Hello! 
I'm making an FFT audio spectrum analyzer. I found a library here. I tried an example code fft_adc on Arduino Nano and it compiled. When I connected op-amp output to the A0 pin and watched the serial monitor, I got hieroglyphs instead of normal output. Baud rate of the serial monitor was set correctly. Maybe someone has tried something similar and have some experience?
:o
The comment at the beginning of the fft_adc code says:
there is a pure data patch for visualizing the data.
It sends binary data to the Serial port and, in that form, you need something to translate it on the PC.
You can modify the code so that it prints readable ASCII to the serial monitor. I have some code here somewhere that does that and I think versions of that sort of thing have been posted on the forum too.
If I find mine I'll post it.
Pete
BTW did you DC offset the output from the op-amp? The Arduino ADC only handles 0 - 5V.
Pete
See message #23 on this page. It has code which prints the results of the FFT and then stops. You can modify it to keep printing but to do that it would probably be best to print the result of one FFT all on one line.
Pete
Thank you, Pete!
Now I can see the output of FFT. Should read this before...
I made a simple audio amp with 2,5 V offset and a first order low pass filter. It seems that I should need higher order filter.