This subwoofer active filter circuit is a 24 dB octave filter with a Bessel character and cutoff frequency of 200 Hz. So, if you are interested in experimenting with audio circuits in subwoofer range, this circuit is for you.
In subwoofer range, all audio frequencies below 200 Hz can be fed to a single speaker box since the human directional perception of sound diminishes at this frequency range. The normal stereo signals above 200 Hz can be fed to 2 satellite speakers.
How does the subwoofer filter works: A1 and A2 buffer the signals coming from right and left channels. Opamp combinations A2/A4 and A9/A10 function as the highpass filters. The outputs are then connected to the final amplifiers of the battelite boxes. Signals from both channels are fed to A5. Opamps A6/A7 function as the lowpassfilter, A8 as the output amplifier for the subwoofer signal.
The signal level can be balanced between the subwoofer and the satellite lines. The power needed for this filter circuit must ne a symmetrical power supply. The opamps can have either JFET or bipolar inputs.