I'm building a simple mixing stage, lowpass filter, and final amp in sigle supply (9v battery). I started from the LM386 circuit on the data sheet. It had a lot of distortion which was solved with the decoupling capacitor (fig.1). As a second step, I introduced an op-amp unity gain buffer to isolate the mixing stage from the filter stage and be able to calculate easily the cutoff range of the pot. I put the input capacitor (C11), matched the input pots with the input resistors (useful?), and used C13 to remove the bias, since the LM386 wasn't working with the biased signal (fig.2).
In fig.1, everything works fine: I tried many input sources and the amp works well from a gain factor of 20 to 200.
In fig.2, it works until the gain factor of LM386 becomes very high. At this point, if I open the filter and let the high frequencies pass there is a significant distortion.
The input and the speaker I used for the test are the same.
Any solution to have the circuit in fig.2 withstanding the same amp gain without distortion as in fig.1?