22 Watt Car Subwoofer Amplifier

This unit is intended to be connected to an existing car stereo amplifier, adding the often required extra `punch` to the music by driving a subwoofer. As very low frequencies are omnidirectional, a single amplifier is necessary to drive this dedicated loudspeaker. The power amplifier used is a good and cheap BTL (Bridge Tied Load) 13 pin IC made
22 Watt Car Subwoofer Amplifier - schematic

by Philips (now NXP Semiconductors) requiring a very low parts count and capable of delivering about 22W into a 4 Ohm load at the standard car battery voltage of 14. 4V. The stereo signals coming from the line outputs of the car radio amplifier are mixed at the input and, after the Level Control, the signal enters the buffer IC1A and can be phase reversed by means of SW1. This control can be useful to allow the subwoofer to be in phase with the loudspeakers of the existing car radio. Then, a 12dB/octave variable frequency Low Pass filter built around IC1B, Q1 and related components follows, allowing to adjust precisely the low pass frequency from 70 to 150Hz. Q2, R17 and C9 form a simple dc voltage stabilizer for the input and filter circuitry, useful to avoid positive rail interaction from the power amplifier to low level sections. Due to the long time constant set by R17 and C9 in the dc voltage stabilizer, the whole amplifier will become fully operative about 15 - 30 sec. after switch-on.

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