mo-seph
Schematic mistake - Piezo -ve
Hi all.
Note there's seems to be a pretty big mistake in the schematic. In this version (versus the original) the piezo -ve pad / path has been removed.
This original connection point isn't connected directly to ground (go look at the schematic) and is likely to have a fundamental effect on performance in terms of not making the input differential (i.e noise) and impedance issues.
The GND point (for the piezo) is for separate shielding of the transducer to reduce noise allowing the +ve/-ve inputs to form the differential input.
Not the maintainer of the circuit, but this is not a mistake. This is simply the unbalanced variant of the circuit.
When making a piezo preamp you can either make it balanced, that means you have a positive and a negative signal input, and a separate shield input (e.g. solder + to the center of the disk, - to the outside of the disk, then insulate the whole thing, then add shielding copper tape around it and then solder the shield to that copper tape. This might reduce the noise a little bit if done correctly. In this case you need to run 3 wires to the piezo.
Now if you use your piezo in very loud applications (e.g. on drums) doing that isn't necessary at all an you can go with an unbalanced version where you only run two wires to the piezo (+ goes to the center of the disk, shield goes to the outside). Both variants are legitimate, but they are for different applications.
