Practical and high-performance preamplifier circuit

The circuit is shown in the figure below (the other channel is the same), Wl is the left and right balance potentiometer, W2 is the volume potentiometer. The Japanese Toshiba audio special tube A970 and C2240 are selected for the first and second stage amplification, working in the single-ended Class A state. C2 and C4 are input coupling capacitors, R2 and C2 filter out unnecessary ultra-high frequency signals, R1 also acts as a bias resistor for T1, C1 and C5 are vibration elimination capacitors, R4 is the bias resistor for T2, and R7, R8, D1 and D2 provide bias voltage for T3 and T4. D1 and D2 are connected in series to keep T3 and T4 in the on state, reducing the crossover distortion of T3 and T4. E1 is the bootstrap capacitor, E4 is the output coupling capacitor, and Rl0 is the output buffer resistor. T3 and T41 are in a static asymmetric state, and the emitters of the two tubes have a static voltage of +3V. The output voltage is filtered by R9, Rl0, and E2 integrally to supply power to the emitter of T1. This current is proportional to the output signal size of T3 and T4 in dynamic state, so that the working current of T1 changes with the change of the output signal amplitude. The signals of T3 and T4 are always output at a constant current, so that the whole circuit reflects the many advantages of working in the current negative feedback mode to improve the sound quality. R3 is the feedback resistor. The remaining components constitute a conventional negative feedback tone network. The network has an adjustment range of ±7dB and ±10dB for high and low bass respectively. The gain of the whole circuit is about 7dB; the circuit working voltage can reach ±28V, which is much higher than the integrated tone circuit. It is very beneficial to the dynamic speed performance of music.