Is there a time you SHOULDN'T add a resistor to the piezo's

[Michael Lemke]

I've read the articles about the oscillations of cheap amps and that adding the resistor will help prevent it, and doesn't hurt otherwise. I'm just curious if there is a time that adding the resistor is actually bad...

[Bill Fitzmaurice]

No, just if you have a high quailty amp it's unnecessary. But what you never need to do is follow the manufacturer recommendation to use a 20 ohm resistor. That primarily serves to pad the 93dB sensitivity of the piezo down closer to the 87dB sensitivity of the average hi-fi midbass.