AMPLIFIER No. 143A
A Telephone for Subscribers with Weak Voices
A special valve amplifier and telephone have been available for some years to assist subscribers with weak voices. This equipment has, however, several disadvantages, and it has now been redesigned using a transistor type amplifier and a 700-type telephone.
Faint speech is a comparatively rare ailment, but for those who suffer in this way it is a severe handicap, particularly for users of the telephone. The loss of voice is sometimes only temporary, as, for instance, during the few months' recovery period following a throat operation. For other people it is a permanent affliction due to defective lungs or vocal chords, and such persons may be able to talk only in a soft whisper, which may be inaudible over a telephone. Several years ago a valve amplifier and a special telephone instrument were developed to assist these subscribers by amplifying their speech signals. This equipment, known as the Telephone No. 262 CB , gives a satisfactory transmission performance, but has the following shortcomings:-
Modern equipment, incorporating a transistor-type amplifier, has therefore been developed to overcome the difficulties outlined above. This new equipment will replace the earlier telephone.
The New Equipment
Circuit Description The amplifier circuit, contains a single transistor connected in the common-emitter configuration. With the ON push-button contacts, KAI and KA2, operated, the handset transmitter is connected to the input circuit of the transistor and the telephone-line impedance acts as the collector load. Metal rectifier MR1 acts as an automatic voltage regulator, compensating for changes of voltage at the amplifier due to changes of line current: as the line current decreases the forward resistance of the rectifier increases and it therefore tends to maintain the voltage across the amplifier. The inductor, L1, prevents the regulator, MR1, acting as a shunt across the amplifier at speech frequencies.
Resistor R1 is a non-linear resistor with a positive resistance/temperature coefficient. It is part of the emitter-bias resistor chain and also carries the full line current. On short telephone lines, where the line current is a maximum, the resistance of resistor R1 rises to its maximum value and a large potential drop is developed across it. This gives a negative bias to the emitter and reduces the total collector-emitter voltage, thus preventing the overloading of the amplifier when it is used on a subscriber's circuit with high line current. The amplifier gives a maximum gain of at least 20dB with line currents in the range of 12-110ma. Full-wave bridge rectifier MR2, consisting of four germanium diodes, ensures the correct polarity of the d.c. power supply to the amplifier irrespective of line reversals. The gain control, VR1, is adjusted at the time of installation so that the afflicted subscriber's speech is transmitted to line at approximately the same level as speech from a subscriber with a normal voice. This is judged by a simple listening test at a distant telephone. When using the amplifying telephone, the subscriber hears the normal level of sidetone in his receiver whilst talking. The presence of the speech amplifier introduces slight attenuation of the incoming speech signals, but the effect on intelligibility is negligible.
Circuit Diagram - SA4150.
The original amplifier, introduced in 1961, with the volume control within the case was called the Mark 1. In 1966 a volume control was fitted externally and this was called the Mark 2.
Amplifier No. 143A (Mark 2)
Amplifier No. 143A - Internal view
Last revised: June 22, 2023