Circuit Description
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.
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 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.
Circuit Diagram
Parts list:
- P1_____________10K Log Potentiometer
- P2_____________22K Dual gang Linear Potentiometer
- R1,R4___________1K 1/4W Resistors
- R2,R3,R5,R6____10K 1/4W Resistors
- R7,R8_________100K 1/4W Resistors
- R9,R10,R13_____47K 1/4W Resistors
- R11,R12________15K 1/4W Resistors
- R14,R15,R17____47K 1/4W Resistors
- R16_____________6K8 1/4W Resistor
- R18_____________1K5 1/4W Resistor
- C1,C2,C3,C6_____4µ7 25V Electrolytic Capacitors
- C4,C5__________68nF 63V Polyester Capacitors
- C7_____________33nF 63V Polyester Capacitor
- C8,C9_________220µF 25V Electrolytic Capacitors
- C10___________470nF 63V Polyester Capacitor
- C11___________100nF 63V Polyester Capacitor
- C12__________2200µF 25V Electrolytic Capacitor
- D1______________LED any color and type
- Q1,Q2_________BC547 45V 100mA NPN Transistors
- IC1___________TL072 Dual BIFET Op-Amp
- IC2_________TDA1516BQ 24W BTL Car Radio Power Amplifier IC
- SW1____________DPDT toggle or slide Switch
- SW2____________SPST toggle or slide Switch capable of withstanding a current of at least 3A
- J1,J2__________RCA audio input sockets
- SPKR___________4 Ohm Woofer or two 8 Ohm Woofers wired in parallel
Technical data:
- Output power (1KHz sinewave): 22W RMS into 4 Ohms at 14.4V supply
- Sensitivity: 250mV input for full output
- Frequency response: 20Hz to 70Hz -3dB with the cursor of P2 fully rotated towards R12, 20Hz to 150Hz -3dB with the cursor of P2 fully rotated towards R11
- Total harmonic distortion: 17W RMS: 0.5% 22W RMS: 10%
