Are you just wiring the piezo direct to the volume and tone controls? Piezos usually are better with an active preamp, at least as a buffer.
My thought on the passive set up is that the tone control, even at max cut, is just adding a capacitor in parallel with the piezo. The Piezo is iteself, electrically very much like a capacitor and so your tone cap is loading all frequencies evenly, ie no treble cut.
If you are getting a basically decent signal from your piezo and want to stay passive, I think that doing tone control downstream, in an EQ box or at the amp or PA will work better. You might just get some more tone control action with your current set up with a resistor in series with the piezo, somewhere in the range 2k to 5k at a guess.
JohnH is correct in that he had to do a lot of prestidigitation to get the sound that he wanted.
This is primarily due to the piezo sounders used as specific frequency output devices. Piezo beepers are used as self-resonant circuits and have a very peaked response.
While a piezo is capacitive in nature, it is modeled as an AC voltage generator in series with a capacitor. Aside from the uneven frequency response output voltage as indicated above, it forms a capacitive voltage divider with the external capacitance.
Unfortunately, the frequency response of a capacitive divider is not the same as an inductive pickup driving a RC tone circuit.
I don't know about the value of the pot, but your diagram doesn't show a capacitor on it. You initially indicated a 47nf cap, but I don't see one in your drawing.
Is there a 47nf cap wired to the tone pot, and how is it wired? You may just have omitted the cap from your diagram but I bring it up because your original post referred to the tone control not working.
yes, but what I meant was, take the circuit that you had, with tone pot tone cap volume pot, and instead of the wiring the piezo directly to it, wire the piezo with a resistor in series with it , between piezo and the controls.
But are you getting any reasonable tone so far? Unless you are close to getting a good sound, this will probably not fix it, because usually an active buffer is needed. What it will hopefully do is to allow the tione pot to do more though
OK, I've just tried putting a 3.3k resistor between the piezo and the volume and it did nothing except perhaps made the output quieter. Then I tried another 47n in parallel with the first capacitor and all it did was seem to make it quieter as I turned it down until there was no output at full ccw. So I tried a 220n in parallel with the first cap and it seemed basically the same.