Make LM1875 sound better

LM1785 is a high-fidelity thin-film power amplifier IC produced by NSC of the United States. It is highly adaptable and can be made into different forms of power amplifiers according to different connection methods of OTL, OCL and BTL. This article modifies an amplifier according to the OCL circuit recommended by the NSC manual. After listening, I feel that the sound range is wide and the sound is charming. In order to further develop the potential function of LM1875 and make the power amplifier sound better, repeated experiments were conducted during the production process, and two points of experience were obtained: First, the selection of peripheral components is very important. Because the power amplifier circuit is simple and there are few peripheral components, relatively high-end components, even high-quality components, are selected. Although the price is more expensive, the total cost will not be too high, but the sound quality is greatly improved. Second, some small improvements in the design and production of the circuit can also achieve very ideal results. The maximum undistorted power of

LM1875 can reach 30W, and the maximum output current is 4A. Therefore, the power supply of the left and right channels is independent. The power transformer power of each channel is about 100VA. Try to choose high-quality products. The rectifier bridge current is ≥5A. The electrolytic capacitor should be imported high-quality products or domestic genuine products. The filter capacitor is best to use 1000µF X 2. The power supply should be directly taken from the filter capacitor terminal, not connected to the rectifier bridge, so as not to cause AC noise.

When designing a printed circuit board, the signal ground and power ground should be routed separately, and star-shaped single-point grounding should be used. In fact, LM1875 has only 5 lead terminals, and it is also very convenient to install it using a wiring bracket. However, it should be noted that it is forbidden to use long wires to connect, especially the signal input line.

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 resistor uses a 1/2W metal film resistor. The volume potentiometer uses a Japanese ALPS audio-specific double-connected potentiometer, which has extremely low mechanical friction noise and good consistency.

The original LM1875 amplifier uses an AC negative feedback circuit. The feedback capacitor with large capacity has a greater impact on the whole machine, so this capacitor can be eliminated. Since the offset voltage of LM1875

is very low, with a maximum of ±2µV, after eliminating the feedback capacitor, the measured DC voltage at the output end is only about 10mV, and the circuit is very stable. The improved circuit is shown in Figure 5-21. An amplifier with a gain of 25dB is assembled using high-quality components. A Sony CDP-211 laser disc player that has been upgraded is used as the sound source to drive a pair of 4-inch woofers and 1-inch treble small imported speakers. The sound is very delicate, the power is sufficient, and the positioning is clear and accurate. Especially when listening to string music and vocals, it is both sweet and real.
Figure 1: