Automotive UMZCH on the TDA7294 chip. Scheme, description

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Automotive UMZCH on the TDA7294 chip. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Automotive power amplifiers

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For high-quality sound reproduction in a moving car, you need at least 50 watts per channel for front speakers, 20 watts for rear speakers, and 100 watts for a subwoofer. Radio tape recorders (CD receivers) sold in stores give out a power of 15 W (20 W when the car is running), no matter what they write on for advertising purposes.

Thus, an external amplifier is needed for the front and subwoofer (the rear will provide an amplifier for the radio). It is such an amplifier that is described in this article.

Automotive UMZCH on the TDA7294 chip. General view of the amplifier

The amplifier has 3 channels - two channels for stereo amplification and one channel for passive subwoofer. To provide the required output power when powered by 12 volts, a voltage converter is used.

Amplifier parameters

Maximum output power into 4 ohms
with a harmonic coefficient of not more than 1%:
subwoofer amplifier - 175 watts.
stereo amplifier - 75 watts per channel
Amplifier harmonics:
on TDA7294 at Рout.=60 W no more than - 0,01%
subwoofer at Pout.=150 W no more than - 0,1%
Supply voltage :
minimum - 11 V
maximum - 16 V
Power consumption:
At rest - 1 A.
At the maximum output power of all channels - 40 A.
Dimensions: 290x153x63
Weight: 2,3 kg

Amplifier

The amplifier circuit is shown in Figure No. 1. Channels of 75 W each are made on TDA7294 microcircuits, with an output power of 70 W they have K harmonics of no more than 0,01%. The supply voltage is + - 28 V for a 4 ohm load. With a load of 8 ohms, + -35V power is required and the circuit can be connected from a + - 38V source, which will simplify the converter circuit. At the input there are switchable low-pass filters with a cutoff of 100 Hz, made on operational amplifiers DA5, DA6. The filter is needed to avoid overloading the built-in speakers with low frequencies for which they are not designed. If desired, the filter is switched off by switch S2. From voltages + - 28 V, stabilizers DA7 type 7815 and DA8 type 7915 obtain + -15 V voltages for powering the preamplifier and filters.

(click to enlarge)

For the subwoofer channel, I had to abandon the use of TDA7294 microcircuits, since the 75GDN-1 head of a homemade subwoofer had a resistance of 4 ohms. Power was required at least 150 watts, one microcircuit will not provide such power. Bridged TDA7294 chips require a load of at least 8 ohms. For those who have an 8 ohm sub head, we can recommend the use of TDA7294 microcircuits in a bridged connection powered by a source of + -28 V. The amplifier is made on the basis of a well-known circuit based on KT825, KT827 transistors. In the final stage, transistors of the type KT8101 KT8102 are used in the final stage. If desired, you can use transistors KT825, KT827, only there will be difficulties with heat dissipation (with this design of the transistor case). The best replacement for output transistors is the Toshiba pair 2SA1302, 2SC3281 and a pair of KT850, KT851 for pre-output.

The subwoofer channel does not require an amplifier with Kg in thousandths of a percent. The supply voltage to achieve the required power was chosen + - 38 V. This required the use of an additional winding on the transformer, an additional rectifier and a filter in the power supply. At the channel input, a filter is installed that cuts frequencies above 120 Hz, made on the DA4 chip. There is no cutoff frequency adjustment, it is selected for specific acoustics. For adjustment, you can set the switch to 2-3 positions, switching RC circuits. The original amplifier does not have loudspeaker output protection against DC voltage in case of amplifier failure. Scheme No. 3 shows such a protection unit. If you use high-quality expensive heads in acoustics, then I recommend supplementing the amplifier with this protection.

inverter. The most complex part of the amplifier (and responsible) is the voltage converter. The converter circuit is shown in Figure 2. It is made according to the scheme of a push-pull transformer cascade. MDP keys are used as power elements. The control circuit with output voltage stabilization is made by the TL494 PWM controller microcircuit, voltage feedback is connected only from positive power sources, stabilization of negative sources is carried out indirectly. The accuracy of maintaining the voltage of positive sources is several times higher than that of negative ones. No negative effects were found with this scheme.

To more accurately maintain the voltage at all outputs of the converter, you can put a compensation choke (this is in all computer power supplies). The inductor is placed immediately after the rectifier diodes, it must have four windings (according to the number of voltage outputs). Many branded amplifiers do not have converter output voltage stabilization at all. Key field-effect transistors are used of the IRFZ44 type, two in parallel. It is possible to use other transistors: IRF1010, IRFZ48, IRFP150, IRFZ46. When choosing transistors, you need to strive to ensure that the resistance Rsi is as small as possible. The converter is switched on by relay contacts for 30 A by a signal from the radio. Such an output is available in most radio tape recorders, it serves to extend the antennas, turn on active antennas and external amplifiers. When the radio is turned on, a voltage of 12 V appears at this output. If there is no such output, you can install a switch on the front panel of the car that will supply 12 V to the enable relay. At maximum output power, the converter draws up to 40 A.

