PUBLIC ADDRESS SYSTEMS 125 



the power amplifier, or for connection to the long distance lines, in 

 those cases where the speeches are transmitted to distant audiences. 

 It is also suitable where connection is to be made to a radio station 

 for broadcasting the speeches. The power amplifier is shown just 

 to the right of the transmitter amplifier. Below it is indicated the 

 power supply equipment by wh'c'i the commercial power is converted 

 to a form suitable for the vacuum tubes in both amplifiers. The 

 output from the power amplifier is taken through a panel where 

 switches and a multi-step auto-transformer are provided for the 

 regulation of several projector circuits. Just above this panel is an 

 indicating instrument, known as the volume indicator, provided in 

 order that the operator may know what volume output is being 

 delivered to the projectors. 



The projectors, at the extreme right of the figure, consist of the 

 motor or receiver unit transforming the speech currents into sound 

 waves, and a horn to provide the proper distribution of the sound. 



It is interesting to note the power amplification which is obtained 

 in the larger of the two systems from the transmitter to the pro- 

 jectors. Referring to Fig. No. 6, a chart will be seen which indicates 

 the power levels through the system drawn to a scale based on miles 

 of standard telephone cable, our usual reference unit. The output 

 of the high quality transmitter is of the order of 65 miles below zero 

 level, this latter being the output from a standard telephone set con- 

 nected to a common battery central office by a line of zero resistance. 

 Expressed in watts the output of this transmitter under average con- 

 ditions of use with the public address system is of the order of 10~ 8 

 watts. Incidentally, this is of the same order as the speech power 

 picked up by the transmitter, or in other words, the transmitter 

 does not amplify the speech power received by it as is the case with 

 the transmitter used for regular telephone service. 



This very minute amount of power in passing through the trans- 

 mitter amplifier may be amplified about 120,000,000 times. Ex- 

 pressed in terms of telephone power levels, this is 17 miles above the 

 zero level previously mentioned, or a few tenths of a watt. 



The power amplifier serves to increase this power from a level of 

 17 miles to about 40 miles, the latter corresponding to about 40 watts. 

 This power is then distributed to the projectors, the amount con- 

 sumed by each projector, of course, depending upon the number 

 connected. 



An idea of what this amount of power at speech frequencies means 

 may be given by the statement that it is sufficient to operate at 

 about the level considered commercial all of the 14,000,000 tele- 



