@lewm
a smaller motor pulley driving a larger pulley (platter vs. sub platter) has many engineering advantages as can be computed in simple belt pulley - pulley drive system configurations. i performed some engineering calculations simulating a 1" motot pulley driving a 12" platter vs. driving a 4" sub-platter. Below are the findings:
1) Bearing force. The platter driven system has a bearing load force of 2.8 lbs. The sub-platter driven system has a bearing load force of 6.7 lbs. or 240% more load. More than twice the force on the bearing means increased wear, noise but more importantly increased drag on the speed of the platter.
2) Cyclic Variation (in belt tension). The platter driven turntable has a cyclic variation of 0.79 lbs. The sub-platter driven turntable has a cyclic variation of ~2.4 lbs, over (3) times as much as the platter drive.
Because the motor and belt in the sub-platter system have to work harder to spin the platter, the tension on the "pull" side is much less than the tension on the "slack" side of the belt. This translates into constant pull / release tension cycling and stretching which will wear out the belt significantly faster than the relatively light belt stretching on the platter driven turntable.
3) Drive ratio. How many revolutions the motor pulley spins in relation to the speed of revolutions of the platter. The platter driven drive ratio is 0.083 while the sub-platter driven system drive ratio is .25, or 3X as much. What this means is that speed variations in the motor of the sub-platter will be magnified 3X as compared to speed variations in the sub-platter motor. In other words, a speed variation of 1% in platter driven system caused by the motor will equal a 3% speed variation in the sub-platter system, all else being equal. A platter driven system has 3X the speed stability of a sub-platter driven system.
4) Drag resistance. This is a simple lever arm calculation. The larger pulley (12" diameter pulley) compared to the sub-platter (4" diameter pulley) has a 3X greater resistance to drag forces. Drag forces in the form of tone arm drag (minimal but measurable) and bearing drag. Bearings are not frictionless. But because the bearing forces are 2.4X higher in a sub-platter system, it has more drag to overcome and 3X less capability to do so. In effect, the sub-platter system is 5.4X more sensitive to drag caused speed variation than the platter driven system.
Summing up. The combination of much higher bearing load forces, belt tension variation, 3X more speed variation sensitivity and 5.4X more sensitivity to drag forces place the sub-platter system with a significant amount of physical and mechanical disadvantages when compared to a platter driven turntable. It's refreshing to see that the high end turntable manufacturers actually have some sound engineering and physics principals to back up their designs.
