Turntable Belt Drive Analysis, Platter vs. Sub-platter

Discussion in 'Audio Hardware' started by avanti1960, Oct 11, 2016.

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  1. avanti1960

    avanti1960 Forum Resident Thread Starter

    Location:
    Chicago metro, USA
    Most high end belt driven turntables connect the belt from the motor to the rim of the platter. Basically a small pulley (motor) driving a much larger pulley (platter).
    Many belt driven turntables also make use of a smaller diameter pulley in the form of a sub-platter. Usually the sub-platter is much smaller in diameter than the platter.
    My intuition told me that the platter driven approach is fundamentally better than the sub-platter driven table, all else being equal. Riding my old ten speed bike told me that the motor (the rider) has a much easier time pedaling in a lower gear (larger sprocket) than a higher gear (smaller sprocket). On a turntable, I wanted my belt to drive the largest pulley possible (the rim of the platter) so that the motor would not have to work as hard and be more stable against drag and speed fluctuations that when driving a smaller sub-platter.
    Engineering calculations prove this is true by a staggeringly large margin as well as other key design elements.
    Below is a belt drive analysis for two turntables- one uses the platter as the larger pulley (the one on the left) and the other uses a sub-platter driven pulley. The values are very close approximations right down to the motor horsepower which on turntables is usually around .0025 HP, in case you were wondering.

    1) Bearing force. The platter driven system has a bearing load force of 2.8 lbs. The sub-platter driven system has a bearing load force of 6.7 lbs. or 240% more load. More than twice the force on the bearing mean increased wear, but more importantly increased drag on the speed of the platter.

    2) Cyclic Variation (in belt tension). The platter driven turntable has a cyclic variation of 0.79 lbs. The sub-platter driven turntable has a cyclic variation of ~2.4 lbs, over (3) times as much as the platter drive.
    Because the motor and belt in the sub-platter system have to work harder to spin the platter, the tension on the "pull" side is much less than the tension on the "slack" side of the belt. This translates into constant pull / release tension cycling and stretching which will wear out the belt significantly faster than the relatively light belt stretching on the platter driven turntable.

    3) Drive ratio. How many revolutions the motor pulley spins in relation to the speed of revolutions of the platter. The platter driven drive ratio is 0.083 while the sub-platter driven system drive ratio is .25, or 3X as much. What this means is that speed variations in the motor of the sub-platter will be magnified 3X as compared to speed variations in the sub-platter motor. In other words, a speed variation of 1% in platter driven system caused by the motor will equal a 3% speed variation in the sub-platter system, all else being equal. A platter driven system has 3X the speed stability of a sub-platter driven system.

    4) Drag resistance. This is a simple lever arm calculation. The larger pulley (12" diameter pulley) compared to the sub-platter (4" diameter pulley) has a 3X greater resistance to drag forces. Drag forces in the form of tone arm drag (minimal but measurable) and bearing drag. Bearings are not frictionless. But because the bearing forces are 2.4X higher in a sub-platter system, it has more drag to overcome and 3X less capability to do so. In effect, the sub-platter system is 5.4X more sensitive to drag caused speed variation than the platter driven system.

    Summing up. The combination of much higher bearing load forces, belt tension variation, 3X more speed variation sensitivity and 5.4X more sensitivity to drag forces place the sub-platter system with a significant amount of physical and mechanical disadvantages when compared to a platter driven turntable. It's refreshing to see that the high end turntable manufacturers actually have some sound engineering and physics principals to back up their designs.

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  2. jupiterboy

    jupiterboy Forum Residue

    Location:
    Buffalo, NY
    Nice post. I suppose some designs balance the various sub-platter dimensions and forces as you line them out with some perceived advantage of having the belt/motor not be in contact with the actual platter, making vibration transfer from the motor one material removed from the platter for the purpose of damping. Obviously this is why a design would remove the motor from the plinth or otherwise attempt to isolate the motor noise from the platter.
     
