Direct drive vs belt vs rim vs idler arm

Richardkrebs
"The perfect TT has absolute speed stability under all load conditions"
I would draw your attention to the following :

Newtons Second Law of Motion - Force = mass x acceleration

or Acceleration = Force/mass

In other words deceleration due to an external force is inversely proportional to mass. The moment of inertia for the following TT's are -

Final Audio high inertia TT - 3920kg/cm2
Kenwood L07D - 1025kg/cm2
Technics SP10mk3 - 1100kg/cm2

This means that the high inertia Final Audio TT requires a 300% increase in stylus drag variation compared to the L07D and SP10mk3 to achieve the same loss of speed.
In other words for a given additional force due to stylus drag variation the Technics SP10mk3 and Kenwood L07D will have a deceleration 4 times higher than the Final Audio high inertia TT.

Now if we look at the design intent of both the Technnics and Kenwood we find some interesting facts :

Kenwood L07D Manual

A moment of inertia of 1025kg-cm2 is obtained ( when the turntable sheet is attached ), resulting in high kinetic energy of rotation. Therefore the platter rotates stably even if the transient load from the stylus varies.

The L07D uses double servo coupling which automatically switches the control method when the speed difference is beyond +-3% from the rated speed: when the speed difference is beyond +-3% the speed is controlled to obtain a large torque and, when it is less than +-3%, the phase is controlled to obtain a wide lock range and large phase gain

Clearly the Kenwood designers believed that the magical figure for moment of inertia of 1000kg/cm2 was sufficient to overcome stylus drag fluctuations.

Now let's look at the Technics SP10mk3 Sales Brochure

Technics SP10mk3 Brochure

1.1 ton-cm2 Moment of Inertia
Huge moment of inertia results from the 10kg platter made of diecast aluminium with a 15mm thick copper alloy surface layer. Therefore there is no change in speed with any fixed load up to 10kg-cm – equivalent to 1000 tonearms tracking at 2g each. Of course, the real benefit is that the platter keeps rotating at exactly rated speed even if you accidentally touch it during play.

Wow and Flutter 0.015% WRMS, Speed Accuracy +-0.001%
Besides the huge moment of inertia of the heavy platter itself, our full cycle detection FG also contributes to rotational accuracy by supplying a more reliable signal for phase comparison with the reference

So the Technics designers have a foot in both camps. On the one hand they believe that a moment of inertia of 1100kg/cm2 will support 1000 tonearms tracking at 2gm but then they go on to say that the fast error correction helps to maintain speed.

Furthermore the Technics SP10mk3 Brochure also explains that purpose of the high torque motor is to ensure for that the platter can be brought up to speed within 0.25seconds and reverse torque can stop the platter in 0.3seconds to meet the requirements of Radio Stations.

With regard to your comments on the Techdas, I would draw your attention to the design. Although the Techdas has a 30kg platter, the motor drive is low torque. They deliberately drop the torque as low as possible just to keep the platter spinning accurately and no more. They have done this to minimise noise and vibration. This is explained on their website.

To reiterate my earlier posts on cogging -
The torque ripple or cogging torque will be lower on a high inertia thread drive because a Direct Drive motor is running much slower.
For example
DD - 33.33rpm x 20 poles = 20 poles per revolution of record.
AC/thread drive - 1800rpm x 4 poles = 216 poles per revolution of record.

In summary then, the benefits of a high inertia thread drive over direct drive are :

Final Audio High inertia TT has a resistance to stylus drag variation 4 times higher than the SP10mk3 & Kenwood L07D
Final Audio High inertia TT has less cogging effect ( and lower amplitude ) due to the higher motor speed.

Speed correction is required on Direct Drive TT's due to insufficient moment of inertia which is required to resist to transient load fluctuations.

