Direct drive/rim drive/idler drive vs. belt drive?

Back on track...

The discussion so far has been about torque, but in my opinion the most critical component of a turntable may be the platter. With all three types, belt driven, idler driven and direct drive, the platter can be the make-or-break piece because it is the final interface to the record, it is key to how inertia works and it is the interface to the rest of the mechanism. That said, I really believe that a turntable should be seen as an entity because, like the audio chain in your system, it is only as good as its weakest link. There is no single part, no matter how seemingly insignificant, that should be overlooked. Designing a decent one is easier said than done for certain.

Win

And to take that theme and refer back to the OPs question, I might ask: how much of the total inertia of the system should the platter represent? What influence does splitting some or most of the inertia away from the platter have on the perceived "drive" of the system?

It seems to me that many of the designs which are known for having lots of what Albert calls "drive" have moderate mass / moderate inertia platters tightly coupled to a high inertia drive system. If I'm not mistaken Win had a similar concept in mind when he desgned his TT.

Mark Kelly

Platter is important but in my experience the motor is the most important and then combination of platter and bearing. After all the motor is the active component here, hence it's called TURN-table for a reason. It has the most demanding job and requires the most sophisticated engineering that involves electrical and mechanical skills. If you get a good motor, you are more than half way there. Of course, if you use a soft rubber belt to conceal the flaws of the motor then the platter is more important. I do agree that a record player should be seen as a whole system not just a collection of parts.

Interesting this inertia talk.

That raises the (old?) question who should control the STEADYNESS of speed - inertia of the platter, or the motor AND IT'S CONTROLLER.

If the motor sux, or the controller sux, you have to look back to the platter's inertia. If you manage to make it THAT heavy (never mind the resulting bearing problems) there must come a point where any conceivable variation in friction-changes of the needle to vinyl interface become SO SMALL as to approach zero.

I have a notion we have to look for some mighty heavy platter to get there. One expert put two Micro-Seiki on top of each other --- and then waits 5 minutes for the darn thing to stabilize the speed.

You see, we are now starting to move big-time out of the practical useful user-application --- which in turn also sux.

So it's back to the more practical idea as was mentioned above platter-weight not much more then ~ 4.5kg, still using a belt for a tiny bit more forgivingness, than a DD which translates its little faults too immediate into the speed stability.
Lastly a motor / controller package with the best available feedback speed / loop to mankind :-)

A thing to note: that bearing friction has to be present at a controlled level to "damp" the controller feed-back loop, preventing feed-back resonance, so to speak.

Now have a look who is doing something like this, and I guess you'd have a 'best of breed' and still user friendly tt.
Greetings,

Axelwahl,

On your first point: That one's easy. I take it as a truth universally acknowledged that if the motor were not running, frictional forces (of whatever source) would cause the system to slow down and in doing so it would lose kinetic energy.

It therefore must be the case that energy is being put into the system for it to remain at constant speed. The only possible source of such energy is the motor, therefore the motor must have the primary role in maintaining constant speed. All that platter inertia does is to reduce the slope of the decelerations (and equally the accelerations).

Your second point falls foul of a fundamental property of feedback loops, that the speed of the feedback loop must take account of the speed of the "forward" portion of the loop. If you interpose a low pass filter such as the belt and platter in the loop then you must slow the loop down to prevent oscillation. If you leave the belt and platter outside the loop then the loop cannot possibly compensate for losses in the belt / platter system such as torque dependent belt creep.

No free lunch.

Mark Kelly