamp in my guitar. I designed piezo pick up pre-amps (amongst other things) at Fishman for a number of years. (I would expect something like -3dB 80Hz -100Hz might be a reasonable starting point, and it could end up significantly higher than that) Excessively extended LF response may result in thumping when you handle the guitar or play it.
#Piezo preamps series
If you use LTspice or other similator, measure the capacitance of your pick up and model it as a voltage source in series with that value of capacitance, that will help you to understand the relationship between input resistance, capacitance, gain and LF extension. You may have to fiddle with both to get it right. Large values of capacitance reduce output levels, but extend the LF response. (This will also help tame the potentially high peak output of the pickup when you play hard, and also lowers the general output level.) If you cannot get the sound you want you may also increase/decrease the input resistor value, lower values of resistance will push the LF corner up, slightly reduce output levels, higher the reverse. Start as is and add a few hundred pF at a time until you get the bass response you want.
^^Looks like a pretty good choice, note also that you can place capacitance across the input to control output level and LF response.
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