PUBLIC ADDRESS SYSTEMS 



133 



Volume Control 



As discussed heretofore, it is necessary to give the operator control 

 of the volume put out by each projector or group of projectors. The 

 equipment provided for this purpose is mounted on a panel uniform 

 with the others and consists essentially of an auto-transformer con- 

 nected across the output of the power amplifier with 11 taps multi- 

 pled to the contacts of eight dial switches, the arrangement being 

 shown schematically in Fig. 10. Seven of the dials control projector 

 circuits on each of which one or more projectors may be grouped, the 



UONITORIXC 



Projector 



Poweh St a if 

 Output T«*n3 



Fig. 10. 



eighth dial being reserved for controlling the volume of the operator's 

 monitoring projector in the control room. A key is associated with 

 each dial for opening the circuit and a master key is provided for 

 cutting off all of the projectors simultaneously. 



The device shown as "Volume Indicator" in this figure consists of 

 a vacuum tube detector bridged across the output terminals of the 

 power amplifier. The rectified current is taken to a sensitive direct- 

 current meter of the moving coil type, the degree of deflection of this 

 meter measuring the output from the power amplifier when con- 

 nected at a proper place in the circuit. The deflections of the meter 

 therefore serve as a basis for determining the adjustment required 

 on the transmitter-amplifier to give the required output when switch- 

 ing from one transmitter to another or for different speakers. 



Receiver — Projectors 



From the control panel the power is taken to the projectors, each 

 of these consisting of a loud-speaking receiver mechanism to transform 



