that's it! (finally)
hope that everything's right, i know i haven't put the protection diode you were talking about because the piezo will be very insulated, with a few rubber and foam, so even with the strongest hit i shouldn't get a 150Vpp 
a final thought: since the piezo capacitance is an unwanted feature (isn't it?), what if i put a cap in series to it in order to decrease its value?
ex: piezo capacitance = 10nF If i put a 5nF cap in series, i'll end up with 1/((1/10nF)+(1/5nF)) = 3.3nF total capacitance, right?