2020 update : JC Verdier La Platine

@ledoux1238 You have done the correct thing in contacting Nick Gorman, he is with knowledge levels that are beyond many EE's, if there is a method and a simplified one available Nick will see it.  

As for being out of your depth, nearly all that I own is bespoke built by EE's and Engineers who excel in certain disciplines within HiFi, this is where the two edge sword swings.

When the sword swings one way, I am most confused by what is under discussion, as a proportion of the methods employed are unique to the individual, with an area of the work to be being undertaken, being unique and their own stamp on a design, drawn from their years of experience.

When the sword swings the other way, the experience of the working design in use and as time of ownership passes, where comparisons have been undertaken and others have received demonstrations of the devices offering their assessments.

This furthering of the experiences does undoubtedly identify that something special has been done for you, and this knowledge is very satisfying, especially when some of your owned devices are being recognised as being more attractive in use than much more expensive 'off the shelf products' they have been compared against.

It is also nice to find a place of rest in the knowing the search for a particular device is definitely seen to be over.     

As all with many years of enjoying music replays on a HiFi System know, we are a 'curious old bunch' and always chasing a rainbow somewhere 😀.

@pindac Thank you for the words of encouragement from the last post. 

Well, after some back and forth, Nick Gorman has agreed to adapt the LDA to a new motor, if I can source one. @dover is there a AC motor that you could readily recommend? Or would I be looking at vintage / idler type motors to adapt to? 

Is there a webpage that I can learn more about this LDA controller?

Thanks!

@thekong  longdog audio

AC vs DC. While I fundamentally agree with Dover, a few points he raises need expansion.

Not all AC motors lock on to the field frequency. What we should be talking about is synchronous motors.  In this design the rotor follows the rotating field in the stator at the same speed, but not the same phase angle. IOW the rotor is running slightly behind the rotating field in the stator, but at the same speed. There is an angular displacement between the two. No displacement, no torque. This angle is determined by the load the motor is seeing and the motors design. More load equals greater angle, where the motor momentarily slows until the torque it produces equals the higher torque demand. Maintain that higher load and the angle will stay at this higher figure. Reduce the load and the angle will similarly decrease and momentarily the rotor's speed will increase. The rotor maintains an average speed dictated by the rotating field, but when driving a dynamic load, its real-time speed is changing at microscopic rates. Some synchronous motors are very stiff. This means that the increase in displacement angle is very small for any given increase in load, others are comparatively loose where the angle increase with load is correspondingly larger. 

Further adding to the mix, the change in angle vs load is not linear. As the angle increases the motor draws more current to self correct, but it first needs to increase the angle, thus slow momentarily. It has go wrong to correct. It is feedback. The correction is sinusoidal so we can assume that this is more benign. But, as above, the point to point speed can be changing when playing a record. There is no free lunch.

Most synchronous motors used in TTs are 2 phase. We need a way to generate a phase shift between the two phases in order to make the motor rotate. This can be done with something as simple as a capacitor all the way up to sophisticated regenerator controllers. If you are targeting smooth, ultra low torque ripple you want to use a 3 phase synchronous motor. Well designed 3 phase synchronous motors are virtually linear torque devises. Of course the controller needs to be capable of driving such a motor by generating three phases, 120 degrees apart 

If one was designing a TT today, where we were aiming for great dynamic speed stability.  A logical strategy would be to use a 3 phase synchronous motor that is very stiff and of sufficient intrinsic torque to dominate the platter. Then wrap a very high speed, but gentle, feedback loop around this to make the drive even more speed stable. 
 

Cheers