Patent GB-888333-A (GB)
Title: Sound amplifying apparatus for telephone circuits
IPC: H04M1/60
Filed: 1958-03-11
Abstract: 888,333. Substation circuits. BEATTY, D. C. March 11, 1958, No. 7820/58. Class 40 (4). Substation equipment may be set by a control switch to give an answer signal to the exchange automatically in response to ringing tone. A loud-speaker, a microphone and associated amplifiers are connected to the line when the signal is made. The line is held until dial tone is received after clearance of the other party. The circuit may be modified by operation of a switch so that the line is cleared after a predetermined period of silence. If a call is received in the normal manner, using a handset, the amplifying equipment &c. may be switched on and off during the call. Power supply.-A mains transformer X4 and a rectifier T7 supply low-tension alternating current on leads 1, 2 and high-tension direct current on lead 20. Substation circuit, " Phase One " and " Phase Two."-The usual handset 200 and ringer 300 are connected to the line L1, L2. With switch S1 open, a call is received and answered normally. With switch S1 closed, relay R1 responds to ringing current and is followed, after a delay, by a thermal relay R2 and relay R3. Relay R3 connects an anti-side tone circuit " Phase Two " to the line and thus signals reply. Relay R3 is held independently of R1 and R2 and is tripped later either manually by switch S2 or automatically by R4. While R3 is up, the amplifier circuits of microphone MIC and loudspeaker SPKR are linked to the line L1, L2 by transformers X1, X2 in the anti-side tone circuit " Phase Two " and conversation can take place without the handset. Relay R3 and the line L1, L2 are released when R4 pulls up. Relay R4 is controlled by R5 in the output stage of a selective control amplifier " Phase Five. " With the switches S4, S5, S8 set as shown, R4 follows R5 when the tuned amplifier responds to dial tone frequency on the line. If the switches are changed over, R5 is normally held up by speech on the line and R4 comes up after a predetermined period of silence, the period being controlled by the condensers C5, C106 and resistors V8, V9. Loud-speaker circuit, " Phase Three."-The loud-speaker SPKR is linked to transformer X2 by two resistance-capacity coupled amplifier stages. Volume is controlled by a potentiometer V19 in the grid circuit of the output tube and the speaker can be cut out by switch S7 in the H position. Microphone circuit, " Phase Four."-The microphone MIC is linked to transformer X1 by a similar two-stage amplifier. Two levels of amplification can be selected by switch S7 and fine adjustment can be made by potentiometer V14. Control amplifier, " Phase Five."- With switches S4, S5, S8 set as shown, the grid circuits of tubes T5, T5A are tuned to the dial tone frequency by inductors Q, Z, and condensers C2, C101. The output of T5A is rectified by a selenium rectifier SR and charges condensers C5, C106 in the grid circuit of output tube 20 which controls relay R5. The condensers discharge slowly through resistors V8, V9, thus delaying release of R5. If the switches S4, S5, S8 are changed over, relay R5 pulls up in response to speech and remains up until a predetermined period of silence elapses. Relay R4 then follows.