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Hi,I'd advice putting a capacitor in series with the piezo - DC kills piezos.
PWM signals are AC A capacitor in series basically stops current from flowing when the voltage isn't changing - which stops DC currents.
... those speakers that Webbot is showing us are similar to the ones in iRobot, which I used it in Axon II and I burned it, when I connected, by mistake, to the pins Vcc/Gnd instead of PWM_pin/Gnd So then I bought a piezo and I developed a program for playing some music using PWM signals and everything worked fine!but no idea about speech synth...
@Soeren,yes you are right and I confirm that the DC kills, (probably) piezos (not tested yet) and magnetic speakers (fried like a cheese stick)
so in case you use magnetic speakers I would use as a protection a capacitor in series yes, and for piezos I don't know.
Also what it is important to know here is that both components work like a low pass filters, converting the square wave into a sine wave signal, so there is no need of pre conditioning the signal.
What size capacitor is good to go with a piezo? Around 0.1uF or more like 200uF?