QUOTE=chris@panteg;497388]The platter on an SP10 MK 3 weighs 10 kg , the motor and control circuitry is a different ball game to the 1200 of course .

I tried to point out some time ago that a high mass platter doesn't work on a 1200 , the compensation circuitry is set in stone .[/QUOTE]


Martin T has commented that he heard hunting when using a “high mass platter”.
Similarly Marco has observed negative effects that he ascribes to “platter mass”.

Well for my part all I have to say is that where DDs are concerned, “High mass platters” don’t in fact do this or that.
Because mass, per se, is irrelevant. Platter mass is static and sits on the bearing. For a heavy platter the bearing might then scream: “I’m not bloody Hercules” but that’s all.

You may have noticed, but when we feed energy into our decks, the platters start to rotate and this changes everything; now we have inertia. It is this that matters because its value is non-linear. Inertial values increase by the square of the increasing distance from the centre of rotation.

We can have a very heavy (massive) platter with its mass close to the centre and it will have a "low" inertial value.
Or a low mass platter with material concentrated at the edge and the inertia can be higher than the case above.
Or they can be balanced to the give same inertial value, when despite our scales telling us they are totally different, as far as the electronics are concerned, they are one and the same!

From our point of view, because we are listening (measuring?) to the mechanical interactions of both constructs, these two designs are again deemed to be different.

So, what we are left with is the fact that in DDs the system has to be optimised to match the power available to the inertial load. If the value goes outside of the loop constants, (high or low) then instability can set in, and hunting can be detected.

In developing professional versions of our platter for as fast a startup as possible for a particular client, we reached a point where servo lock was lost and hunting occurred - quite impressive.
The best solution for recovery we found was mechanical and not electronic.

The Technics engineers optimised the 1200's R-C values for the (maximum) power available from the motor and matched it to the platter(‘s inertia).
Tinkering with the values will not yield improvements as drawing any more power from the coil winding will only shorten the motor’s life – I’d say blowing up fits into that condition.
For this reason, one can argue the design is indeed set in stone.

if you want more power, then an SP10 is a pretty good place to start...

In developing our platters, therefore, we have always stuck within the designs constraints, both with our original Type I and now also with Type II. They are two totally different animals, so you can have you cake and eat it. I say this only because of previous comments regarding the deleterious effects of "mass".

As a relevant adjunct to this post, I'll tell you why belt drives are attractive: As the motors operate at much higher speeds and produce many times more error pulses per platter revolution.
This then permits for much smaller and hence less intrusive correction to take place.
With DDs the correction has to be applied with great finesse lest it be heard. And with 1200 I think we already know the answer!

That apart, in truth, there is no real difference between the two philosophies. The only difference is that we have designs fixed and this prevents us from exploring. We are then left saying that a belt drive DOES this and a DD DOES that. To my mind, both work.

Arthur