132 BELL SYSTEM TECHNICAL JOURNAL 



power amplifier has been developed for this use. It is arranged to 

 connect directly to the transmitter amplifier and the output is taken 

 to the projectors through the volume control panel. It has a gain of 

 17 miles or a power amplification ratio of about 33. The maximum 

 output is about 4 watts, or about one-tenth of the power obtainable 

 from the high-power amplifier. 



The schematic of this amplifier, is shown in Fig. 9. The input coil 

 is the same as is used in the high-power amplifier. The push-pull 

 connection of the tubes is also used in this amplifier, although but 

 two power tubes are used. The filaments of these tubes are supplied 

 from a 12-volt storage battery while the plate circuits are supplied at 

 350 volts direct current from a motor generator set which will be 

 described later. 



Speech Input Amplifier — Second Type 



A speaker using the system may read his speech from his home or 

 office and in such cases it is unnecessary to use the push-pull carbon 

 transmitter in the distant-talking manner. For use when this trans- 

 mitter is spoken into from a distance of a few inches, a second form 

 of speech input amplifier has been made available having a gain of 

 the proper value to supply either of the power amplifiers, or a long 

 distance line if desired. This gain is relatively small as the output 

 of the transmitter when used for close talking is about 10,000 times 

 that when it is used for distant talking. 



Fig. 8 shows the schematic of this amplifier which is a single-stage 

 one, employing one tube and having the same over-load characteristic 

 as the first form of speech-input amplifier. A two-way switch permits 

 the connection of the transmitter or an incoming long distance line to 

 the amplifier. To the right of this switch is a potentiometer for 

 regulation of the gain. To the right of the tube is a second two-way 

 switch for connecting the output either to the power amplifier or 

 through an output transformer to an outgoing long distance line. 

 The power supply for the tubes and transmitters, is the same as was 

 described under the first form of speech input amplifier. 



The switching means provided on this amplifier allow it to be used 

 in a number of ways. Announcements from a close talking trans- 

 mitter may be made from the projectors through a power amplifier 

 or may be sent out on the telephone lines to a distant public address 

 system installation or a radio-broadcasting station. In addition to 

 these uses, incoming speech over the long distance lines may be put 

 out on the projectors through the power amplifier or may be sent out 

 on the long distance lines to a distant installation. 



