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[Source : Electronics Now, October 1995] For many systems, replacing poor quality speakers with better ones is the best way to upgrade the sound without spending a lot of money. After that, adding a subwoofer tops the list. A high quality automotive subwoofer system can be assembled in cheap price. A subwoofer provides two main advantages : First, it provides a dramatic improvement in bass, and second, it improves the higher frequencies by letting you turn down the bass frequencies that are fed to the other speakers. That lowers distortion and lets you play the other frequencies louder. A subsonic filter prevents subaudible energy from reducing dynamic headroom and distorting the bass. It also eliminates annoying turn-on thumps. To be effective without sacrificing the deep-bass response, the subsonic filter should have a slope or at least 12 dB per octave. Following is the schematic diagram :

The inputs through Q1 form a differential summing amplifier with switch S1 functioning as a polarity inverter. A 24dB per octave switched capacitor filter (IC1) is the heart of the continuously variable filter. Potentiometer R13 controls the cutoff frequency of IC1 by controlling its sampling frequency. Because of the inherent sampling action of switched capacitor filters, an anti-aliasing filter is required at the input of IC1. Transistors Q2 and Q3, along with the surrounding components, form this second-order, low-pass, anti-aliasing filter. The subsonic filter with a boost stage follows the output of IC1 at pin 5. When switch S2 is closed, the boost is added. Additional subsonic filtering action is provided by C1 and C2 at the inputs of the crossover circuit. A reconstruction filter that eliminates sampling artifacts is formed by R18 and C10 at the output of IC1.The power supply circuit, based on the 78L08 voltage regulator IC2, provides both an 8.6 volt main supply and a 4.8 volt bias supply. Diode D1 protects against negative voltage spikes and incorrect hookup. Diode D2 biases the 78L08 regulator reference pin at 0.6 volt to provide an output of 8.6 volts rather than 8 volt.

Parts lists : (all resistors are 1/8 watt, 5%, unless noted).

R1-R4: 47k ohms

R5,R6,R19,R21 : 10k ohms

R7: 20k ohms

R8: 16k ohms

R9: 3.3k ohms

R10,R11:9.1k ohms

R12,R14,R16,R23: 2.2k ohms

R13: 10k ohms, linear taper potentiometer

R15:100k ohms

R17: 5.1k ohms

R18: 1k ohms

R20: 47 ohms

R22 : 18k ohms

Capacitors : C1,C2 : 0.22 uF

C3: 0.047 uF

C4,C6,C7 : 0.1 uF

C5: 0.01 uF

C8,C9: 0.22 uF

C10: 1 uF, 25 volts, radial electrolytic, 105 degress C

C11,C13: 10 uF, 25 volts, radial electrolytic, 105 degrees C

C12 : 220 uF, 25 volts, axial electrolytic

Semiconductors

Q1, Q3, Q4, Q5 : 2N3904 NPN transistor

Q2 : 2N3906 PNP transistor

D1,D2:1N914 Diode

IC1 : MF4CN-100 switched capacitor filler

IC2: 78L08 voltage regulator

Other

S1 : DPDT switch

S2: SPST switch

J1,J2: RCA jack, PC mount

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