Turntable speed accuracy

Lewm
Do you think that adding mass to the platter of an existing dd turntable( assuming the spindle bearing is up to the task) could also achieve the same effect as "loosening" the servo response.
Perhaps the periphery clamp of the LO7D helps to achieve this.

Here is my vid of me touching the side to test pulsed speed stability... it is a hung suspended table so you may see it move alittle actually its just the suspension moving laterally
http://www.youtube.com/watch?v=PB2TaN2FPRU&feature=youtu.bey... no movement on the strobe!

Lharasim
Which turntable are you using for the demo in your video ?
Watched it but do not recognize it.
Thanks for sharing.

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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