AN AMPLIFIED TELEPHONE HANDSET
W. G. SOAR - Audio-Frequency Research Department
The new amplified Etelphone-type handset described has been developed to assist the telephone user who is hard of hearing. Requiring no separate batteries, the handset can be readily interchanged with the normal handset of many existing telephone instruments without modification of the telephone circuit.
Many subscribers with impaired hearing will welcome the availability of a new light-weight telephone handset which, incorporating a transistor amplifier, provides a high standard of amplified reception. The handset has been designed to replace the Post Office Repeater Telephonic No. 17A - a valve amplifier housed with its attendant battery supplies in a separate wooden box situated near an associated special telephone instrument. This instrument had a gain control mounted on a bracket at the rear, h.t. and l.t. switching contacts added to the switch hook springset, and a separate bell-set. The whole formed a bulky piece of apparatus requiring frequent maintenance visits for battery replacements.
| Fig. 1
Complete Handset with Amplifier alongside
The new model is built into the Etelphone, the entire amplifier fitting in the hollow handle of the handset (Fig 1). No auxiliary supplies are needed power to operate the amplifier is derived from the telephone line current. The only evidence that this is not a normal telephone handset is the projecting edge of a miniature gain control situated at the receiver end. The instrument circuit does not require modification, the amplified handset being connected into the circuit in the same manner as a normal one.
Merely changing the handset transforms an ordinary telephone into an amplified telephone. The absence of any large additional equipment will appeal to deaf users who wish to conceal their disability. The lower initial cost, absence of batteries and less frequent maintenance should result in lower rental charges. The amplifier uses all the advantages offered by transistors, i.e. small size, low operating voltage, low power consumption, high efficiency and reliability.
Investigation showed that the maximum useful gain that could be put into the receiver circuit was approximately 20db at l000c/s. More gain caused instability leading to oscillation and, consequently, a target gain of 20 db was adopted.
Circuit diagram of Amplified Handset
The amplifier, the circuit diagram of which is shown in Fig. 2, uses a single transistor with grounded emitter, base bias provided through R4, and the telephone receiver as the collector load. The received signals appearing at terminals Al and A2, pass through T1, RV1 and C3 to the base of the transistor. A terminating resistor R1 provides a correct impedance match through T1 at Al and A2 for side-tone balance.
Current/Voltage characteristic of MR2
Power to operate the amplifier is derived from the telephone line current which passes through MR2 and the transmitter. The rectifier arrangement MR2 has a non-linear current/voltage characteristic which tends to reduce the voltage change produced across it by variations of current through it (Fig. 3). Two units are combined back-to-back to cater for either direction of line current. Placing MR2 in series with the transmitter introduces a transmission loss of about 4 db. This loss is reduced to less than 1db by C1, a large capacity, non- polarized capacitor placed in parallel with MR2.
The voltage developed across MR2 is applied to the amplifier through the bridge rectifier MR1 to ensure correct polarity of the amplifier supply voltage. Resistor R3 and capacitor C2 form a filter circuit giving decoupling from the line additional to that provided by Cl.
The gain control RV 1 is series connected to prevent complete signal cut-off in the minimum position. It has a semi-logarithmic law to give a smooth control range of 30 db, i.e. from full gain of 20db above normal to about 10db below normal listening level.
To prevent acoustic shock to the user, the maximum output power developed by the amplifier is limited to a little over one milliwatt, corresponding to a sound pressure of 200 dynes per square centimetre from the receiver.
Amplified handsets of this type are made for both local-battery and central-battery working and are supplied as part of the telephone or as a separate item. They are available in the full range of Etelphone colours.
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