PHONOGRAPHS 



235 



transducer actuated by a phonograph record and deHvering energy to an 

 electrical system, the electrical current having frequency components cor- 

 responding to those of the wave in the record. The systems for converting 

 the mechanical vibrations into the corresponding electrical variations are 



100 1000 



FREQUENCY IN CYCLES PER SECOND 



Fig. 10.7. Velocity response frequency characteristic of a phonograph record with constant 

 voltage applied to an equalized recording amplifier. 



as follows: magnetic, carbon contact, condenser, dynamic and crystal. It 

 is the purpose of this section to consider examples of some of the most com- 

 mon phonograph pickups in use to-day. 



1. Crystal Pickup. — A crystal pickup'^ is a phonograph pickup which 

 depends for its operation on the piezoelectric effect. The crystal in use 

 to-day is Rochelle salt. A cross-sectional view of a typical crystal pickup 

 used in commercial phonographs is shown in Fig. 10.8. The needle, driven 

 by the record, is coupled to the crystal. The elements of the system and 

 the equivalent electrical circuit are shown in Fig. 10.8. The displacement 

 of the crystal can be determined from the equivalent electrical circuit of 

 the mechanical system and the velocity of the generator obtained from 

 Fig. 10.7. The voltage output of the crystal is proportional to the dis- 

 placement. Therefore, the open circuit voltage at the low frequencies is 

 accentuated as shown by the response characteristic A, Fig. 10.8. The 

 internal electrical impedance of the crystal increases with the decrease in 

 frequency since the crystal is essentially an electrical capacitance. The 

 open circuit voltage characteristic renders the low frequency compensation 

 problem exceedingly simple. The response frequency characteristic of a 

 pickup with a resistance shunting the crystal is indicated by 5, Fig. 10.8. 

 The high frequency response of the crystal shows a cutoff around 7000 



7 Williams, A. L., Jour. Soc. Mot. Pic. Eng., Vol. 32, No. 5, p. 552, 1939. 



