Manufacturing method of designing BTL circuit using LM1875

Application of negative feedback OCL circuit of DC current
LM1875 application experiment and current feedback BTL circuit design
According to the "Electronic News" 2001 No. 27 "LNl875 power amplifier production experience" a circuit (Figure 1) experiment, the results are indeed as stated in the text, after the circuit is DC and changed to current feedback, the frequency response is widened, bass strength Significantly enhanced, high-frequency resolution increased, mid-range sound quality enhanced, sound quality is much improved compared to standard circuits, so you can listen to it for a long time. The 15-square-meter room has an output of about 10W. The sound is very close to the new Dirk machine, and the sound power is empty and the resolution is comparable. The LMl875 treble is a bit more delicate, and the new Dirk 6800 vocal has a strong thickness-a little, the difference between the two is small, which is unexpected.
However, the deficiencies of the LMl875 DC current negative feedback circuit also make people feel lost: the power is too small, there is audible hum when it is static, and the measured output of the LMl875 is a few to dozens of mV at static.


LM1875 DC current negative feedback BTL design LM1875 application circuit LM1875 application experiment and current feedback BTL circuit design
The DC current negative feedback BTL circuit is shown in Figure 2. C12 and C22 in the standard BTL circuit are eliminated to make the circuit DC; the resistors R16 and R26 are sampling resistors, and the current feedback signal enters the amplifier A1 through R15, R16, R25 and R26 respectively. The inverting input of A2, the resistance of R13, R14, R15, R16 determines the size of the amplifier gain.

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 no static output noise. There is only a slight "pit" sound in the speaker when turned on, and no noise can be heard in the speaker when turned off. It can be known from the experiment that the operation of this input circuit is very stable. Even if the input terminal is unplugged and then plugged in when the volume is turned on or static, the circuit will not be self-excited. Capacitor C11 has a great influence on the sound quality. When this capacitor is removed, it will be bright immediately, the middle and high frequencies become clear and delicate, and the bass is full of flexibility and strength. The DC current negative feedback BTL circuit inherits the advantages of the DC current negative feedback OCL circuit sound quality, the distortion is further reduced, and the output power is increased to 3 times the original, reaching more than 60 watts, overcoming the impact of its switch speaker Acoustic and static have the shortcomings of hum, which is the ideal optimized circuit of LM1875.

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