Linear feedback-free power amplifier circuit (b)

For the power output and distortion servo circuit o
    VTii, ~VTI l5 forms a composite tube complementary push-pull output stage, and its characteristic frequency directly affects the conversion rate of the whole machine.
High-power tubes with fT between 20 and 100MHz should be selected, such as 2SC2922/2SA1216 and 2SC2461A/2SA1baSA. The emitter resistors
R138 and R139 of the power amplifier output tube should be 3~5W carburized resistors. Ordinary carbon film resistors should not be used, otherwise there will be a risk of burning out due to large current. On the other hand, the carburized resistor is a non-inductive resistor , which can reduce the switching distortion of the final stage.     The output terminal is connected by R14a, Liol, R149. The filter network composed of , Cl30, R15¨C131, etc. has a good effect on suppressing high-frequency self-excitation and absorbing the back electromotive force and impedance mutation of the speaker.     Iclol forms the core of the distortion servo circuit, which is a voltage comparator composed of a high-speed operational amplifier LF356. The input signal is input to the non-inverting terminal of the comparator via R125, and the output signal is input to the inverting terminal t of the comparator via RP103 and R14sRl46 . The voltage difference between the two signals after comparison is output as a distortion correction signal. Distortion correction is performed through the input end of the output tube of the power amplifier circuit through an adder circuit composed of RI27~Ri, R133, and R134. C113, C114, VDl04, VDios, Ri∞, and R124 constitute the floating power supply of the circuit to ensure that the operational amplifier It works within the rated voltage range without causing distortion due to too high peak value at the input end. C117 and C118 are high-value decoupling capacitors. ⅥD106 and VD107 are input protection diodes. C119 , C1.20, C122, and C124 are acceleration capacitors. When a transient signal arrives, the final output tube is turned on in time to improve the transient characteristics .     VD109~VI14 form a bias protection circuit to prevent VT112 and VTii{> from entering the cut-off state and the bias is released after power failure to protect the output tube.