GB637549A - Improvements in tone control arrangements for audio frequency amplifiers - Google Patents

Abstract

637,549. Valve amplifying circuits. COLE, Ltd., E. K., and BURNARD, C. L. Sept. 3, 1947, No. 24262. [Class 40 (vi)] In an audio-frequency amplifier having a tone control of the type using negative feedback, at least at one end of the audio-frequency range a band of frequencies is cut and the frequencies immediately adjacent to the band and lying nearer to the centre of the range are accentuated in amplitude in relation to the middle frequencies in the range. As shown, the anode circuit of the input valve V1 is connected through resistor R3 and condenser C1 to the control grid of the pentode output valve V2, and through a condenser C2 to earth, the condenser C2 being shunted by a resistor R4, in series with a condenser C3. The negative feedback path from the anode of V2 is provided by a condenser C4 shunted by a resistor R5, resistors R6 and R7 connected to the input side of the coupling condenser C1, and a condenser C5 connected between the junction of R6 and R7 and earth. Switches S1, S2, S3 are preferably ganged and provide a variation of tone control. With the switches as shown and by suitably proportioning the components, condenser C2 byepasses the highfrequencies, but in the feedback circuit the next highest frequencies are byepassed to earth by condenser C5, giving these frequencies a boost. At the bass end of the signal spectrum, by connecting the feedback to the input side of condenser C1, the lowest frequencies are cut whilst the frequencies immediately adjacent are boosted by the reduction of feedback by C4. Design data and typical values.for components are given.