On Aliexpress you can buy cheap amplifier modules but I want to do something with noise cancellation (fan noise in particular) so I was wondering whether 180° inverted amplifier modules are a thing (I can't find one) or is it something you can only get if you make it yourself? Would it be practical to modify the module below to be inverting (the microphone module I have has variable pre-amp so I don't need to be able to adjust the inverting amplifier). Thanks!

Edit: actually, when I look at the datasheet for the PAM8403 it looks like the op-amp configuration is baked into the IC and therefore not changeable, right?

 

Maybe this one instead?

 

Why not just reverse the wire on the speaker? That should effectively invert the out put.

I do know that what you show in the last schematic is wrong, or at least part of it. The cap being replaced by a resistor. The cap is there to keep any DC signal from going into the amplifier, sound waves are AC/sine waves after all. And caps pass AC and stop DC.

 

Can you really cancel sound just by reversing the speaker wire polarity? Well slap my head and call me Sally...

 

Can you really cancel sound just by reversing the speaker wire polarity? Well slap my head and call me Sally...

Probably not.
You can reverse the sound polarity 180° by reversing the speaker leads but that doesn't necessarily mean the noise will be cancelled.
The phase of the noise signal you are trying to cancel varies with location and distance of both the mic and speaker relative to the source of the noise.
Also a fan is not a point source for noise but generates it over the whole fan area, while the mic and speaker are point sources.
All this makes it quite difficult to actively cancel noise.

You can try but your results are likely to be disappointing.

 

didn't look at them but these may help to get some ideas - 1950s – With U.S. Patent 2,866,848, U.S. Patent 2,920,138, U.S. Patent 2,966,549 by Lawrence J. Fogel, systems were created to cancel the noise in helicopter and airplane cockpits. From, https://en.wikipedia.org/wiki/Active_noise_control