GB721653A - An Automatic Switching Device for Radio Amplification and similar Systems - Google Patents

Abstract

721,653. Loud-speaker systems; sound reproducing circuits. SUNDIN, A. T., and NILSSON, H. N. Nov. 14, 1952 [Nov. 19, 1951], No. 28741/52. Class 40 (4). In a system in which signals from a first source normally pass from a first input circuit of the system to a load circuit, e.g. a loudspeaker, on the application of signals from a second source to a second input circuit the latter signals are amplified and a portion thereof is rectified, the rectified voltage, if greater than a threshold value, serving to render an amplifier in the first signal path non-conducting and to increase the gain of the amplifier in the second signal path, the remaining portion of the output of the lastmentioned amplifier being applied to a final amplifier common to both signal paths and thence to the load circuit, so that presence of the second signals suppresses the first signals and only the second signals appear in the load circuit. As described, the first signals, which are audio-frequency currents from the A.F. stage of a radio receiver, are applied at 8 and are normally amplified in amplifier 9 and applied via the final amplifier 10 to the load (not shown) which may be a loud-speaker in a public service vehicle. If it is desired to interrupt the radio programme with an announcement, the attendant speaks into microphone 11. The speech currents are amplified at 12 and 13, part of the output being rectified in diode 14 provided the peak voltage exceeds that across the cathode resistor 15. The voltage appearing across resistor 16 is smoothed and applied to bias back the amplifier 9, thus reducing the gain of that amplifier and hence the voltage across 15. The gain of 12 is therefore increased and eventually valve 9 is cut off and the gain of 12 is considerably increased. The output of amplifier 13 is fed via final amplifier 10 to the load.