Subwoofer car amplifier 100W

[Vermes]

The entire system fits one PCB with dimensions of 75mm x 125mm. It consists of three blocks:

1. Pulse converter. This is a copy of a good and proven system AVT2732, with all overload and voltage security. Pulse transformer was taken from 250W AT power supply and it is enough to move the right amount of power for best performance of the tip. 5V primary winding and 12V secondary was used. That gives the gear equal to 2,4. With this gear and supplied voltage 14,5V (with the engine running), there is almost +-33V idle at the output of the converter. After the load with the power amplifier, the voltage is slightly down to a value almost perfect for operation of the power amplifier at impedance 4ohms. Operating frequency of the keys is 50KHz, the core warmed considerably at lower frequencies. The keys do not need cooling when the converter works properly.
2. Preamplifier and active filter with adjustable cut-off. The filter is powered by a symmetrical voltage of +12V, taken from the converter output and limited by two Zener diodes 12V.
3. Power amplifier. It is TDA7294 system in its typical application. MUTE circuit and the ST-BY made accordingly to the control application one line, here permanently attached to the + power. Two capacitors 1000uF/50V located next to the feet of the tip power are simultaneously output capacitors of the converter. The tip needs of course a substantial heat sink.

Notes on installation and starting. Two connections made by thick wires – from rectifier diodes for filtering capacitors must be performed on the PCB. There is no need to thicken paths. Capacitor C4 should have a value of at least 4700uF, the power of the inverter depends on its capacitance and efficiency. The converter should be run with oscilloscope. First run should be additionally done with power supply with current limitation or include a light bulb (e.g. 12V/55W) in series with power source. 10A fuse must be installed on the positive supply line. In case of failure it will protect the amplifier and the car against the fire. The power cord should have a minimum of 2,5mm^2. Converter and amplifier mass are not connected, so the supply mass is get from a metal element of the trunk and signal mas is from the car radio. Connecting them near the amplifier is not advisable. A resistor with a value of 10K-100K chosen empirically, should be given to a series with signal. The preamp and whether or not the amplifier will operate, depends on the resistor.
Power transformers often have common mass and must be separated for primary and secondary windings – otherwise the mass of the power amplifier and audio input is connected to the positive pole of the battery – which disqualifies the system for use in the car.
Dual potentiometer is linear potentiometer, interchangeably a potentiometer with the 2x22K or 2x47K can be used. In this case, to keep the range of adjustment of the cut-off, filter capacities should be changed to 400nF and 200nF. For 2x10K there are 400nF and 200nF, for 2x22K there are 200nF and 100nF, for 2x47K there are 100nF and 47nF – so that when the resistance is doubled twice – the capacity decreases. Resistors R24 and R25 should also be changed (increase in so far as increasing the potentiometer), but it is not necessary, we will simply have more control up.





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