This is a project for complete car amplifier based on TDA7294 amplifier chip. This is a powerful car amplifier for subwoofer, it’s based on push-pull converter so it’s more difficult to build. Build-in low-pass filter, all on one one-sided 75mm x 125mm dimension PCB.
It consists of three blocks:
1 – Push-pull converter. This is a copy of very good and tested AVT2732 circuit (published in polish magazine) with all the current and voltage protections included. This converter is based on the TL494 driver. Over-voltage protection takes care of the power amplifier safety – driver will shut-down if voltage exceeds 15V on input (converter has no voltage stabilization). Under-voltage protection takes care of the car battery discharge – driver will shut-down if voltage drops to 9V and below. Current protection takes care of switching transistors and general security of whole circuit. This is a very nice, simple, and reliable solutions – if switching transistors are more loaded, then we get higher voltage on IN+ of build in comparator. This is measured by simple circuit consist of 1N4148 diodes and RC (10K, 10nF) filter. On IN-, we can adjust compare voltage with PR1 potentiometer and if voltage on IN+ exceeds that adjusted one, driver will shut-down. Green diode means normal work, red diode means that one of protections has disabled the driver. To restart, switch off and then turn on supply or REMOTE voltage. Soft-start circuit allows slowly starting converter and charging a big capacitance at the output.
As for the switching transformer, you can make your own or take one from ATX (or AT) pc power supply. Use the 5V and 12V lines, it has a very nice transmission – 2,4x. That means, if we put +14 battery voltage on 5V line, we get 2,4x more voltage on 12V line – about +/-33V to power audio amplifier. This is very good solution, but not every transformer is good to use. First of all we must to separate common grounds for output lines (12V and 5V lines) – it is very important, otherwise our +14V battery voltage will be connected to power amplifier ground – this eliminate using this circuit in a car – because amplifier ground is a signal ground. Switching frequency is 50KHz, this is good for ATX/AT transformers. Lower frequency will cause transformer core to heats up. If you use your own transformer, you can lower switching frequency to reduce losses in switching. Do that by mounting bigger capacitor on pin5 of driver, e.g. 1nF will give frequency about 50KHz, 1,5nF give 30KHz. Switching transistors do not requires any kind of cooling if converter works properly. You can replace the IRFZ44N for other transistors, we need only 100W of output power and IRFZ44N can provide up to 300W. Converter driver is powered from REMOTE line, and it drains not more than 50mA from that line.
2 – Preamplifier and low-pass filter adjustable. This is a simple circuit with one operational amplifier, see schematic for more. Supplied with symmetric voltage +12V/-12V provided from 12V zener diodes voltage limiter.
3 – Power amplifier. TDA7294 chip in its typical application, see datasheet for more. Two capacitors at the power amplifier chip are also push-pull converter output capacitors.
Make two air-jumpers on TDA7294 car amplifier PCB, from rectifying diodes to amplifier capacitors – use thick wires. Input capacitor C4 must have at least 4700uF, on its capacity efficiency depends the output power. NECESSARILY use a 10A fuse on battery line. This is not for the beginners, launching of push-pull converter requires some knowledge and equipment (i.e. oscilloscope). However you can build this SMPS + Amplifier using exactly same values components like on the schematic and simply use it. When doing a first launch, use power-supply with current limiting.
International users, click here.