An Improved Circuit for the Telephone Set

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 transmission 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 611