Your drawing basically shows the functional operation of the Graph Tech preamp.

The only thing missing is the selector switch at the preamp output.

(just as a note, the humbucker selecter switch should be after the controls, not before)

If you are concerned with keeping the piezo signal pure, the preamp will do that. The preamp selector switch is a convenience that allows you to leave the controls at their settings and switch between pickups without having to re-adjust them. Without the switch, you would have to roll the volumes all the way back on the unused pickups, and reset them again when used.

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