LM1875 DC current negative feedback BTL circuit

The DC current negative feedback BTL circuit is shown in Figure 2. C12 and C22 in the standard BTL circuit are canceled to make the circuit DC; the resistors R16 and R26 are sampling resistors, and the current feedback signal enters the amplifier A1 and A1 respectively through R15, R16, R25 and R26. The inverting input terminal of A2, the resistance values ​​of R13, R14, R15, and R16 determine the gain of the amplifier.

Using the circuit experiment in Figure 2, no matter how the power supply and input signal are turned on and off, there is always no DC output at the output end, and there is no static output noise. There is only a slight "beep" sound from the speaker when it is turned on, and there is no noise audible from the speaker when it is turned off. Through experiments, it can be seen that this kind of input circuit works very stably. Even if the input terminal is unplugged and plugged in when the volume is turned up or in a static state, the circuit will not self-excite. Capacitor C11 has a great influence on the sound quality. After removing this capacitor, my eyes immediately brightened up. The mids and highs became clear and delicate, and the bass was elastic and powerful. The DC current negative feedback BTL circuit inherits the sound quality advantages of the DC current negative feedback OCL circuit, the distortion is further reduced, the output power is increased to 3 times of the original, reaching more than 60 watts, and overcomes the impact of the switch speaker. It has the shortcomings of humming sound and static sound, and is an ideal optimized circuit for LM1875 .