In 1961, Acoustic Research developed a turntable whose simplicity, outstanding performance, and low price earned it a unique place in the hi-fi marketplace-and something close to classic status among knowledgeable phonophiles. Aware of the degradation of sound quality caused by acoustic feedback, AR's founder, Edgar Villchur, designed a turntable (actually a complete record player, since it was available only with an AR tone arm) that was virtually immune to external mechanical excitation.
In addition, the AR turntable essentially eliminated audible rumble by using a very small, low-torque twenty-four-pole a.c. motor (similar to those in electric clocks), which turned at only 300 rpm instead of the 1,800 rpm of the four-pole induction or synchronous motors commonly used at the time. The motor's fundamental vibration frequency was thus lowered from the very audible 30 Hz of the higher-speed motors to an inaudible 5 Hz. Since the torque of the AR turntable's motor was relatively low, its platter mass was kept correspondingly small, and it was driven via a light rubber belt. Speeds were changed (between 33-1/3 and 45 rpm) by removing the platter and manually shifting the belt to a different-diameter pulley.