A757 power amplifier circuit

With the emergence and popularization of digital audio sources (such as ∞ and VCD), higher requirements have been put forward for power amplifiers.
On the one hand power amplifiers are required to have high technical performance so that they can match the high sound quality and large dynamics of digital audio sources. , low noise
; on the other hand, it must be low-cost and low-priced so that it can adapt to the current national conditions and be accepted by the working class
. The domestic power amplifier LHGA757 is a model that has appeared to meet this requirement. Its principle is shown in Figure 2-14.
    This machine adopts the design principle of simplicity first, and through careful design and strict material selection, it reaches a high technical level.
    The circuit adopts a fully symmetrical and fully complementary OCL circuit structure. The input circuit adopts a symmetrical double differential circuit, and VTi and
VT2 form NPN. The polarity differential stage, voltage regulator tube VI): I, code, Rll, C constitute its constant current source; VT3, VT4
constitute the PNP polarity differential stage, 1Ga, Rio, Rl2 and island are its constant current source. Current source o The emitter of each differential tube is connected with resistors
R5~Ra, and the base of each tube is connected with resistors R3.R4 and R1"R18 to ensure complete symmetry of the circuit and ensure
the stability of the circuit. Rg, Rio resistors A higher value can improve the common mode rejection ratio of the differential amplifier and enhance the anti-interference performance of the circuit
{_f VTs and Vr6 form a voltage amplification stage to provide sufficient voltage gain. This stage
is served by Toshiba audio dedicated pair tubes 2SA940 and 2SC2073, and Working in a good linear area, VT7 and VT8 form a push stage, which together with
the power output tubes VT9, VTri, VTio, and VTi2 form high-amplification NPN composite tubes and PNP composite tubes to
output sufficient power. Power output tubes Toshiba audio special tubes 2ⅨⅪ280 and 2SA1301 are used, and two are
connected in parallel to reduce the output impedance and improve the load characteristics.
    VT13, R23, and R24 form an output stage bias circuit. Adjusting R24 can change the output current of the power amplifier stage. This circuit can adjust the voltage
between the ce poles of VT13 to about 2~2.2V.     Rl"‰ and C form a feedback loop to determine the closed-loop gain of the amplifier.     The entire circuit adopts direct coupling from the input stage, which can reduce the loss of weak signals. Extremely small, the ability to analyze details in music is greatly improved.