Amplifiers* 



By E. O. SCRIVEN 



Appreciable care is required in the design of a system which must amplify 

 with great fidelity practically the whole range of audible frecjuencies and 

 be capable of delivering a high level while at the same time providing a 

 wide volume range. Some of the problems involved are discussed, particu- 

 larly as applying to the equipment used in the reproduction in Washington, 

 D. C, of the Philadelphia Symphony Orchestra playing in Philadelphia. 



VACUUM tube amplifiers have been closely identified with the 

 extension of the channels of communication since, with com- 

 pletion of the initial transcontinental telephone line 20 years ago, they 

 first enabled New York to converse with San Francisco. There are 

 now thousands of audio frequency amplifiers in telephone circuits and 

 in sound picture theaters, public address systems, and other similar 

 services as well as in the millions of radio receiving sets. 



Along with the extension of the field of usefulness of audio amplifiers 

 there has been continuing progress toward more faithful reproduction, 

 better transmitters, better receivers, and better amplifiers. Those 

 first telephone repeaters, although quite adequate for their immediate 

 purpose, transmitted a frequency band only a few octaves wide. 

 Very few radio sets even now cover a range above 3,000 c.p.s. without 

 distortion, and the most up-to-date sound picture installation rarely 

 can be depended upon for accurate reproduction of frequencies above 

 7000 or 8000 c.p.s. The requirements as to frequency range and 

 freedom from distortion for any particular service are, in the last 

 analysis, determined by public demand. 



However, when one undertakes to reproduce an orchestra like the 

 Philadelphia Symphony and to reproduce it in such a manner as to 

 satisfy the critical ear of the director, or that of the devotee of sym- 

 phonic concerts, one has to provide something out of the ordinary in 

 audio amplifiers. 



In his paper, which forms a part of this symposium. Dr. Fletcher 

 has pointed out that only the elimination of those frequencies below 

 40 c.p.s. and those above 15,000 c.p.s. produces no detectable differ- 

 ence in the reproduction of symphonic music. This, then, is the 



* Fourth paper in the Symjiosium on Wire Transmission of Symphonic Music 

 and Its Reproduction in Auditory Perspective. Presented at Winter Convention of 

 A. I. E. E., New York City, Jan. 23-26, 1934. Published in Electrical Engineering, 

 January, 1934. 



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