This is the way of the turntable world: manufacturers have made their choices between hi and lo mass and hi and lo torque, and between direct-drive and belt-drive (and idler-drive). Then comes the technical justification for what each has done. Techdas, as you know, is by far not the first manufacturer to choose hi-mass platter combined with lo-torque motor. I think the first guys to go all the way with that were Lloyd Walker and David Fletcher (Nottingham Analog). The consortium in the American West that gave rise to Teres, Galibier, and Redpoint can also take some credit. It is merely up to us to listen and choose. Chronologically, I don't know where Final Audio fits; perhaps they were leaders too in implementing that idea. (Is "Techdas" coming from Dertonearm, by the way? I had not heard of this new brand, but I do know DT announced plans to bring a turntable to market, and he does preach very high mass/low torque.)

"The torque ripple or cogging torque will be lower"... I think a better way to put it is that the cogging of a motor spinning at 1800 rpm will occur at a higher frequency, for a given number of poles, and perhaps (really, perhaps) is less likely to be audible for that reason. But on the other hand, such a more rapidly spinning motor will be more likely to emit vibrations and noise, due to structural imperfections. Enter the belt-drive. Further, Kenwood, Pioneer Exclusive, Dual, and a few others back in the day used coreless motors to minimize if not eliminate totally the issue of cogging in their direct-drive turntables. Another wag on these pages or on VA has opined that cogging is essentially inaudible. I have no opinion on that.

Do you happen to know what is the rotational inertia of the L07D with the optional peripheral ring weight installed? I am using it with mine. Interestingly, when one uses the ring, one is also told to flip a switch on the outboard PS which I guess changes the servo so as to recognize the additional mass.

If you prefer your thread drive to any and all direct-drives you have ever heard, that is all well and good. I am sure it is superb. But I don't think you can prove from first principles that it is inherently superior to all direct drive.

I looked it up and now know Techdas is Japanese. "Techdas" sounded German to me.

Dover
With the many faults of analogue playback I'm guessing there would be few on these forums including those in the audio press that would debate stylus drag as a principal fault, but maybe I'm wrong.
Speaking for myself I think this is why experimenting with some vintage direct drives has started me on a better path to enjoying more from my records then ever before and at a fraction of the cost of my previous belt drive tables.

More on serious faults as you are well aware, another speed instability problem that grossly effects pitch is off centre spindle and worn holes which effects pretty much every record causing velocity changes.

:pitch rules:

These new fancy tables of today ,that is, the ones you would take a mortgage out on,.Do any of these new age designers tackled this problem. The wow from a off centre record can be significant, worst then a warped record.

A while ago one of these table manufactures put up a dramatic test on youtube I think, featuring several people rapping on the platform of his $150,000.00 ,less arm, table while it played! Anyone here ever witness Mich Cotter in one of his demonstration's doing basically the same thing............ 30 years ago.

Finally for those with a LO7D that want to use a Time line,you can easily fabricate a disk to fit over the spindle of the table and set in a second spindle that fits the hole of the timeline.

Dover,

Final Audio High inertia TT has a resistance to stylus drag variation 4 times higher than the SP10mk3 & Kenwood L07D

Yet you already admitted that you have to reset the speed on your Final Audio TT to accommodate for stylus drag?

So my test procedure was :
1. Set the speed with the KAB with no record playing.
2. Use the Timeline to validate the speed at both inner at outer grooves.
3. Reset the speed again with the KAB with the record playing tracking at 2g
4. Use the Timeline to validate the speed at both inner at outer grooves.

Now I'm reasonably happy about the need for belt-drive decks without monitored speed correction.....to be set up for 'stylus down' accuracy.
But I have yet to see (via video)......a belt-drive (even with massive inertia) pass the Timeline test with and without stylus in the groove?
I think you agree with that:-

I think your test methodology would prove that only turntables with error speed correction built in will pass. We know that stylus drag exists and should always set speed with the stylus playing.

So I'm not quite sure where you're going with this 'High Inertia' argument unless you can demonstrate differences in the effect of 'stylus drag'?

Regards