Here is the circuit diagram of a simple subwoofer filter that can be operated from a 12V DC supply. This circuit is very useful in car subwoofer applications. The circuit is simply a low pass filter whose pass frequency can be adjusted between 60 and 160 Hz.
The circuit is designed around the dual TL072 IC BIFET. Outside the two operational amplifiers in the chip, IC1A is connected as a buffer. The audio inputs left and right after the mixture is fed to the input of the switch S1 IC1A with bipolar. Switch S1 is the phase control switch that can be used to make the subwoofer in phase with the other speakers. When S1 is in position 2, 180 degrees of phase change induced.POT R7 can be used to control the level. IC1B a low pass filter whose pass frequency can be controlled by adjusting the dual band POT R13.