236 



MISCELLANEOUS TRANSDUCERS 



cycles. This cutoff can be made any value up to 15,000 cycles by a suit- 

 able choice of constants. 



^TJOT^ 





CRYSTAL 

 NEEDLE ''IcCmcI'mc 



CROSS-SECTIONAL VIEW "" '"" FREQUENCY 



Fig. 10.8. Cross-sectional view and equivalent circuit of the mechanical system of a crystal 

 pickup. In the equivalent circuit: Zmr the mechanical impedance of the record. Cmo 

 the compliance of the needle. m\ the mass of the needle holder. Cm\ the compliance of the 

 shaft. Cuirui and CmzTmi the compliances and mechanical resistances of the crystal sup- 

 ports, mc-, Cmc and tmc the mass, compliance and mechanical resistance of the crystal. 

 m^ the mass of the pickup and tone arm. Ju the force generated by a constant velocity 

 generator. The graph shows the voltage response characteristics with the record charac- 

 teristic of Fig. 10.7. A. Open circuit response characteristic. B. Response characteristic 

 with 500,000 ohms shunting the crystal. 



2. Magnetic Pickup. — A magnetic pickup ^'^ is a phonograph pickup 

 whose electrical output is generated in a coil or conductor in a magnetic 

 field or circuit. A typical magnetic pickup is shown in Fig. 10.9. The 



Cm2 



EQUIVALENT CIRCUIT 



1000 

 FREQUENCY 



Fig. 10.9. Magnetic pickup and the equivalent circuit of the mechanical system. Intheequiv- 

 alent circuit: Zmr the mechanical impedance of the record. Cmo the compHance of the 

 needle, mi the mass of the needle holder and armature. Cmi the compliance of the needle 

 holder pivot. Cm2 and rui the compHance and mechanical resistance of the damping ma- 

 terial. OT2 the mass of the pickup and tone arm. /m the force generated by a constant ve- 

 locity generator. The graph shows the voltage response frequency characteristics. A. Re- 

 sponse with the record characteristic of Fig. 10.7. B. Equalized response characteristic. 



8 Kellogg, E. W., Jour. A. I. E. E., Vol. 46, No. 10, p. 1041, 1927. 

 ^Hasbrouck, H. J., Proc. I. R. E., Vol. 27, No. 3, p. 184, 1939. 



