I have problem at output of the first opamp . About 3 -4 minutes , the opamp's output voltage decreases by 0.1V . Although I reduced gain of opamp to 2, but the opamp 's output was still reducing 0.1V about 3 -4 minutes . Could you me about this ? Thank You !
It almost sounds like there's no DC path to ground at all terminals of your op-amp to allow bias currents to flow so the capacitances are just charging up slowly and killing the signal...except I don't see that in your circuit anywhere.
But Ron does have a point about your capacitor being backwards in the circuit if you are using a polarized capacitor (electrolytic or tantalum).
You can find out if the voltage is positive all the time, negative all the time, or goes + and -. Measure across the cap. not to ground. Some types of caps can only handle DC and some can work with AC.
AND
What AudioGuru said.