Working principle: Figure (a) is the left channel preamplifier circuit, and Figure (b) is the right channel preamplifier circuit. The circuit uses a dual op amp OPA2111. The left and right channels use one of the dual op amps OPA2111. Therefore, there is good matching between the left and right channels (the gain difference is 3dB, the drift current is 0.5pA, and the drift is ±0.5μV/°C), which is very suitable for forming balanced stereo preamplification. As can be seen from the figure, the left and right channel circuits are identical. The capacitances in the circuit are 0.01μF, 0.03μF and 1μF to improve the sound quality. The adjustment range of the semi-variable capacitance CT is about 0.01~0.047μF. The IF gain is approximately 26dB.
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