The necessity of a plinth

I previously posted a question on the need for a massive plinth for the SP-10 on a couple of sites but did not receive any definitive replies. While there is extensive information of plinths for Garrards, Lencos, Thorens, and other rim-drives, there is not nearly as much for Technics or other quality DD tables.

My premise was that because of the mass needed for rim-drives, most DD owners assumed that would benefit their tables as well. But now we have more information and extremes are reported. At the low mass end is Raul who's SP-10 utilizes only an extension for arm mounting and three suspension feet, while the high mass approach is represented by Albert Porter and Oswalds Mill, among others.

So obviously both approaches can work. Too bad no one has reported on a direct comparison.

Here's the way I try to think of the difference between an idler and a DD as regards plinth design: In the idler, the motor and idler itself are "external" sources of noise and vibration, whereas the rotation of the platter and the bearing can be just as inherently silent as that of a belt-drive table. A high mass plinth can drain away the motor/idler noise before it reaches the platter/bearing. That makes sense to me. On the other hand, in a DD table, the motor is a priori and inseparably associated with the bearing/platter. In a way, it's a closed system. So it is not obvious to me how a high mass plinth per se can efficiently interdict the transfer of noise to the platter from the motor. That's why I admire the thinking that has gone into Albert's plinth; there is an attempt to drain spurious noise into a heavy iron block via a threaded rod that contacts the base of the motor/bearing assembly. So instead of going upward into the platter, the spurious motor and bearing vibrational energy has a low impedance path downward into the iron block. In theory, it makes a lot of sense and similar strategies could be adapted to other DD tables. The high mass plinth may just be icing on the cake, to dampen chassis vibration and provide a solid base for the total structure. (I guess other plinth-makers have used a similar strategy to Albert's; I am not assigning a patent on the idea.)

Hi Lewm, the idea of dampening of spurious vibrations is a nice one, but you actually don't need high mass to accomplish this! What you need is a low impedance path for the redundant kinetic energy, which should have a constraint layer construction. Such a construction is not by definition high mass --> look at the theory behind the Symposium Ultra Platforms.

Chris

Interesting points Lew. Wish I remembered more of what I was supposed to have learned in Physics, i.e. energy travel.

From what I understand from a discussion with Albert, his brass rod contacts the bottom of the bearing housing under the motor. How much energy is then transferred downward to the iron block sink, thus not upward through the spindle, platter, and record I have no idea. But it is easy to accept that this will have some positive effect.

Part of my lack of understanding is the assignment of a diode effect on energy travel to some isolation devices by a number of hobbyists. For example I think the effectiveness of "tip toe" style devices relies more on the concentration of mass than on transmitting energy in one direction but not the other. Like the old story that a 120 pound woman in high heels exerts more pressure per square inch than an elephant.

So for now it seems we are left to trial and error in our own systems.

As much as we wish they were, cones do not act as diodes.