Patent GB-1127136-A (GB)
Title: Tone control
IPC: H03G5/06
Filed: 1965-11-18
Abstract: 1,127,136. Tone control circuits. D. HAFLER. 18 Nov., 1965 [18 Dec., 1964], No. 49095/65. Headings H3T and H3W. [Also in Division H1] A tone control system for an amplifier comprises separate frequency selective circuits for introducing boost and cut respectively and a single variable resistance control device which in operation has one position in which both frequency selective circuits are disabled; in other positions one or other frequency selective circuit is disabled, tone controlled signal being derived from whichever one is not disabled. In an embodiment giving control of bass, signal is applied at terminal 10 to a circuit comprising resistor 12, capacitors 15, 16 and resistor 13 in that order, to ground: output is taken from the join of capacitors 15, 16, point 17. Point 17 is joined also to the sliders of a variable resistance device, the ends of the track of which are connected across capacitors 15, 16. The variable resistance device comprises a pair of interconnected slider arms 21, 22 rigidly connected to one another and the track includes three series-connected sections, the middle section 19 being of resistive material and each of the outer sections 18, 20 respectively being of negligible resistance: the arrangement is such that in the position shown the slider terminal 17 is directly connected to both ends of the track, so that capacitors 15, 16 are shortcircuited. Embodiments of the variable resistance device are described in the Specification (Figs. 5, 5a, not shown, see Division H1). A network of the type described may be connected between anode and grid of a valve to provide tone control by means of negative feedback (Fig. 2, not shown). In an arrangement giving control of top, input is applied to one end of a potentiometer comprising resistors 50a, 50b, Fig. 3, and output is taken from the tapping point 49. Connected across the potentiometer is a series network comprising capacitor 43, a variable resistance control device, and capacitor 47 in that order; the variable resistance control device comprises a slider 48, connected to junction point 49 and a track in the form of two resistance portions 44, 46 respectively, separated by a gap 45. The latter is wide enough for slider 48 to rest upon it, disconnecting both the capacitors 43, 47 from circuit. A combined passive bass and top control may be made utilizing the arrangement of Fig. 1, in conjunction with that of Fig. 3 (Fig. 9, not shown). In another embodiment comprising a two stage R.C. coupled valve amplifier, the second valve feeds into the tapping point 17, Fig. 4, of a network of the type shown in Fig. 1, an output being taken from one end terminal 11, across resistor 13. The other end terminal 41 provides negative feedback, over a network 68 of the type shown in Fig. 3, to the cathode 55 of the first valve. Transistor circuits.-A feedback amplifier with temperature stabilization comprises NPN transistor 101, Fig. 7, direct-coupled to PNP transistor 105, which has resistor R6 as collector load; the emitter circuit of transistor 101 comprises resistors R3, R5 in series, resistor R5 being decoupled by capacitor C5. Negative feedback for D.C. only is applied from the collector of transistor 105 to the emitter of transistor 101 by means of resistor R8, and for both D.C. and A.C. by means of resistor R4. Resistor R8 may be omitted (Fig. 6, not shown), while resistor R5 and capacitor C5 may be replaced by a Zener diode. 'The Specification also describes a three stage direct-coupled amplifier in which alternate transistors are of opposite conductivity type and in which stability is attained by D.C. negative feedback produced by means of a decoupled resistor common to the emitter circuits of the first and last transistor (Fig. 8, not shown). As applied to a tone control circuit, transistors 101, 105, Fig. 10, are direct-coupled and provided with combined D.C. and A.C. negative feedback over resistor R8 as in the arrangement of Fig. 8 (not shown); transistor 105 feeds into the tapping point 17 of a network of the type shown in Fig. 1, an output being taken from one end terminal 11 across resistor 13. The other end terminal 41 provides A.C. negative feedback over a network 48 of the type shown in Fig. 3, to the emitter 102 of the transistor 101.