Our MAP Drive couples a traditional synchronous AC motor with modern digital speed control techniques including real-time measurements of the platter speed, with an optical sensor feedback to control the platter speed and make micro adjustments as needed. Most turntables in its class use the 60Hz of the AC mains to turn the motor at a fixed speed and then rely on pulley size ratios to achieve the desired platter speed (33 ⅓ or 45 RPM). In this topology, there is no way to account for belt slip, pulley wear or the effects of temperature change on the system. In addition, the mass of the platter design is constrained by the “stall torque” of the motor. A platter of desirable (large) mass would require a motor of undesirable size and noise.
By synthesizing the sinusoidal drive waveforms required to turn the motor, the Fern & Roby MAP Drive system is not limited to a fixed motor speed or fixed drive power. This allows for a design with extravagant platter mass coupled to a modestly sized motor. From a standstill, the platter is accelerated gradually to the desired speed in order to stay below the motor’s stall torque. The platter’s speed is determined 48 times per revolution with a precision-cut optical interrupter wheel mounted on its underside.
A green LED indicator beside the mode selector switch is used to indicate “in-bounds” platter speed. Any time the platter speed is more than 0.23% above or below the target speed, the LED indicator switches from green to orange while the motor control compensates to correct the speed into the desired rpm.