AMPLIFIERS 281 



arrangement. Often modifications of tentative circuit or apparatus 

 must be made to obtain satisfactory operation. 



The provision of a volume range of some 75 db on an energy basis 

 became largely a matter of the suppression of a.-c. hum. The low 

 inherent electronic noise effect of the Western Electric No. 262A vac- 

 uum tube and the relatively high level from the microphones kept 

 electronic tube noise well in the background. Careful and in some 

 cases rather elaborate shielding of audio transformers and leads and 

 the segregation of the 60-cycle power equipment coupled with the 

 use of vacuum tubes having indirectly heated cathodes and specially 

 designed to have small stray fields prevented a.-c. hum trouble in the 

 early stages. However, the Western Electric No. 242A vacuum tubes 

 used in the push-pull final stage have filamentary cathodes, and when 

 such tubes have raw a.-c. filament supply, a very appreciable 120-cycle 

 component appears in the space current. Although theoretically in a 

 perfectly balanced push-pull amplifier this component would be 

 eliminated, in practice an exact balance cannot be obtained. As a 

 final step in noise elimination, advantage was taken of the fact that 

 each channel employed two amplifiers in parallel. Under such condi- 

 tions and with proper phasing of the power supply to the two ampli- 

 fiers the net a.-c. noise output of the two amplifiers in parallel will be 

 less than that of either one alone. 



Having reduced feed-back and noise to tolerable values, it remains 

 to determine the operating conditions for maximuum output. The 

 vacuum tube is not strictly a linear device, but, when properly used, 

 the total harmonic content can be held to a low figure. For a high 

 quality system the total harmonics produced in the system should 

 not exceed one per cent of the fundamental. This requires that 

 impedance and potential relations in the vacuum tubes should be 

 adjusted to give approximately linear operation; and also that the 

 design of audio transformers, particularly those carrying considerable 

 levels, must be scrutinized carefully to insure that they operate over 

 an essentially linear portion of the magnetization curve of the core 

 material. 



An instrument really essential to the design of high quality amplifiers 

 is a high sensitivity harmonic analyzer that is capable of quickly and 

 accurately resolving a complex wave into its simple components. By 

 this means the effect of variations in circuit relations can be evaluated 

 and the optimum condition for maximum distortionless power output 

 determined. 



It may be desirable at this point to examine the make-up of the 

 audio amplification system used in the Philadelphia-Washington 

 experiments. It should be noted that the arrangement of equipment 