(click to enlarge)

An LC noise suppression filter is installed at the power input. The 2DR1 filter inductor can be wound on a piece of a ferrite rod with a PEV wire with a diameter of 2 mm, the number of turns is 10-20. A good inductor is obtained on a piece of ferrite from a line transformer of TVs. Ferrite 2000 NMS1 is used there. Transformer 2T1 is wound on two rings K42x28x10 of grade 2000NM1 stacked together. It is better to wind with a bundle of several thin wires (by dialing the required cross section) than with one thick one (firstly, it is easier). The winding technology is as follows: by selecting an existing wire, for example 0,8 mm, we calculate the number of wires based on an average current of 20 A.

We take the current density of 5 A per mm2. It turns out 8 wires. We make a bundle of 16 wires of the required length and wind the primary winding with it, trying to distribute the winding evenly over the core. With a dial, we divide the tourniquet in half, connect the beginning of one half to the end of the other. We wind the secondary winding in the same way. Before winding, the sharp edges of the ferrite rings must be rounded off. The number of turns of the primary winding is 2x6, the secondary winding is 2a 2x16 turns, the winding is 2b 2x22 turns. Rectifier diodes must be high-frequency (such as KD213A, KD2997), it is better if they are Schottky diodes. So I have KD213A installed on the board without radiators and heat up at maximum load, but I considered the heating to be not very strong and the maximum load mode was short-term. Otherwise, they need to be put on the radiator.

Noise suppression LC filters are installed at the output of the converter. Filter chokes 2DR2-2DR5 can be wound on segments of ferrite rods with a diameter of 6 mm with a PEV wire of 0,8 mm, the number of turns is 20. Capacitors at the output of 4700 microfarads and 2200 microfarads are quite sufficient, since filtering occurs at a high frequency, they can be reduced, but no more than 2 times.

Design. The entire circuit of the amplifier and inverter is mounted on one printed circuit board made of one-sided foil fiberglass 2 mm thick and 280x120 mm in size. The printed circuit board drawing is shown on Figure #3, arrangement of elements on Figure #4.

The board is installed in an aluminum U-shaped case of complex shape with ribs. The housing drawing is shown in Figure No. 5. All power transistors, stabilizers and TDA7294 microcircuits are screwed through mica insulating gaskets to 6 mm thick intermediate aluminum panels, which are bolted to the case (using heat-conducting paste). Therefore, by unscrewing just a few (5) M5 bolts, the board with panels can be easily removed from the case. The screws that fasten the microcircuits and transistors to the plate must not protrude beyond its plane.

Rice. 5 (click to enlarge). 1 - Mica gasket. 2 - Insulating sleeve. 3 - Screw M3. 4 - Chip TDA7294. 5 - Transformer. 6 - Fee

The body is anodized, and the outside is also painted. The ends are covered with aluminum plates 2 mm thick, which are screwed directly to the board. Input connectors are mounted on one of the plates and holes are made for the knobs of the switch and volume control.

Automotive UMZCH on the TDA7294 chip. Type of amplifier
Fig. 6

Adjustment. It is expedient to carry out adjustment of low-frequency amplifiers by feeding them from an external laboratory power supply unit with adjustable output voltage separately. If the power supply does not have short circuit protection, then we do the first turn on of each ULF by connecting it to the PSU through 2 20-50 Ohm resistors. Thus, it is possible to save the terminal transistors from failure due to installation errors. In amplifiers on the TDA7294, we make sure that there is no constant voltage at the output, we check the quiescent current.

Applying a signal from the low-frequency generator to the input, we check the output signal using an oscilloscope. After that, the resistors can be replaced with jumpers. In the amplifier for the subwoofer, we also make sure that there is no constant voltage at the output, it can be within plus or minus a few millivolts, with resistor R44 we set the quiescent current of transistors VT5, VT6 to about 20 mA, the quiescent current of the terminal transistors VT9, VT10 should be zero. Applying a signal from the low-frequency generator to the input, we check the output signal using an oscilloscope, bringing it to the limit. After that, the resistors can be replaced with jumpers. We check the output power by connecting a PEV resistance of 25-50 W 4 ohms to the output, so as not to burn the resistance, it can be lowered into a jar of water.

I recommend starting the adjustment of the converter with non-soldered key transistors (or without supplying power to the transformer). After checking the operation of the TL494 microcircuit, we supply 14 V power to the transformer through a 12 V 60 watt lamp. Only after fully debugging the operation of the converter with a small load, we replace the lamp with a fuse. This will save the key transistors from failure in case of various errors. For adjustment, you must have a powerful power supply 12-14 V for a current of 10-20 A or a car battery. Final adjustment is made in the car.

Recommendations for mounting the amplifier. Typically, the amplifier is mounted in the trunk of a car, although it can be located elsewhere, such as under the driver's seat. Since the amplifier consumes current up to 40 A, a copper wire with a cross section of at least 12 square millimeters is required to connect the +12 V and -10 V power supply. The +12 volt wire is laid directly to the battery and connected to the positive terminal through a 50 A fuse. It is necessary to strive to keep the wire from the fuse holder to the terminal as short as possible. The minus 12 V wire is connected to the car body at the nearest point or also laid to the battery. Signal shielded wires from the radio are laid on the floor of the car under the carpet.

Automotive UMZCH on the TDA7294 chip. Panel

Automobile UMZCH on the TDA7294 chip

Literature

V. Vilchinsky. Power amplifier with power supply. - Radio, 1990, No. 5, p. 52

Printed circuit board

Publication: cxem.net

See other articles Section Automotive power amplifiers.

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