I had this preamplifier schematic archived copied from somewhere and inspecting it ;

Would like to know :
C2 - is that a minimum value or a calculated need ?
Are R2 and R3 supposed to be equal value or they are fine as shown ?
What is C3 doing, just DC stabilization at near half supply ?
Is IC1A just a high impedance input buffer and the amplification is only on IC1B ?
Does R1 affect frequency response ?

Do you see something unusual in its design ?

 

R2 and R3 and setting a DC level at 40% of V+ for the benefit of the op-amp inputs. Whether it's 40% or 50% depends on where you should center the valid input voltage range for the amplifier chip.
C3 is holding that DC level steady and providing an AC ground for the input signal.
C2 i a high frequency noise filter and bypass capacitor which is probably soldered right next to the amplifier chip.
I don't know how to calculate those, just that they are mighty helpful for fast switching circuits, and a piezo is, by definition, a fast leading edge event.
yes, yes, no.
R1 is a load for the piezo transducer. It keeps the input voltage from going to hundreds of volts which the chip can not amplify.

 

[QUOTE="#12, post: 1136803, member: 105487"R1 is a load for the piezo transducer. It keeps the input voltage from going to hundreds of volts which the chip can not amplify.[/QUOTE]
In addition R1 supplies the bias voltage to the + input of U1A. R1, in combination with the capacitance ofthe transducer, sets the low frequency response of the transducer.