GB1349734A - Arrangements for amplifying and reproducing audio frequency signals - Google Patents

Abstract

1349734 Loudspeaking telephone set; public address equipment POST OFFICE 19 April 1971 [7 Nov 1969] 54751/69 Headings H4J H4K and H4R In order to prevent singing between the loudspeaker and microphone of a loudspeaking telephone or a public address amplifier system, frequencies above the speech band are amplified to a greater extent than speech frequencies whereby any oscillation in the system, which is more prevalent at high frequencies, can be detected and the gain of the amplifier thereby automatically reduced. As shown a telephone line 1, 2 is connected via surge-limiting resistors 3, 6 and full-wave bridge 7 to a loudspeaking set which incorporates integrated circuit amplifiers 45, 26 for respectively a loudspeaker 46 and an electret microphone connected to terminals 27, 28 possibly via a not shown preamplifier. Incoming signals reach amplifier 45 via capacitors 11, 21, transistor 17 and capacitors 33, 44. Outgoing signals reach line via common base transistor 8, part of whose collector output is fed via transistor 17 to the loudspeaker so as to provide sidetone. In addition some of the output of amplifier 26 is applied to transistor 16 whose rectified (43) output serves as a control signal for FET 36. Capacitor 41, which is used to store this control signal, together with its associated resistors, have a time constant sufficiently short to avoid blocking of incoming speech by the hangover effect. The impedance of the FET obviously determines the effective gain of the incoming speech amplifier. The latter has a negative feedback network which includes a bandstop (18-20 KHz) filter 47 so that maximum gain is provided at this frequency. The FET is thereby chiefly controllable by a signal in this frequency band if such should be fed back via an acoustic path from the loudspeaker to the microphone. The FET is preferably of the N-channel depletion mode type. Power is taken from Zener diode 9 and smoothing capacitor 14. Current-sensitive resistor 12 tends to reduce changes in sidetone level caused by variation in line resistance. Capacitor 40 besides providing an A.C. path to earth from the source of FET 36 also in conjunction with diode 37 prevents instability that could arise if power supply voltages from line would reach the input of amplifier 45 via FET 36. Transistor 17 which isolates A.C. signals from the D.C. supply applied to transistor 8 may be replaced by a transformer. In alternative arrangements, the high frequency filter may be incorporated in the input circuit of amplifier 45 or separate amplifiers for the A.F. and ultrasonic signals may be provided.