Thanks, alec_t.
Similar circuit as the previous one on #3
±4dB along 300Hz to 3KHz is just fine, 5% distortion is tolerable. I believe the resonance of the piezo disc is around 4KHz, not of concern.
Design notes I previously referred to, prefers a source resistor as well as the drain resistor. Please educate me what they differ in behavior.
Also, gate resistors suggested were to + and to gnd instead of + bias only. Please educate me what they differ in behavior.
What differs in biasing the gate from drain or directly to + ? Some AGC behavior ? Please educate me.
The 2N7000 data sheet suggested 2.1V at the gate, if I remember well. Please educate me on optimally biasing this particular circuit, or if the single 1MΩ is just fine.*
* Edited : OK, the Q point should be above the Vth, into the triode/ohmic region, and the single bias resistor to + should be fine.