Direct Drive vs. Idler Drive vs. Belt drive

By the way, most of the vintage lathes are worm drive.

Furthermore, DC motors have zero torque at constant speed.

@dover  These two statements are false. I own a Scully lathe which is typical of what was used to make many older recordings. Its not worm drive. It employs a 1/8th horsepower syncronous motor which drives an anti-vibration coupling which then drives a transmission (which allows for shifting from 33 to 45 rpm). The transmission employs bevel gears in an oil bath but no worm drive. Its output is a shaft which drives another vibration isolation coupling; that in turn drives the platter directly, which rests on a set of carefully machined bearings (which require frequent attention).


DC motors have torque even at constant speed, so long as there is a load. This link might be helpful:

https://www.motioncontroltips.com/torque-equation/
However its a bit of a misnomer to equate a servo controlled multi-pole motor with a simple DC motor, and its also worthy of note that AC motors have a similar property of lockup being the point of highest torque output (as I'm sure you know quite well). I'm guessing that what might have been conflated/misconstrued in your comment above is the fact that when a motor has no load, its **current draw** is its lowest. But I think you'll find that how much torque it has is another matter altogether! Certainly a motor at constant speed will be making torque...
Its one thing to 'scope the waveform that is driving the servo operation, its quite another to see how the system responds to that waveform. I very much doubt that the platter speed variation can be represented by a triangle wave!

Atma-Sphere, I have noticed on several recent re releases and aggravating degree of rumble transferred to the record. I have always assumed this was from poor maintenance of an older lathe. Your last post would confirm this. Does anyone still make lathes? Can you get parts for older lathes and keep them running at spec? Is a high quality modern turntable quieter than a lathe?

I think everyone should take a deep breath and put on a record. Between tracks turn up the volume and listen to the noise. What happens to the noise when the music starts. How quiet does the music have to get before you can hear the noise again? I think we can all agree that that background noise is a form of random distortion, probably in the order of 3 to 100% depending on the volume of the music. But, when the music gets loud enough it disappears. Try as hard as you can you can not hear it (unless there is a huge scratch) If you can hear it, it is time to throw that record away and get a new copy. 

My point in all this is that this noise is several orders of magnitude worse than any aberrations cause by even the least expensive audiophile turntables available today. These discussions are fun and alot of these arguments have merit and may influence your purchasing decisions.
The best turntable is always the one you just bought. Why else would you have bought it? In the end speed stability and wow and flutter are trivial issues next to rumble and isolation. How often has someone complained on this web site about wow and flutter? How often does someone complain about their woofers "pumping?" How often do people c/o foot fall skipping?

AC motors are king because they auto correct on the run. They will add or subtract torque to maintain whatever frequency they are being fed. As long as that frequency is perfectly stable you are in business. 
Correction of DC motors always occurs after the fact and is a more complicated endeavor. Can you hear a difference? You can't even hear all that crap underlying the music on your records. 
  

Can you get parts for older lathes and keep them running at spec? Is a high quality modern turntable quieter than a lathe?

With my lathe its helpful knowing a machinist :)  No worries keeping it at spec. I think the Technics SP10 Mk3 is in the same ballpark of speed stability; most turntables are not. But the Technics motor was meant for LP mastering anyway.


With regards to surface noise I agree most of that is in the groove, not the turntable. The faults of turntables seem to be speed stability (heard as a shimmer in the soundstage; if you get to play reel to reel you'll understand what I mean right away) and resonance in the plinth, tonearm tube and platter. With regards to the platter, it has to be dead, and also control resonance in the LP itself- which is not a trivial matter! While the former seems to be understood in high end machines, the latter seems to get little notice, even though its just as important.


The surface noise is mostly an artifact of the pressing plant, although I do notice when the mastering engineer speeds up the cutter threads to cut the leadout grooves at the end of the LP. Otherwise though I simply don't get rumble despite the electronics having full bandwidth to 2 Hz.

Dear friends : I want to return to that " supposed "/hipothetic DD servo " problem and I want to do it because @mikelavigne  was very emphatic about in this thread.

This what Mike posted:

"""   when we consider belts, idlers, and direct drive........2 of those have the choice of no servo’s.

i’ve owned a number of top flight direct drive turntables; including the Rockport Sirius III, the SP-10 Mk2 and Mk3, and now the Wave Kinetics NVS for the last 9 years. taken singularly; none of those were obviously lacking in speed solidity and musical flow. but over these last 6-9 months, in direct side by side comparison to the Saskia model two idler, and the CS Port LFT1 Belt drive, neither of which have servo’s......this idea of the human ear hearing the musical cost of feedback is very real.

execution of an idler or belt to the degree to take full advantage of the lack of feedback is paramount, but when you do your ’ears’ will thank you.
but the separator turns out to be the lack of servo on the belt and idler. i’m drawn to those two emotionally more. now that i’ve lived with those i’m always conscious of that aspect of my NVS direct drive.  ""



and this my answer I posted to him:

https://forum.audiogon.com/discussions/direct-drive-vs-idler-drive-vs-belt-drive/post?postid=1952212...

He in very plite way as usual from himjust tell me he disagree totally with what I posted.


In what I posted in wbf things were that the gentleman ( that's an American Sound distributor ) posted there that the only DD he likes is the DP-100 that comes with servo control, this goes against his TT servo " repulsion. ).

Today no one can't prove that the servo control in DD TT develops a " problem " that we can hear, not even Mike can do it because he as the wbf gentleman can't listen that hiphotetic problem through their BD/ID units when all the LPs they and we are listening where cutted through DD motors that all were/are speed servo controled .
So any trouble about servos must be reflected in the LPs we listen to and this never happens or at least is what Mike shared when listening through his BD/ID units.

The information I shared in my last two posts were unknowledge by Mike when he posted about.

I don't know with all the evidence in the subject what he could think today and maybe could be interesting for all of us that he can be so gentle to share his opinion even if he could/would confirm what he already posted. Could you?, appreciated.

Regards and enjoy the MUSIC NOT DISTORTIONS,
R.



Btw, @alexberger in this thread posted:

""    Yes, the belt drive have advantage of very low noise.
BUT, when you listen an orchestra,on one hand you can listen a 10th violin separately and other small details, but on the other hand the rhythm and flow of music is broken by belt drive momentary speed fluctuations. I think the rhythm accuracy and flow of music is much more important than small details.  "

Way interesting too.

rauliruegas

This what Mike posted:
"when we consider belts, idlers, and direct drive........2 of those have the choice of no servo’s."

Not true. There are belt-drive 'tables with servo control, such as the Oracle Delphi III that I once owned. Some SOTAs too, I think.