According to the "Electronic News" 15th issue of 2000 "Amplifier Fever Pickup", I produced a LM1875 current negative feedback amplifier. I found that the background noise is very large, there is some hum, and the LM1875 heats up seriously when there is no signal. After analyzing the electrical schematic diagram, it is believed that the unbalanced DC bias circuit at the positive and negative input terminals of LM1875 is the source of the fault. The LM 1875 non-inverting input terminal ①pin is grounded through a 22kΩ resistor, and the inverting input terminal ② pin is connected in series with R2 after being connected in series with R3 and R6, that is (22kΩ + 0.39Ω) ∥680Ω <680Ω. If the bias circuit is artificially set to be unbalanced, the heat will reduce the common mode rejection ratio of the amplifier, which will have a bad influence on the sound quality transparency and the signal-to-noise ratio, and it is also extremely unfavorable for the stable output midpoint zero potential. The original LM1875 inverting input was grounded in series with a capacitor and a resistor. Its purpose was to maintain the zero potential of the output midpoint, so that the amplifier can work stably and normally. Now that this capacitor is cancelled, the setting of the DC bias circuit appears very Important. According to the above idea, the inverting input of LM1875 is grounded in series through R3 and R6, that is, 22kΩ + 0.25Ω≈22kΩ, which is almost equal to the grounding resistance R1 (22kΩ) of the in-phase input. In addition, after canceling R2, the current negative feedback signal will have no shunt of R2, and will be directly fed back to the inverting input terminal of LM1875. If the original current negative feedback resistance of 0.39 Ω remains unchanged, the relative power loss is too large. Therefore, the current negative feedback resistance is reduced from 0.39Ω to 0.25Ω to ensure proper negative feedback and output power. After the above changes, the IC temperature touched by hand is only slightly warm. Trial release of Cai Qin's "New Feelings and Old Memories", I feel that the music background is very clean, the volume is low, but the bass is still regrettable, and it seems to be endless. For some discs with poor quality, the size of the music background noise can be easily identified, and the high-frequency resolution is evident. 
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