Pure Class A DC power amplifier circuit

This circuit is a DC power amplifier circuit using fully symmetrical complementary push-pull output , as shown in Figure 2-35. The whole circuit consists of a dual differential
input stage, a common base + common emitter voltage amplifier stage, a push stage and a parallel complementary push-pull output stage, as well as a bias circuit and a DC servo
circuit.
    VTi ~ VT4 form a dual differential input stage. xrrs and VT6 are their constant current sources.
A feedback resistor with a larger resistance is connected to the emitter of the differential tube. In addition to balancing the differences in the pairing parameters of each tube, It also has the effect of extending the dynamic
range, broadening the frequency band and improving the common mode rejection ratio.
    Vr7, VTg, VT8, and VTio form a common-base, common-emitter push-pull amplifier, which is used as a voltage amplification stage. VD3 and VD4 are
VTg, the base bias of vTio is
    VTiz, VT13 is the output push stage, and VIll is its bias circuit. VT14~VT17 form a parallel complementary push-pull
output stage.
    ICt, Ⅵj, and ⅥX are DC servo circuits, among which Ⅵbu and VD8 are input protection circuits.
    Circuit adjustment parameters: the voltage drop across the differential stage emitter resistor Rs is 0.2V,
the voltage drop across the collector load resistor island ~ R12 is 4Vo, the voltage drop across the emitter resistors R21 and Rzz of the amplifier stage is 3, 2Vo adjusts RP so that
the voltage drop on any resistor of R28 ~ R31 is 0.4V, then the quiescent current of each output tube should be 0.8Ao.
    Component selection: VTi ~ VT6 use Toshiba high frequency tubes 2SC1775E, 2SA872E or Hitachi 2SD756E,
2SB716E, etc. . The error of matching Lu and V island must be less than 5%.
    VT7~VTio use complementary tubes such as EOG128, ECG129 or BC560.BC550, and the requirements are the same as above.
    VDi~VD4 and VI should use voltage regulator tubes with good characteristic curves and the same voltage stabilization value PCrn>400mW.
    The power output tubes VT14~VT17 use audiophile-grade power amplifiers. The paired tubes 2SC2922/2sA1216 are paired with a DC
    servo LC of LF356Ho.
    The output power of this machine is 50wo.