Audio frequency power amplifier on the LM1875 chip. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Transistor power amplifiers

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Small-sized music centers are in great demand, usually consisting of a broadcast receiver, a cassette deck and a CD player, assembled in a compact design, complete with two miniature speaker systems. Such equipment, at a relatively low price, provides a fairly high sound quality, but the output power usually does not exceed 3-5W per channel. In combination with small-sized speakers with low sound pressure level, the volume is insufficient.

You can increase the power and sound quality if you make output jacks in the music center connected to the output of a pre-amplifier (or "audio processor", in which the functions of amplification and electronic volume control, tone control, etc.) are concentrated. And the signal from these sockets should be fed to more powerful self-made active speaker systems or power amplifiers with remote speakers.

Fig. 1

Figure 1 shows a well-tested circuit of one of the channels of a power amplifier that develops power up to 25 W at a load of 4-8 ohms. With a power of 20W, the coefficient of non-linear distortion at a frequency of 1 kHz does not exceed 0,04%. The signal-to-noise ratio is no worse than 105 dB, and the frequency range at 1 dB unevenness extends from 14 Hz to 100 kHz (measured at an output power level of 1W).

The current consumption of the LM1875 microcircuit is not more than 4 A (at maximum power), there is a built-in protection that turns off the output stage when the current exceeds this value. Also, there is protection against short circuit in the load and overheating (when the crystal overheats more than 170 ° C).

The input impedance of the amplifier is 22kOm, the use of a non-polar capacitor C1 at the input made it possible to reduce the noise level (most "electrolytes" make a lot of noise, especially when working as transients at low signal levels). The R3-C2 circuit forms a low-pass filter that suppresses radio frequency interference that can penetrate the amplifier input as a result of interference in the connecting cables or from a signal source that has powerful high-frequency circuits in its circuit (switching power supply, CD player circuits, etc.).

The LM1875 chip, like many other integrated power amplifiers with bipolar supply, is a powerful operational amplifier. Therefore, the voltage gain, like that of any op-amp, here depends on the depth of the OOS, between the output of the microcircuit and its inverse input. In fact, it is equal to the ratio R4 / R5, and is equal to 22. If necessary, by changing the ratio of these resistances, the gain can be adjusted within a fairly wide range. However, you should be aware that a significant increase in the gain, firstly, invariably leads to an increase in the coefficient of nonlinear distortion, and secondly, there is a danger of self-excitation of the amplifier. Therefore, an increase in gain is permissible only within small limits.

Power is supplied to the microcircuit through conventional fuses P1 and P2. Blocking capacitors C5-C8 are located on the amplifier board, right next to the microcircuit. Their task is to suppress current ripples that occur on the wires through which power is supplied to the amplifier, caused by a change in the current consumption of the microcircuit that occurs during its operation.

Capacitors C9 and C10, which smooth out the ripples of the rectifiers, are located outside the board.

The power supply is bipolar. Usually, to obtain a bipolar voltage, one transformer is used, the secondary winding of which has a tap from the middle. Unfortunately, it is quite difficult to choose a ready-made transformer with such a winding, or the transformer needs to be rewound. It is much easier to find two identical transformers of lower power, with single secondary windings. One transformer is used to receive a positive voltage, and the other is a negative one. The primary windings, if they are 220V, are connected in parallel. One of the positive aspects of such a scheme is that you can use "illiquid" 120V transformers by connecting their primary windings in series. In any case, the transformers must be the same.

You can use one 80W transformer with a 36V secondary winding, but its secondary winding will need to be rewound so as to be divided into two equal parts.

Fig. 2

The details of the amplifier (up to capacitors C9 and C10) are located on a printed circuit board made of one-sided foil fiberglass, the image of which is shown in the figure.

The microcircuit must be installed on a heat sink (radiator) with a surface area of ​​at least 400 cm2. When installing, use mica insulating pad and heat conductive paste. Place an insulating washer under the fixing screw. Before turning on, check if there is any electrical contact between the heatsink and the heatsink plate of the chip.

Publication: radioradar.net

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