To create a 2.1 audio system, 5.1 channel home audio or active subwoofer, low pass filter/ Subwoofer controller is required to pass the low frequency and eliminate the high frequency. Because of the low frequencies that spread in all directions, the music recordings are mono low frequency, namely the right and left it's same. So it takes only one subwoofer, this low pass filter circuit should be able to adjust the frequency of the crossover so that the overall response frequency can be flat and can adjust to the frequency response of the subwoofer speakers used. Below the circuit design of subwoofer controller:
U1 A/B NE5532 as a pre-amp which amplifying a voltage of about 3X. Then the output is mixed so it becomes a mono signal. Then the signal goes into U2A NE5532 which functions as a subsonic filter, ie signal with the frequency below 20 Hz is eliminated. Signals with frequencies below 20 Hz can not be heard and often cause damage the subwoofer speaker.
Then signals with frequencies above 300 Hz are discarded by U2B, whose output is fed to a low pass filter whose crossover frequency can be adjusted from 35 Hz to 150 Hz with U3A NE5532. With the U2B, the slope filter becomes sharper. U2B and U3A functioned as a second order Butterworth filter.
The output of the above filter is inputted to a phase regulator whose phase can be adjusted from 0 to 180 degrees. The goal is for subwoofer sound phase like the sound of satellite speakers. Due to the placement of the subwoofer which is often not parallel to the satellite speakers so that the required phase correction.
For making this subwoofer controller easier, I share the PCB layout design for it below:
If you have any question please make a comment on comment box below.