If you are using a standard piezo pickup, all of them I've seen are single ended (one signal wire and a ground). So if you are using a commercial pickup, you'll probably have to re-wire it for a balanced connection. The cable shield can be left open at the pickup and grounded at the amplifier end.
The pvdf film device I am using is just a generic device which has a very very high output impedance. I had to crank my amp for straight amplification without a pre-amp. Then the problem became that the amplification was SOO high, that all the background radiation was amplified instead of the signal, even with proper shielding (on the sleeve, still have to make a copper jacket for the device. I wouldn't have thought the surface area of the pvdf would have been big enough to make that much noise.)
Also, the PVDF film doesn't like to be stretched too far without amplification. On the instrument I have a piezo ceramic crystal that is attached through 20 feet of cable before being amplified and it works fine, the pvdf on the other hand is still a whisper even with the amp AND a pre-amp cranked way high. I am trying my best not to have to put a amplifier on the instrument for weight concerns.
With the input buffer, your differential amplifier (technically it's not a comparator) can have much lower impedance, maybe 5K would be good.
Good point... I had a bunch of 400kohm smd resistors laying around and I guess I tried forcing myself into using them. Luckily I found a reel of 5k's though.
You don't need all those 3.3V/2 bias circuits. I'd bias the op-amps at half the 12V supply. C6 will block that higher bias and R15 & R18 will re-bias to 3.3V/2.
Doh... Forgot about the blocking resistor on the output stage. That would explain why the output is still a little jittery in simulation.
Just to verify though, using a voltage in the middle of the voltage range of the opamp will give it better frequency response correct? Because its allowed to transition from 6v to 0, instead of 1.65v to 0? Something to do with capacitance and me not wanting to do the calculations.
You need a resistor in series with C6 (maybe 1K) to limit the current out of the op-amp when/if your protection diodes kick-in.
The TL072 are designed for continuous short circuit current source and sink, or is it still good practice to do this? Ill have to check but the output of the TL072 is well below the 1N4148's.