The most important activity in the field of sound reproduction centred around the advent of long-playing records, and the reaction of designers to the various methods of reproduction available. Record changers with speeds of 33⅓, 45 and 78 RPM are being fitted to most radio-gramophones, and, to cope with the different stylus sizes and needle pressures required by standard and microgroove records, most manufacturers are fitting interchangeable plug-in pickup heads. Turnover heads with a 0.001 inch radius stylus on one side and 0.0025 inch on the other are available with magnetic or crystal movements, and have been adopted by a few firms; Ferguson, on the other hand, have fitted a single fixed head with the compromise stylus tip radius of 0.0017 inch.
Long-playing records have a pronounced rising characteristic towards high frequencies compared with standard shellac records, and a different reproducing response curve is required in each case. Most makers seem to have left this to the existing tone controls in their sets and to the ear of the customer, but there are one or two notable exceptions. Decca fit their pickup heads with three-pin plugs, the connections of which are arranged to select automatically the appropriate correction circuit in the amplifier; a similar arrangement is adopted in K B radio-gramophones. In their model A172R, Murphy, who use a moving coil for 78 RPM and a lightweight moving-iron pickup with higher sensitivity for 33⅓ RPM records, have taken advantage of the spindle extension on the speed-change control of the Garrard RC72 record changer to switch-in appropriate alternative correction networks via a Bowden wire control.
See also Stereophony from Discs