AMPLIFIER No. 143A

A Telephone for Subscribers with Weak Voices
Amplifier No. 143A
by W. T. LOWE (taken from POEEJ, 1962)

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:-

• The dry batteries used for the valve H.T. and L.T. supplies require frequent replacement.

• The amplifier, with its ON/OFF key, is housed in a plinth underneath a specially modified 200-type telephone but the amplifier is not readily adaptable for use with the new 700-type telephones.

• The equipment was developed for use on direct exchange lines and is not suitable for other types of subscribers' installation.

• The telephone is only available in black, as it is not economical to stock a special telephone, for which there is only limited demand, in a range of colours.

• The amplifier ON/OFF key can be left inadvertently in the ON position at the completion of a call. This is an embarrassment if the afflicted subscriber is not the sole user of the telephone.

• The valve used in the amplifier is now obsolete and maintenance replacements are unobtainable.

• The complete equipment is bulky and draws attention to the subscriber's affliction.

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
The new amplifier (Amplifier No. 143A) is a single stage transistor-type amplifier which draws its power from the telephone line; no additional power supply is required. The amplifier is fitted into a small plastic case 6in. x 5in. x 2in., suitable for fixing to a wall or skirting board, and has only to be connected to a standard telephone (Telephone No. 710) by a 9-way cord. Two push-buttons (marked ON and OFF) are fitted in the telephone, leaving two vacant push-button positions in the telephone available for other purposes. The ON push-button is arranged so that it is automatically released when the handset is replaced on the cradle, thus ensuring that the amplifier is switched out of circuit at the end of each call. If the ON push-button is not pressed, the telephone operates as a normal instrument.   The Mark 1 with internal volume control is pictured).

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.

Conclusion
It is apparent that the new amplifier unit is compact and unobtrusive and that the telephone has the general appearance of a standard telephone. As a result, this new equipment will not draw attention to the subscriber's affliction.

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