LM4766 Dual 40W+40W Power Amplifier

LM4766 is a dual-channel high-power amplifier integrated circuit launched by National Semiconductor Corporation of the United States. Each channel can output 40W average power on an 8Ω load, and the distortion is less than 0.1%. In the product series of National Semiconductor Corporation, LM4766 is classified as the "Overture" series, which is the most high-end single-chip dual-channel audio power amplifier integrated circuit.

The indicators of LM4766 are very attractive. The distortion and signal-to-noise ratio of a good power integrated circuit are very good. LM4766 can achieve only 0.06% distortion and noise value at 30W power output in the audible frequency band. The design ability of National Semiconductor's power integrated circuit is evident. We have reason to believe that if combined with excellent circuit design, the effect that LM4766 can play may not be evaluated only by whether it "reaches HI-FI". Of course, it does not mean that as long as the LM4766 power integrated circuit is used, the HI-FI effect will be achieved! The design of the power amplifier is a systematic project. If you just connect it carelessly, you will naturally not get any good performance.

The integrated block also has perfect protection measures inside: overvoltage, undervoltage, overload, overtemperature (output automatically shuts down at 165℃, automatically resumes work at 155℃) and the company's patent-safe working area SPIKE peak protection (guaranteeing that the integrated amplifier's final stage works in the safe working area of ​​the characteristic curve, etc.). It can be said that the power amplifier successfully designed with LM4766 can generally achieve considerable safety performance. In addition, the two channels inside the LM4766 have independent mute circuits, which are led out through two pins. Its function is to turn off the input of LM4766 so that the internal power amplifier has no signal output. After these two pins are connected in a certain way, the impact during the startup process can be eliminated. From the DATASHEET, we can see that when the power supply voltage of LM4766 reaches +-30V and there is enough signal excitation, then LM4766 will output 40W of power to drive the speaker unit. This power can already drive most multimedia speakers. What's more, when people usually enjoy music, 1 or 2W of power is enough. According to past experience, although the power of LM4766 is not very large, its thrust is strong, and the sound is thick, rich, graceful, soft and pleasant to listen to. Using it to drive small and medium-sized speakers can effectively avoid the shortcomings of small and medium-sized speakers' sound being thin and weak, and even enjoy the sound similar to that of a tube machine.

The following figure is the application electrical schematic diagram of LM4766:

(II) Production Guidance

After the printed circuit board is made according to the drawing, the components can be welded. When welding, the parts can be inserted first and then welded, starting from the smallest parts, and the parts should be as close to the surface of the circuit board as possible. Pay attention to the positive and negative polarity of the electrolytic capacitor, otherwise it will be broken down. The metal film capacitor has no polarity difference, but it is best to face the same direction, which is more neat and beautiful. Because the electrolytic capacitor does not perform well at high frequencies, 104 polypropylene capacitors are added to improve the sound quality. There is no similar replacement available. The higher the withstand voltage, the better. The principle is to arrange it in parallel with the 4700UF electrolytic capacitors in large, medium and small sizes.

The two inductors can be made by winding 10 turns of No. 18 enameled wire around a 10Ω2W resistor, scraping off the paint on the two ends, and soldering them to the two legs at both ends of the resistor. Since No. 18 enameled wire is thick and difficult to wind, you can leave a longer section of the enameled wire and wind it 5 turns from the middle of the resistor to both sides. This is more labor-saving and more beautiful. The inductance value in the figure is 0.47mH, but most of the inductors available on the market cannot meet the requirements of low DC impedance (which can be roughly judged from the body shape). The requirement of the inductor in this example is low DC impedance, and the accuracy of the inductance value is secondary, so we use thicker enameled wire to wind the inductor ourselves. Because the DC impedance of the inductor is too large, it will block the signal, so it is better not to install the inductor (directly short-circuit the resistor) than to buy those chip inductors.

(III) Installation and debugging

The size and style of the heat sink in the circuit can be purchased according to your own preferences. It is best to buy a 10CM X 11CM X 3MM heat sink. Apply thermal paste between the heat sink and LM4766 to help conduct heat.

After everything is installed, you can test it. Turn the volume to the maximum, use the DC voltage file of the multimeter to detect the output end, and the voltage should be between 0-0.2V. The ideal value of the output end is zero (because the positive and negative power supplies are applied symmetrically), but there must be errors in the parts, so there will be a slight voltage output. Then adjust the multimeter to the lowest AC voltage file, touch the input end (just touch IN, not the ground), and give some human body induction noise. At this time, the multimeter indicator should swing. If the measured voltage is too large, it may be that the 47KΩ resistor is not connected between the input end and the ground. After it is connected, the situation should be improved.

Four 4700UF large-capacity filter capacitors, equipped with high-end toroidal transformer, with a power of 150W and an output voltage of 18V+18V.