This unit is designed to be connected to an existing car stereo amplifier, adding that often require extra "punch" to music by driving a subwoofer. As the very low frequencies are omnidirectional, a single amplifier is necessary to promote the dedicated speaker.
The power amplifier uses a good and cheap BTL (bridge tied load) 13 pin IC from Philips (now NXP Semiconductors) that require a very low of the parties and capable of delivering about 22 W on a charge of 4 ohms in the battery voltage of 14.4 standard car.
The signals from the stereo line output amplifier car stereo mix in the entry, after the control level, the signal enters the buffer and can be IC1A reversed phase through SW1. This control can be useful to allow the subwoofer to be in phase with the speakers in your car radio today.
Then a variable 12dB/octave filter low pass frequency around IC1B, Q1 and components listed below, which allows precise adjustment of the low pass rate of 70 to 150Hz.
Q2, R17 and C9 form a simple voltage stabilizer for the input and filter circuits, useful to prevent interaction of the positive rail power amplifier to the low-level sections.
