AMPLIFIERS 279 



frequency spectrum that the ampHfier must be designed to handle. 

 Also, it is important that there shall be uniform amplification of all 

 parts of the frequency range and that no extraneous frequencies shall 

 be introduced. 



Of importance commensurate with the distortionless amplification 

 of the complete frequency range of the orchestra is the provision of an 

 equivalent volume of sound. The amplifier must be capable of supply- 

 ing to the loud speakers without distortion an amount of energy that 

 will produce a sound volume at least equivalent to that produced b>' 

 the orchestra (the Philadelphia-Washington installation was designed 

 to produce about 10 times this amount). And equally important, the 

 amplifier must be so free from internal disturbances and from self- 

 induced electrical fluctuations that the softest music, the weakest input 

 to the microphone, can be reproduced without appreciable background 

 noise. According to Fletcher the ratio of the heaviest playing of a 

 large orchestra such as the Philadelphia Symphony Orchestra to the 

 softest music such as that of a violin is about 10,000,000 to 1, or 70 db. 

 Thus it is required that any noise be at least 75 db below the loudest 

 tones; that is, there must be at least a 75-db volume range. 



The sources of noise may be divided into 2 groups. In the first 

 group are included the 60-cycle alternating current power supply, 

 vibration or jar of mechanically unstable vacuum tubes, contact and 

 thermoelectric potentials, and similar disturbances, which may be 

 reduced to practically any degree depending upon the lengths to which 

 one is willing to go to reduce them. In the second group are those 

 electronic irregularities intimately associated with the operation of 

 the vacuum tube and which depend somewhat upon the design, manu- 

 facture, and method of operation of the vacuum tube; and which, 

 when sufficiently amplified and fed into a loud speaker, may be heard 

 as noise. In general, the maximum volume range of an amplifier is 

 reached when all other disturbances are reduced to the level of this 

 tube noise. 



It is evident, then, that under ordinary circumstances the limiting 

 volume range of an amplifier is a function of the amount of ampli- 

 fication following the first tube. In other words, the magnitude of the 

 signal voltage with respect to the noise voltage in the plate circuit of 

 the first tube in a multistage amplifier determines the limiting volume 

 range obtainable with that amplifier. 



It will appear that in a sound reproduction system a highly efficient 

 microphone simplifies the amplifier volume range requirements, and 

 that loud speakers of high efficiency reduce the volume required from 

 the amplifier. 



