GB846344A - A thermionic-valve audio-frequency power amplifier - Google Patents

Abstract

846,344. Valve amplifying circuits. MURRAY, J. S. March 6, 1959 [March 6, 1958(2)], Nos. 7268/58 and 25252/58. Class 40(6). An audio-frequency power amplifier comprises a pentode or a cascade stage followed by a longtailed pair phase splitter and a push pull output stage, wherein the control grid of one valve of the phase splitter is D.C. coupled, either directly or through a cathode follower, to the anode of the pentode or the upper valve of the cascode stage, that of the other being maintained at the same D.C. potential but decoupled to earth for signal frequencies. Bias for the pentode or cascade stage is obtained by D.C. connecting the screen grid of the pentode, or the control grid of the upper valve of the cascade, to a point on the common cathode resistance of the phase splitter through a network which decouples the grid at signal frequencies but not at very low frequencies or D.C. In the embodiment shown in Fig. 1 the coupling network Consists of a capacitor C and resistor R which applies negative feedback at very low frequencies and at the same time effects a phase shift to the feedback signal, increasing the stability of the circuit at very low frequencies. Overall negative feedback may be applied from the output stage of the amplifier through the loop 32. In the embodiment shown in Fig. 4, a pentode input stage is used and a cathode follower 41 is included between the pentode stage and the long-tailed pair. The bias coupling network containing C, R may be connected by the lead 43 to the common cathode impedance of the phase splitter, or by lead 44 to the cathode of the cathode follower stage 41. The modification of Fig. 3 (not shown) has a pentode input stage preceded by a pre-amplifier circuit having a tone control. The cathode return circuit of the pentode stage is modified to include a balance-potentiometer, the circuit being intended to form part of one channel of a stereophonic system.