Subwoofer for Cars

This system needs to be attached to an existing car stereo amplifier to add extra 'boom effect' to the music. It has a dedicated loudspeaker with a power amplifier to boost low frequencies that are normally omnidirectional.

Fig. 1: Circuit diagram of the subwoofer for cars

Fig. 2: An actual-size, single-side PCB for the subwoofer

Fig. 3: Component layout for the PCB

The power amplifier used is an integrated class-B output amplifier in a 17-lead single-in-line (SIL) power package. It contains 4×12W single-ended (SE) or 2×24W bridge-tied load (BTL) amplifiers. The output speaker is connected in BTL configuration in this system for better quality.

Circuit and working
Fig. 1 shows the circuit diagram of sub-woofer for cars. It is built around low-noise JFET-input operational amplifiers TL072 (IC1), car radio power amplifier TDA8561Q (IC2) and transistors BC547 (T1 and T2). IC2 has two 24W BTL output channels, one of which is connected to loudspeaker LS1. BTL is an output configuration for audio amplifiers, a form of impedance bridging where two channels of a stereo amplifier are fed the same audio signal, with one channel’s electrical polarity reversed. Such an arrangement can double the voltage swing at the load.

Both channels from the car stereo are fed to the system through connectors RCA1 and RCA2. Audio from both the channels is mixed after level control. After mixing, the signals are fed to the buffer stage implemented with op-amp A1 of IC1 and the output phase can be reversed using switch S1A and S1B. This control can be useful to allow the subwoofer to be in phase with the loudspeakers of the existing car radio. A 12dB octave variable frequency low-pass filter (LPF) built around op-amp A2 of IC1, transistor T1 and related components allows to adjust precisely (using dual-gang potmeter VR2) the low-pass frequencies between 70Hz and 150Hz. Resistor R17, capacitor C9 and transistor T2 form a simple DC voltage stabiliser for the input and filter circuitry. The filtered output is fed to inputs INV3 and -IN4 of power amplifier IC2, and the amplified output can be heard at LS1. The power supply of the circuit is 14.4V obtained from car battery. The logarithmic potmeter VR1 is used to control volume level.Construction and testing
An actual-size, single-side PCB for the subwoofer is shown in Fig. 2 and its component layout in Fig. 3. After assembling the circuit on PCB, enclose it in a suitable plastic case. Use suitable heat sink for IC2.

Fix 2-pin connector for connecting 8-ohm, 24W woofer LS1 and connect the power supply at CON1. The left and right stereo channels are to be connected to RCA1 and RCA2 through external wires. Ensure that VR1 and VR2 are accessible from outside for volume and LPF frequency control. Use voltage levels mentioned in the test points table for debugging, if required.

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The author is a B.Tech (ECE) from Prasad Institute of Technology, Jaunpur, UP. He is currently working in ESP Safety Pvt Ltd, Delhi as a senior hardware R&D engineer 

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