Directly coupled push-pull power amplifier circuit

VT1 and VT2 constitute a differential input stage. VT7 is its constant current source. R1,
VDl, VD2, VD3, and RP2 jointly determine the static operating point of the input moderating machine. The two output terminals of the input stage are directly
connected to the bases of the voltage amplifier stages VT3 and VT4. The power amplifier stage VT5VT6 uses two Darlington NPN
transistors with the same conductivity polarity. The bases of the two tubes are directly connected to the collectors of the voltage amplifier stages VT3 and VT4 respectively . VT3. VT4. VT5. VT6
are all biased in Category A and B. In the working state, BL is a dual voice coil speaker. The impedances of the two voice coils are equal and they are directly
connected in series with the emitters of power amplifier stages VT5 and VT6 in opposite phases .
    When the input signal enters the positive half cycle, according to the principle of the differential amplifier circuit: the output signal voltages of the collectors of the two tubes VTl and VT2
are equal in magnitude and opposite in polarity. lcl increases, the voltage drop on the collector load resistor R5 increases, the forward bias of the VT3 emitter junction
increases, lcz decreases, the voltage drop on the collector load resistor R4 decreases, and the forward bias of the VT4 emitter junction decreases. Therefore,
VT4 is cut off. . VT3 is turned on, and the collector current changes with the input signal. After further amplification by the power amplifier tube VT5, rE flows through
the RI_1 voice coil of the dual-tone cone speaker, and gets the corresponding positive half cycle on it, pushing the speaker cone to move outward. (set outward) o
When the input signal enters the negative half cycle, VT3 is cut off and VT4 is turned on. The collector current changes with the input signal. It
is further amplified by the power amplifier tube VT6 and flows through the RL2 voice coil in the dual voice coil speaker. Get the corresponding negative half cycle on it, pushing the speaker
cone to move inward (RI.1 and RI.2 have opposite phases). In this way, although the two power amplifier transistors
conduct electricity , each outputs half a cycle. A signal cycle, but a complete signal cycle is directly synthesized on the dual voice coil speaker, and
the electroacoustic conversion is completed at the same time.