PUBLIC ADDRESS SYSTEMS 



129 



are to be delivered to the audience. It was found that four units 

 would provide for all the conditions occurring in practise, two of 

 these being speech input or transmitter amplifiers with different gains 

 and two being power amplifiers of different power ratings. These 

 units and other equipment used with the system, are made up in 

 panels, of uniform width, in order that the proper equipment for any 

 installation may be assembled on two vertical angle iron racks ar- 

 ranged to be fastened to the control room floor. 



Speech Input Amplifier — First Type 



The first of the speech input amplifiers is shown schematically in 

 the upper part of Fig. 8. It is a three-stage amplifier. Two po- 



SPEECH INPUT AMPLIFIER 



For Distant Talking Transmitter 



To Medium or High 



c=r« «2L x-g ; P t2? o^,,™,.™,,^^ 



To Long 

 Lines 



Connected for 350 V. 



Long Line 

 Transmitter Receivin 8 



Control Panel 



'12V. 9 

 Supply 



SPEECH INPUT AMPLIFIER To Medium orH i gh 



For Close Speaker Transmitter High and Low Power Amplifier ~^ 

 Or Long Lines Receiving 



Gain Switch 



High Quality 



Transmitter .vAV^oiRrHw^ 



Gain Regulating 

 Potentiometer / 



Input Transformer L _ ]_ 



-vwa^OOO^ 



Output Transformer 

 To Long Lines ^ 

 Transmitting 



12 V. 



Supply 



Fig. 8. 



♦350 or' 

 130 V. 

 Supply 



i 



tentiometers provide adjustment of the gain over a large range, and 

 switching arrangements allow the output to be connected directly 



