FEEDBACK APPLIED TO A LOUD SPEAKER 



135 



double voice coil is used. The response characteristic of this loud speaker 

 with a double voice coil is shown in Fig. 7.13. 



7.14. Feedback Applied to a Loud Speaker. — Feedback in a transmis- 

 sion system or a section thereof is the returning of a fraction of the output 

 to the input. Negative feedback is feedback which results in decreasing the 

 amplification. Among the sources of nonlinear distortion and nonuniform 

 response in a reproducing system may be the power amplifier and loud 

 speaker. It is possible to reduce distortion and improve the response as a 

 function of the frequency of an amplifier by making the amplification 

 deliberately higher than necessary and then feeding the output back in 

 such a way as to throw away excess gain. In the same way this system 

 may be made to include the loud speaker. It is not an easy proposition 



PICKUP 



COIL ./VOICE 

 CONE f COIL 



MICROPHONE/ 



^ 



Fk5. 7.14. Loud speaker and amplifier feedback systems. A. The output of the pickup coil 

 is fed into the input side of the amplifier. B. The output of the microphone is fed into 

 the input side of the amphfier. The graph shows the pressure response characteristics of 

 system A: 1. without feedback, 2. with feedback, 3. with feedback and compensation. 



to employ feedback in this way because of the very special control required 

 of phase shifts in the amplifier and loud speaker system. Unless certain 

 phase relations ^-^ are maintained, oscillation will occur. Figure 7.14 shows 

 feedback applied to an amplifier and loud speaker. In Fig. 1 .WA a pick- 

 up coil is attached to the cone. The output from the pickup coil is fed 

 into the input of the amplifier out of phase with the signal input. The 

 response characteristic of the amplifier loud speaker without feedback is 

 shown in Fig. 7.14. The same loud speaker and amplifier with 15 db nega- 

 tive feedback from the pickup coil are also shown in Fig. 7.14. It will be 



« Nyquist, H., Bell Syst. Tech. Jour., Vol. 11, No. 1, p. 126, 1932. 

 9 Black, H. C, Bell Syst, Tech. Jour., Vol. 13, No. 1, p. 1, 1934. 



