620 



THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1953 



108 



106 



104 



g,02 



ID 

 Q 



z 100 



-I 



UJ 



92 



90 



0.02 0.04 



0.06 



0.08 0.10 0.12 0.14 



LOOP CURRENT IN AMPERES 



0.16 



0.18 



0.20 



Fig. 6. — Effect of Vi and V2 varistors on sidetone loudness levels of 500D 

 telephone set for average speech. 



varistor is opened, almost all the effects of sidetone balancing are lost. 

 It requires the presence of both Vi and V2 to obtain the impedance 

 matching necessary for good balance. 



From all that has been said, it should not be thought that the side- 

 tone balance of the 500D set is perfect under all plant conditions. The 

 impedance looking toward the loop varies so greatly in magnitude and 

 phase that the best overall balance involves compromise. However, the 

 balance provided is as good as with the original design of 500 set, which 

 was considerably better than that attained by any previous design of 

 commercial telephone set. 



OTHER MODIFICATIONS 



The introduction of the new equalization and sidetone controls in 

 the telephone set has required a number of changes in other components 

 of the set to realize all possible economies. Fig. 7 is a photograph of 

 the transmission network (425A) which was employed in the earlier 

 500 A and B type sets. In the new arrangement shown in Figure 8 



