138 



BELL SYSTEM TECHNICAL JOURNAL 



O oj 



30 



500 1000 5000 



FREQUENCY IN CYCLES PER SECOND 



10,000 20,000 



Fig. 2 — Voltage amplification-frequency characteristic of an interstage transformer 

 for a high quality program amplifier. 



vacuum tube) for portable recording equipment where light weight is 

 an important consideration. The voltage amplification-frequency 

 characteristic of this transformer is shown in Fig. 3. There is shown 

 also in this figure, for purposes of comparison, the very much poorer 

 characteristic realized when an alloy of about 50 per cent nickel as 

 commonly used is substituted for the special permalloy. In addition, 



50 





note: — THE RELATIVE AMPLIFICATION IN DECIBELS 

 IS 20 LOG|0^2/e2 WHERE 62 IS THE VALUE OF 65 AT 

 1000 CYCLES. THE RATIO OF ^i/fe, IN DECIBELS 



i.e. 20 LOGio e2/e| is 29 decibels 



CURVE 1 - SPECIAL HIGH-PERMEABILITY 



PERMALLOY 

 CURVE 2 - ALLOY OF APPROXIMATELY 



50% NICKEL 

 CURVE 3 — SILICON STEEL 



500 1000 



FREQUENCY IN CYCLES PER SECOND 



5000 10,000 20,000 



Fig. 3 — Curves showing the relative effect of different core materials on the volt- 

 age amplification-frequency characteristic of a small transformer designed for port- 

 able recording apparatus. The composition of the special permalloy is approximately 

 4 per cent chromium, 78 per cent nickel, and the remainder iron. 



