Passive subwoofer filter
The schematic shown, does no need any power supply because is a simple low frequency passive filter. It could be usefull if you want to drive an extra subwoofer from your stereo system.
The circuit filters the frequency ranges above the 150Hz. The schematic is calculated to work with a 50 watts audio amplifier, but can be used with higher powers as well. The frequency can be adjusted with 2k2 potentiometer from 50 Hz to 160 Hz.