Experience in making LM1875 amplifier-EEWORLD

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According to the article "Amplifier Fever Pickings" in the 15th issue of "Electronics News" in 2000, an LM1875 current negative feedback amplifier was made. It was found that the background noise was very large, there was some AC noise, and the LM1875 heated up seriously when there was no signal. After analyzing the electrical schematic, it was believed that the imbalance of the DC bias circuit at the positive and negative input ends of the LM1875 was the root cause of the fault. The LM1875 in-phase input pin ① was grounded through a 22kΩ resistor, and the inverting input pin ② was connected to the ground in parallel with R2 after being connected in series with R3 and R6, that is, (22kΩ+0.39Ω)∥680Ω<680Ω. Artificially setting the bias circuit to be unbalanced will inevitably reduce the common mode rejection ratio of the amplifier, which will have a bad impact on the sound quality transparency and signal-to-noise ratio, and is also extremely unfavorable for stabilizing the output midpoint zero potential. The original LM1875 inverting input terminal is connected to ground with a capacitor and a resistor in series, the purpose of which is to maintain the zero potential of the output midpoint so that the amplifier can work stably and normally. Now that this capacitor has been removed, the setting of the DC bias circuit becomes very important. According to the above idea, the inverting input terminal of LM1875 is connected to ground in series through R3 and R6, that is, 22kΩ+0.25Ω≈22kΩ, which is almost equal to the grounding resistor R1 (22kΩ) of the inverting input terminal. In addition, after removing R2, the current negative feedback signal will not be shunted by R2 and will be directly fed back to the inverting input terminal of LM1875. If the original current negative feedback resistor of 0.39Ω is retained unchanged, the relative power loss will be too large. Therefore, the current negative feedback resistor is reduced from 0.39Ω to 0.25Ω to ensure appropriate negative feedback and output power. After the above changes, the IC temperature is only slightly hot when touched by hand. I tried playing Tsai Chin's "New Feelings Old Memories" and felt that the background music was very clean. Although the volume was low, the bass was still impressive and seemed to have endless power. For some poor quality CDs, it is easy to identify them from the size of the background noise of the music, which shows its high-frequency resolution.