New Turntable Advice

RB, Look up “effect of AC voltage on torque of an induction motor”.

@richardbrand

Re feedback, IMO it is fundamentally the same. We are comparing an output to an input and correcting if needed. Sure in a TT we are looking for a constant output, while as, you say, in an amp we are looking for a constant multiple of the varying input. But in both cases we are correcting, if necessary, a difference in output compared to input. I think that effectively we are saying the same thing.

I have very limited experience with idler drives. so temper what follows with that in mind.  

This is how I see it. If the idler is using a synchronous motor, there is what could loosely be called feedback in that the rotor is following the stator's rotating field synchronously. The rotor lags behind this field in phase but travels at the same average speed. If a higher torque demand event occurs, this phase angle increases a little in response but on average the rotor still rotates at the desired speed. Reduce torque and the angle decreases but, on average it still rotates at the desired speed. The rotating field is thus compliant. So microscopically this angle is modulating about the mean torque demand which also modulates the current demand. The result is tiny momentary speed changes. The platters inertia helps smooth this but does not totally eliminate it. 

The same principle applies  with a BD or DD. The drive is preconditioned to the present torque demand and the energy stored in the platter is fixed, at that moment in time. Take energy out and the platter must slow down because all drives are in some way compliant, feedback, if used, is not instant and the platter's moment of inertia is not infinite. The core difference being a BD has an extra compliant link in the drive chain. You could argue either way whether an idler wheel is compliant or not.

Years ago I observed this vicariously by scoping the motors current demand in a Goldmund Studio. It was not steady when simply spinning a platter and was less steady when playing a record.

You will not see this speed change by looking at a strobe. And the GMTs speed errors are measurable. 

Cheers.

@badbruno I am one of the demographic you have discouraged from posting, as Subjective Analysis is my thing, as for some reason I am recognised having a discerning ear and a system that pretty much reveals the nuances as each layer of the Onion is revealed.

I know lots of Techie' Info, but am not at liberty to discuss it publicly it is not my IP.

Other who you encourage to partake, are in the larger volume with a business interest and can only be limited with their IP, how far that extends beyond Manufacturers Published Data and investigation of those performance measurements I can't forecast.

I don't see much more evolving as there are lookers in, that like to steal ideas for their own remuneration purposes.

I can be easily wrong though, back to my Earl Grey and Cucumber Sandwich.      

@badbruno I have re-read your OP and a selection of replies.

I have seen your initial inquiry and subsequent posts have been adequately supplied content to assist with raised queries. 

Your latest Post is not in any way associated to the OP and other Posts. 

This latest request for Technical Discussion and your being interested in other areas that are connected to audio electronics,, is extending beyond creating  a system and being a listener of recorded music., is one that has quite a limited interest from the broader audio community.

To myself it looks like your last post is a New Thread Request where Off Topic contribution can be dismissed. 

This Thread is full of ’ Off Topic’ when considering  what is asked for in your last post.

The Breakfast Club ’might not meet’, if a New Thread is produced, only way one to find out.

Again, I may be wrong, Is it getting near the recess to have my afternoon Tea and Cake 🍵🍰

On the hunt for a Sony PS-X9