An Improved Circuit for the Telephone Set 



By A. F. BENNETT 



(Manuscript received August 16, 1952) 



A telephone set known as the 500 type has been in prodiiction for Bell 

 System use for some time. The successful outcome of an intensive study 

 has made it possible to simplify the circuit and some of the components of 

 this set, and thereby to increase dependability and life and significantly to 

 reduce the manufacturing cost. This change now has been completed and 

 telephone seos embodying the necessary modifications are in production. 

 This paper discusses some of the problems involved in this work and outlines 

 the improvements which have been effected. Presented also is information 

 concerning the superior performance of the 500 set over the preceding 

 telephone set when used in noisy locations. 



INTRODUCTION 



One of the outstanding characteristics of the 500 type set is a 10 db 

 increase in combined transmitting and receiving performance on long 

 loops. This gain is equally divided between receiving and transmitting. 

 This improvement has resulted largely from the use of a transmitter 

 and a receiver which are not only more efficient, but also have better 

 frequency response characteristics. To take full advantage of these trans- 

 mission gains, two new elements were introduced in the original 500 set 

 design. One of these elements was a better sidetone balancing circuit to 

 offset the more sensitive transmitter and receiver, and maintain sidetone 

 at a level no greater than it was with the previous design of set (known 

 as the 302 type). The other was a tungsten filament and thermistor 

 element to control automatically the transmitting and receiving levels 

 so that the desired gains occur on a graduated basis as the loop length 

 increases. This combination of filament and thermistor bead was called 

 the transmission equalizer. While the transmission objectives were met 

 with telphone sets having these elements, this additional component 

 increased the manufacturing cost of the set appreciably, and therefore 

 more economical means of attaining the desired results were sought. 

 Such means have been found in the form of an arrangement in which a 



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