Vintage DD turntables. Are we living dangerously?

Lewm -
Life is a constant learning exercise, that is why I read and participate in these forums. I regularly listen to and enjoy music on all types of drive ( other than digital ). You are quite wrong on the Final - I would be disappointed if there were not a better turntable out there - because I enjoy the constant upgrade process and the joy of rediscovering old records in a new light when one accomplishes gains in the playback system. I do not claim it is the best, I simply haven’t heard anything better yet. There are many TT’s I still have not heard. The comments I have made in this thread are only related to TT’s that I have heard in depth. Ditto with Rauls MM thread - I still prefer MC’s but have gleaned much knowledge and tried a few new MM’s in the past year or two. I continue to listen to other turntables in my system - it’s enjoyable and usually learn something.
A few corrections on the Final - it has a very powerful AC motor, no active speed correction. Speed is controlled by precision oscillators regenerating sine and cosine waves and a power amplifier to drive the motor. It has separate controllers for 33 & 45 and I can set the speed between 0 & 85 rpm on each as well as vary the level of torque applied. Speed relies on inertia (26kg platter), powerful motor and AC phase locking. As regards belt creep the Final uses silk thread - the stretch is negligible compared with rubber belts, otherwise I agree with your comments on belt drives (rubber) and so far the only decks that have approached the Final have been idlers.
Speed and power wise, the most astonishing TT I have heard in the past couple of years was a Denon RP52 Idler. The owner, a DJ, demonstrated side by side with a Technics DD in the same system that you could could start and stop the Denon with a low compliance MC sitting in the groove and hear virtually no slurring of the music on start up. Doing the same test on the Technics DD resulted in much slower start up such that you could hear the music coming up to speed. I wish I had made a video of it. I suspect the EMT 927 would be in that league - huge torque coupled with a 50lb flywheel.
The Technics DD motor used for driving lathes, the SP20, was designed specifically to operate in conjunction with the Neumann 60lb flywheel and has significantly more torque and poles than the SP10mk3 - this would be an interesting DD platform to listen to.    

Dover
I have listed the time line of my upgrade development on another thread. There I outlined why I persisted with the SP10. To repeat. It's "drive " is intoxicating. Halcro has used a different descriptor for his TT-101 but he and I agree on the point.

BTW a sine / cosine generator is necessary to create a rotating field in the two phases of your synchronous motor.
In its simplest form it can be approximated by a capacitor.
There have, for decades been more accurate devices available to generate the necessary phase shift, so what you have in the Final,  while being elegant, is not a big deal.

Many DDs have three phase motors. This requires a precision triple sine wave generator. Each wave shifted in phase by 120 degrees. These are then fed into a 3 channel power amplifier.
Since they are direct drive these three phase signals must be very accurate, if they were not the platters rotation would be very erratic.

Most quality DD TT manufacturers did this 4 decades ago.
Again it is not a big deal, but they at least provided the purchaser with the power amp.

Also a synchronous motor like yours has local feedback. The rotor lags slightly the rotating field.
With a varying load this phase angle changes and then corrects, hence incremental speed changes.

There is no free lunch when we try to drive something at a constant speed.

Richard -
Last time I looked under the bonnet of the oscillator preamplifier on the Final, there was a bit more than a capacitor. Time to move on and focus on direct drive.

A direct drive motor turns once per revolution of the record. By comparison the Final motor rotates 54 times for each revolution of the record. This means that the error correction and servos in a direct drive application are constantly error correcting over a larger time period per revolution between poles. To put it into perspective a typical direct drive with say 8 poles has only 8 pulses per revolution of the platter compared with 218 pulses per revolution ( motor speed 1800rpm x 4 poles /33.33rpm ) with the Final. This along with the high inertia generated by the Final 26kg platter results in an audibly superior sound.

This same principal applies to idler drive type turntables where the platter is typically driven from a high speed motor with small pulley driving a large platter. In engineering we call this gearing.

Cutting lathes also use this gearing principle, high torque motors and high mass flywheel to achieve stability.     

Dover.
yeah yeah we have all heard ( read ) the arguments extolling the virtues of BD,TD and ID over DD.
My point was that a synchronous motor in a thread drive has local error correction and under dynamic loads it is in play (correcting) all of the time, just like global error correction in a DD. Error correction, something that you seem to dislike with a passion, is clear and present in your own TT.
Different in nature yes, but it is feedback 

Further the fact is that the Final slows when you lower the tone arm. This you have published. You say that this can be adjusted out, but this adjustment must only be a best guess average. So there will be dynamic speed changes since modulation level and the radius read are not constant on an LP. IOW the retardation torque is far from constant.

This is the dynamic speed problem that I hear on the Final.

While one can debate the efficacy of the time line in showing what goes on within a single revolution, Halcro has quite clearly shown us all that a good DD TT maintains speed under load. The Final does not and high platter inertia isn't enough to resist this slowing 

Yes some cutting lathes use a gear principle and then many are DD.

Dover, that may have been a typo on your part.  The SP-20 was another in their series of studio and consumer motor units.  The mastering lathe model was the SP-02.

http://www.lathetrolls.com/viewtopic.php?f=1&t=1223

Huge difference. ;^)