opAmps are fed by +/- 9V.
I have two more filters like this one, in total three filters.
If I use a signal generator as input signal and observe output via an oscilloscope; all those filters give me satisfying results(not the same with simulated results, but close enough considering I am using %5 resistors and other possible non-idealities.)
However, in reality, the input signal comes from a microphone output. And mic takes sound from a 10 cm away speaker(it is being so loud that, microphone gives around 600 mVpp directly(***), without any amplification done.) At this point, the problems start to occur.
(***) - is this normal? I mean I always saw that microphones give output in the range of maybe tens of milivolts while I was doing research on the web. The microphone is an electret one, and I don't have any datasheet or so.
The filters which works fine before, starts to behave problematic. For example, even if I don't connect any input signal(the output of mic), the filters output a sinusoidal-ish waveforms around 400-500 mVpp at their(filter's) designed center frequency when the loud speaker is on. I even connected input to ground, but did not help. One thing I could not understand is why filters interact with sound when there is no physical(electrical) connection with the microphone?
Also, when connected with microphone, the filters still does not behave as before. I think it might be resulting from some kind of loading effect. There is a clear difference between mic out and signal generator out, which I could not figure out. Two things I suspect are,
The filter opAmps are NE5532 which has a very low input impedance of 30k ohms or so.
I think adding a voltage buffer to microphone output may help since it will(?) make output impedance more similar to the output impedance of a signal generator, but I was not sure and did not try.
I am also attaching the mic driver circuit, which is pretty simple.
It is the exact circuit in my implementation with +9V instead of +5V.
The microphone output itself is fine, it gives pretty similar waveforms to loudspeaker output.
I might look combining two questions in one, but I could not be sure if they are related whith each other or not.