OCL power amplifier without differential input

Most OCL power amplifier circuits use differential circuits as the input stage in order to make the circuit work stably. Because
OCL power amplifier circuits mostly use direct coupling, the zero-point drift of this circuit is more serious. In order to overcome the zero-point drift,
the input stage of the circuit mostly uses a differential circuit. The differential circuit requires that the components must be symmetrical and the error must be small, especially the differential pair of
tubes. , must be paired under the same test conditions. This becomes a big problem for amateurs. This circuit uses
a single-tube input, push-pull amplification and resistor-capacitor coupling input stage, which solves the problem of differential tube pairing. The actual circuit has
reached a high technical level and is a very excellent power amplifier circuit.
    The circuit principle is shown in Figure 2-8. It consists of three parts: input stage, intermediate moderated power output stage o VT] ~
VT3 forms the input stage, VTi is the emitter output device, VT2 and VT3 form a complementary push-pull amplifier. In order to improve
the symmetry , the two tubes are The emitter is equipped with current negative feedback resistors Rio and Rl1. Ri is the negative feedback resistor of this stage, Ri~R3 is
the bias resistor of input tube VTi. The base bias of VTi is taken from the output terminals of VT2 and VT3. It is actually the small
loop stage feedback of this stage. It makes The entire input stage has very good operating characteristics. RPt is used to adjust the symmetry of the circuit.
    VT4 ~ VT6 form an intermediate voltage amplification stage with the same structure as the input stage. VTs and VT6 form a complementary push-pull amplification
stage, and are equipped with negative feedback resistors ‰ and Han at their emitters, but there is no small loop negative feedback in this stage. , but
is replaced by the large loop negative feedback from the output stage, consisting of R14, Rl5 and Rn.
    The final power output stage consists of VT8 ~ VT13, of which VTs and VTg are mutual push stages, and VTio, VTlz and
VT'er and VT13 respectively form a quasi-complementary composite tube push-pull output stage. The connection method between the pusher stage and the composite tube output stage of this circuit
is unique, that is, the pusher stages VT8 and VTg are connected in a common emitter configuration and have a certain voltage gain, thereby
adding a small inter-stage loop through ‰~% Negative feedback offers possibilities. This not only reduces the burden on the front stage, but also makes the gain of this stage for positive and negative half-cycle signals more symmetrical and stable, greatly reducing the requirements for the pairing of each tube in the final stage, which is a major feature
of this circuit.     In order to improve quasi-complementarity Due to the upper and lower symmetry of the output PNP tube and NPN tube, the diode VDi is connected in parallel to the emitter resistor of rrlo , and its volt-ampere characteristics are similar to those of the output controller, so that the emitter load of VTio is similar to 1rll. The emitter load is balanced. Adjusting RP4 can not only adjust the gain of the upper and lower arms, but also reduce the high-frequency even-order harmonic distortion at maximum output power.     In order to ensure that the frequency response is broadened under the premise of stable circuit operation, a double-pole compensation circuit is adopted. The first pole frequency is determined by VTs.VT6 collector junction capacitance and q5, about 35kHz, and the second pole frequency is determined by R16 and island, about 2MHz . R14 and C7 are low-pass filters to limit the passband of the power amplifier.