Electronic – arduino – How to hook up a simple MOSFET to amplify the PWM signal from Arduino

Put a really big inductor (like 30 mH or so, get a bunch of 5 or 10 mH inductors and start putting them in series one at a time) in parallel with the piezo. And cover your ears. You may need to play around with the frequency and the inductance a bit to get the best oomph. I got a surprisingly loud tone out of a piezo at about 4 kHz on about 4 volts with 30 mH of inductance. I'm sure you can do even better with 12 volts.

The idea is that the piezo's capacitance will resonate with the inductance, producing a much larger sound than you would be able to otherwise.

Actually, for 40 kHz you might want a much smaller inductor, maybe around 5 or 10 mH.

Ah you didn't read the reviews for this amp - it seems that it does cause loud pops at power-up and one user has implemented a switch that disables the speakers but this could be done with a delaying relay circuit fairly simply.

As for the "hum" - if it is "hum" and not degradation of the signal or high frequency whistles (etc.) this is likely caused by your power supply having too much ripple on it. The amp is a class D type and these are notoriously poor at coping with power supply ripple. Power supply ripple will come from a standard transformer/bridge/smoothing capacitor and is inevitable in these configurations - you can try putting a 10,000 uF cap across the supply rails close to the power amp.

Why are class D amps poor at rejecting power ripple - the output transistors are basically used as switches and they alternate rapidly (above audio frequencies) and this means any power supply ripple is superimposed onto the speaker wires. A conventional amp can easily reject ripple because it operates linearly and will only push ripple to the output when driven at very high levels (because the transistors saturate when the ripple causes the power voltage to be cyclically at a minimum).

It could be wiring but it's easy just to put a big capacitor across the 12V power rails first then take it from there if that doesn't solve the problem to an acceptable level.