Turntable speed accuracy

While I agree with Albert on the importance of the mass and the material composition of the plinth as a determinant of dd performance, you should know that there is a whole "school" of thought on the other side of the question, that holds that "no plinth" sounds best. Those guys place the naked DD chassis on nothing but a set of isolating feet, like for example the old AT feet. Then they mount the tonearm on an outboard pod. To me, this is a violation of many different "rules". But I think it may indicate that a bad plinth can be worse than no plinth at all.

Dear Lespier, Good point. adding mass to the platter definitely has an effect on the servo, but I have to think whether that would reduce the damping effect or enhance it, i.e. under- vs overdamped. If I start with the idea of no platter, the servo mechanism is rapidly hunting for correct speed, so maybe that is the condition of underdamped. Ergo, adding mass to the platter, e.g., via a heavy platter mat as is done by many, would tend to overdamp the servo response, I think.

The L07D has a secondary switch on the PS chassis that one engages only when using the accessory peripheral ring weight. This switch alters the servo response so as to account for the added weight. I checked my L07D using the KAB strobe both with and without the ring. The speed was accurate both ways.

Lewm: "If I start with the idea of no platter, the servo mechanism is rapidly hunting for correct speed, so maybe that is the condition of underdamped. Ergo, adding mass to the platter, e.g., via a heavy platter mat as is done by many, would tend to overdamp the servo response, I think."

I don't think the damping factor of a servo circuit is determined purely by mass, that is, higher mass = overdamped or lower mass = underdamped. It is probably preset by the electronics how responsive or how quick the response time to the load or mass deviation, I supposed there's a "reference" load much like a zener diode in a regulated power supply, which is a form of feedback. Some turntables have very "loose" or gentle response to mass deviation, that is, even without the platter it can still rotate smoothly. I think it is predetermined by the electronics in the over all design. I think sometimes by adding mass to the platter can sound "smoother" is to purposely overwhelm the servo to allow more platter inertia to do the job so it's actually underdamped (less hunting, perhaps?). I guess it's a balance between letting the mass or letting the motor/servo/electronics do the work. It really comes down to engineering decisions. Back to the same idiom that there are many ways to skin a cat, I guess?

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Hiho, The point is that if you don't know enough about the circuit to fiddle with the servo at the level of its electronics (which includes me), you can at least affect damping by changing the rotating mass of the platter, which is a parameter built into the servo system. Easiest way to do that is to alter the mass of the platter mat. Adding mass does not "overwhelm" the servo; it adds damping to the system as a whole, if you take the "no platter" condition as "undamped". I think in engineering terms this is the correct way to think of it, but I am more than willing to be corrected by an engineer.

But I agree with the sense of what you wrote, mostly. The L07D and perhaps the aforementioned TT101 may be examples of servos that don't attempt to control speed as tightly as does, for example, the Technics system. The engineers that worked for Denon, Kenwood, Victor, Technics, Pioneer, etc, each had a go at deciding upon the optimum way to achieve the same goal, and they came up with different answers.

Doesn't this discussion on DD servos highlight the question, does a very high mass platter, with very high inertia, driven by a high torque motor with a belt, thread or fluid drive with built in slippage, such that the platter mass will drive through any load fluctuations sound better than a DD with its constant speed correction.
Correct me if I'm wrong, but isn't the need for servos because DC motors inherently do not respond in of themselves to changes in loads.

For those that are keen to experiment - here are a couple of decks worth trying :
Rossner and Sohn - latest special custom-made turntable is called “The Mott” (The Mother of Turntable). The rotating platter itself weighs 232 kg. Choice of air or oil bearing.
Sati - 50kg platter, interesting comments on motors on their website.

Dear Dover, You wrote, "Correct me if I'm wrong, but isn't the need for servos because DC motors inherently do not respond in of themselves to changes in loads." OK. You are wrong. First you are wrong because your statement carries the assumption that all DD motors are DC motors. I am not sure you meant to say that or perhaps you meant to type "DD" and out came "DC". In fact, a major fraction of DD motors are 3-phase AC synchronous types. DC motors are also used in some pretty fancy and expensive belt-drive turntables. Second, you have failed to define what you mean by "load"; I assume you refer to variations in forces due to stylus drag, etc, that occur during the course of playing an LP. Third, you are wrong because your statement carries also an assumption that all DD turntables use servo feedback. Some of the new ones don't, e.g., The Beat. In any case, I think that the adoption of servo mechanisms has to do with the fact that a DD turntable motor has to turn constantly at the relatively slow speed of 33 rpm, and using servo feedback is one way to reach that goal. As we have been saying, there are many many different ways to apply servo feedback. Belt-drive motors run much faster and for some the belt itself is used to smooth over small changes in speed. Also, it's cheaper not to use any feedback. But major point is that in principle DD motors qua motors (thank you, Nandric) are no more and no less inherently speed constant than are belt-drive motors. I do not think it is possible to argue from generalities that one way is better than another.