Just an interesting thing... everybody know the small 12VDC piezo syren with 120dB loudness. I'm just curious what kind of driver contain it, and disassemble: it have a simple dual oscillator (syren sound) with BD139! and transformer as driver. The voltage (pp) on the piezo element was 300V!
I think: this is a special piezo, but have tried with a poor quality (made in Hungary) piezo, and work successfull! (first i think, i make a big BANG, because the datasheet say: max pp voltage 20V)
Another thing: Murata and others made piezo buzzers with in-built (one or two transistor) oscillator and a small inductor seriel with power input. (the poor quality piezo-buzzers haven't inductor, and produced less than half sound)