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In the valve era, the Bland radio company made a centralized PA system for schools in Australia. The central unit consisted of a preamp for a microphone, turntable, AM radio tuner, and a push-pull amplifier, and switches that would switch it to drive speakers located in the classrooms, and out in the school yard.〔In the living memory of many former school students who went to school in especially the 1970s〕 The speakers in the class rooms were a wedge type cabinet design, and were mounted on the wall up high, containing a 20 cm Rola loudspeaker. The "yard" speaker was also a 20 cm Rola, but housed in a large steel horn,〔Examining of the equipment in private collections in Adelaide〕 that was mounted high up on either the wall of a 2 story school building,〔Yard Speaker still mounted at one Adelaide school〕 or sometimes on the roof of a school building. The amplifier delivered a 70 volt line type output that coupled to the speakers via 500 ohm matching transformers, mounted in the speaker inclosures at each locality. ==Its Use== Usually this system would be used with the ABC's radio programmes for schools. The teachers would tune the tuner into the programme, and switch it to the appropriate class room or class rooms. This would sometimes also be done, for assemblies outside where the yard speaker would be switched in. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Operatic school PA」の詳細全文を読む スポンサード リンク
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