LM1785 is a high-fidelity thin film power amplifier IC produced by American NSC company. It has strong adaptability and can be made into different forms of power amplifiers according to different connection methods of OTL, OCL and BTL. This article modified an amplifier based on the OCL circuit recommended by the NSC manual. After audition, I feel that the range is wide and the tone is charming. In order to further develop the potential functions of the LM1875 and make the power amplifier sound more beautiful, the experiment was repeated during the production process, and two points were gained: First, it is very important for the selection of peripheral components. Because the power amplifier circuit is simple and there are few peripheral components, more high-end components and even high-quality components are used. Although the price is more expensive, the total cost is not too high, but the sound is greatly improved. Second, in the design and production of circuits, some small improvements can also achieve very satisfactory results.
The maximum undistorted power of the LM1875 can reach 30W, and the maximum output current is 4A. Therefore, strive for independent power supply for the left and right channels. The power of the power transformer of each channel is about 100VA. Try to use high-quality products. The rectifier bridge current is> 5A. The electrolytic capacitors should be imported high-quality products or domestic genuine products. The filter capacitor is preferably 1000µF X 2. The power supply should be taken directly from the filter capacitor terminal, and not connected to the rectifier bridge, so as not to cause hum.
When designing a printed circuit board, follow the principle of separate signal ground and power ground, and use a star point ground. In fact, the LM1875 has only 5 terminals, and it is also very convenient to install with a wiring bracket. However, it should be noted that long wires should not be used for connection, especially signal input lines.
Capacitors C2, C3, C4, C9 and input coupling capacitor C1 have a great impact on the sound quality of the whole machine. If French SOLEN products are used, the effect is very good. The resistance selects 1 / 2W metal film resistance. The volume potentiometer adopts Japan ALPS audio dual-connected potentiometer. The potentiometer has very little mechanical friction noise and good consistency.
The original LM1875 power amplifier uses an AC negative feedback circuit. The feedback capacitor with larger capacity has a greater impact on the whole machine, and this capacitor can be eliminated. Because the offset voltage of the LM1875 is very low, with a maximum of ± 2µV, the measured DC voltage at the output is only about 10mV after the feedback capacitor is removed, and the circuit is very stable.
The improved circuit is shown in Figure 5-21, using high-quality components to assemble an amplifier with a gain of 25dB. Using the Sony CDP-211 laser record player that has been mounted as a sound source, it pushes a pair of 4-inch bass and 1-inch treble small imported speakers. Especially when listening to string and vocals, it is both sweet and real.
Figure 1:
![]()
The maximum undistorted power of the LM1875 can reach 30W, and the maximum output current is 4A. Therefore, strive for independent power supply for the left and right channels. The power of the power transformer of each channel is about 100VA. Try to use high-quality products. The rectifier bridge current is> 5A. The electrolytic capacitors should be imported high-quality products or domestic genuine products. The filter capacitor is preferably 1000µF X 2. The power supply should be taken directly from the filter capacitor terminal, and not connected to the rectifier bridge, so as not to cause hum.
When designing a printed circuit board, follow the principle of separate signal ground and power ground, and use a star point ground. In fact, the LM1875 has only 5 terminals, and it is also very convenient to install with a wiring bracket. However, it should be noted that long wires should not be used for connection, especially signal input lines.
Capacitors C2, C3, C4, C9 and input coupling capacitor C1 have a great impact on the sound quality of the whole machine. If French SOLEN products are used, the effect is very good. The resistance selects 1 / 2W metal film resistance. The volume potentiometer adopts Japan ALPS audio dual-connected potentiometer. The potentiometer has very little mechanical friction noise and good consistency.
The original LM1875 power amplifier uses an AC negative feedback circuit. The feedback capacitor with larger capacity has a greater impact on the whole machine, and this capacitor can be eliminated. Because the offset voltage of the LM1875 is very low, with a maximum of ± 2µV, the measured DC voltage at the output is only about 10mV after the feedback capacitor is removed, and the circuit is very stable.
The improved circuit is shown in Figure 5-21, using high-quality components to assemble an amplifier with a gain of 25dB. Using the Sony CDP-211 laser record player that has been mounted as a sound source, it pushes a pair of 4-inch bass and 1-inch treble small imported speakers. Especially when listening to string and vocals, it is both sweet and real.
Figure 1:
Home Diesel Generator,Full Power Range Of Generator,Silent Power Genset,Diesel Generator
Jiangsu Lingyu Generator CO.,LTD , https://www.lygenset.com