Thans for pointing that out crutschow. I've just been skimming through a lot of papers lately and i missed that. So I see that there is a dual stage LM324 amplifier with low pass filter capacitors. According to the pictures on page 8, this is supposed to give out a clear square wave signal when passed through a schmitt trigger. But i googled the LM324 audio applications and read that the opamp causes heavy crossover distortion. Do you think that LM324 is still gonna be suitable for this project? Can LM741 be a good choice?