GB1302534A - - Google Patents

Abstract

1302534 Public address systems; direct call systems SAAD ZAGHLOUL MOHAMED GABR 20 Oct 1970 [24 Oct 1969] 52268/69. Headings H4K and H4R In a public address system (e.g. a paging system) a central amplifier 10 (Fig. 1) supplies an audio signal to a line pair 12. Address stations 18 connected across the line pair 12 each have a local amplifier 20 which feeds a loud-speaker 22. Each local amplifier 20 is powered by D.C. on the line. Each address station 18 may have a variable or preset tuner 23 also powered by D.C. on the line. The central amplifier output may be in parallel with the line pair or in series with one leg of the line. Some of the address stations 18 and the central amplifier station 10 may comprise loud-speaking telephones and any one of the loud-speaking telephones may selectively address one or more of the others. Central amplifier station 10 (Fig. 2) has the output from a local amplifier 30 connected in parallel with the line 12 via a transformer 32 and D.C. isolating capacitor 36. The amplifier input 34 is supplied from a microphone (not shown). A remote exchange central battery (not shown) supplies D.C. power to the line 12. The D.C. is applied to the amplifier 30 through a stabilizer 44 and the amplifier power input 54 is decoupled from audio signals by an inductance 56 and capacitor 58. Alternatively the stabilizer 44 may be replaced by a D.C. supply whose output is connected to line 12 and, via the decoupling components 56, 58 and another stabilizer, if required, to the central amplifier station 10. Another embodiment (Fig. 3, not shown) has the amplifier output in parallel with the line (12) but capacitor 36 is eliminated. Another central amplifier station 10 (Fig. 4) has the secondary winding 40 of the transformer 32 in series with line leg 16. A capacitor 60 forms an A.F. signal shunt to apply signal to the line 12. In another arrangement (Fig. 5, not shown) the capacitor 36 is eliminated. Address stations 18 each have the input of a local amplifier 70 (Fig. 6) connected to the leg 16 of line 12 via a D.C. isolating capacitor 74. The D.C. supply is supplied from the line leg 14 via an inductor 78. A Zener diode 90 acts as a stabilizer. The loudspeaker 22 forms output load a D.C. isolating capacitor 82 being in series with the loudspeaker 22. Alternatively the loudspeaker 22 may be arranged between the amplifier output and the D.C. input 76, the D.C. isolating capacitor then not being required.