646 



BELL SYSTEM TECHNICAL JOURNAL 



The indi\"idual channel filters are all of the band-pass type as 

 pre\'iously indicated and have a free transmission range of approxi- 

 mately 2,500 cjxles. Outside this free range they have a high im- 

 pedance so as not to act as a shunt for the other channels. They are 

 all of substantially the same construction, although the constants of 

 the component parts necessarily vary as the filters for the different 

 channels transmit different frequencies. 



The transmitting and receiving amplifiers, which are practically 

 identical, are shown schematically in Fig. 9. They consist of two 

 push-pull stages connected in tandem. Each half of the second or 

 output stage consists of two parallel tubes of high output capacity. 

 In this way a comparatively high gain and a large energy output 



Fig. 9 



may be secured without overloading. This is very important as 

 these amplifiers are common to all six channels and any tendency 

 to overload would produce objectionable distortion and inter-channel 

 modulation. In order to adjust the over-all gain for the entire system, 

 each amplifier is provided with an input potentiometer. 



As has been previously indicated, the transmitting and receiving 

 circuits are joined to the cable by means of hybrid coils. Probably 

 the most difficult problem encountered in the installation of this 

 system was the securing of an adequate balance. The difficulty of 

 doing this may be better appreciated when it is realized that this 

 balance must cover all frequencies from 3,000 to 30,000 cycles, and 

 must have a value of from 30 to 45 T. U., the higher value which 

 represents an impedance unbalance of approximately one per cent, 

 being required at the upper frequency. In order to secure such a 



