For the 2.1 amplifier project I prepared a 4558 operational amplifier preamplifier-subwoofer filter circuit. The circuit is based on 2 4558 operational amplifiers, where the main audio input and the bass output volume are adjusted. Power supply balanced + -15VDC
The PCB drawing was not very regular, because I had to prepare it very urgently, but I did it without any problems.
The original schematic was a bit confused above, I cleaned the schematic by using the bass output potentiometer in the PCB drawing Stereo 22k. (For the tests I used mono potentiometer)
The bass frequency of the circuit is not adjustable I don’t know what the frequency range is, but I didn’t calculate it, but it gave a very good bass sound.