  3. avanti1960

    avanti1960 Forum Resident Thread Starter

    Location:
    Chicago metro, USA
    One thing I thought of is that direct drive turntables have a 1 to 1 drive ratio. The motor spins 1 revolution and the platter spins one revolution. Any speed deviation in the motor gets transferred directly to the platter.
    At least the sub-platter system is a 1:4 drive ratio, where the motor spins 4 revolutions for every 1 rotation of the platter, so speed deviations at the platter are reduced to only 25% of the motor's deviation.
    But the clear winner is the platter belt drive where its drive ratio spins the motor 12 revolutions for every one platter spin. Any speed variation in the motor is reduced to 12% of it's value at the platter.

    Example- with a motor speed variation of 1%=
    Direct Drive, 1% platter speed variation.
    Sub-platter Drive- 0.25% platter speed variation.
    Platter Drive- 0.083% platter speed variation.

    In my experience this is what I have heard. My platter driven turntable has never exhibited an audible speed variation in the music.
     
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  4. avanti1960

    avanti1960 Forum Resident Thread Starter

    Location:
    Chicago metro, USA
    Absolutely. Motor isolation from the platter and tonearm is a whole other topic but is possible with either drive design,
     
  5. SandAndGlass

    SandAndGlass Twilight Forum Resident

    It's refreshing to see scientific principles pop into the picture every now and again.
     
  6. avanti1960

    avanti1960 Forum Resident Thread Starter

    Location:
    Chicago metro, USA
    No wonder why the u-turn table is so popular. it uses a platter drive system.
    thinking about it in economic terms, you would eliminate the need for any speed controller in the system if you had a platter driven turntable. Now we know why speed controllers are so popular with turntables with small sub-platters.
     
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  7. The FRiNgE

    The FRiNgE Forum Resident

    A very nice post, but I have to question a few points.
    The greater bearing force (side force) occurs when? During start up? or maintaining speed? As we ride a 10 speed bike or 18 speed, the small sprocket is not hard to pedal on flat ground, minus wind drag (as wind drag does not apply in this comparison) The higher ratio requires less effort at cruise, is more efficient, and also applies a fly wheel inertia to help maintain a steady, effortless speed.

    A sub-platter design applies the same physics. The flywheel effect of the main platter assists in overcoming any variations in the motor, such as vibration or perhaps dust or debris on the pulley, all which would have far less effect on the platter speed, resulting in a more steady, flutter free rotation. The perimeter belt drive may have advantage in slow "wow" production, as the platter has a tendency to speed up and slow ever so slowly as the belt stretches and relaxes slightly. This type of speed variance is very slight, and if ever so slightly perceptible would not be objectionable.

    Platter weight is a major factor. The heaviest platters should be perimeter drive as belt stretch becomes more of an issue.
    For the low priced mass consumer belt drive with light aluminum platters, the sub-platter drive should be the better design We need the additional flywheel inertia of the lighter platter to help keep flutter to a minimum, and belt stretch is not a problem.

    Somewhere in the middle would be a Thorens, the original AR XA, a Rek-O-Kut Rondine (hybrid belt and idler wheel) Some Garrard idler wheel turntables are sub-platter driven (the platter drive ring in a similar location as a sub platter would be) This design also minimizes flutter, but at the disadvantage of longer start up time, and perhaps increased idler wheel wear. (as most wear occurs during start up) My AR XA was a splendid performer, extremely accurate speed and stability.. and it is a sub-platter driven design. Belt replacement was approx every two years regardless of the hours on the turntable.

    I agree with the physics, yes belt wear will be increased by the greater load during start up, but that increase would be so small as to not make any difference.

    I respectfully disagree on these terms, that the ratio of the drive with assistance of flywheel momentum does not increase belt wear, does not significantly affect belt stretch on light to medium mass platters, but DOES make a difference in reducing flutter. If a platter takes a little longer to start up, we can just give it a hand push to help get it going.
    Thanks for reading, :wave:
    Steve VK
     
    Last edited: Oct 14, 2016
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  8. Classicrock

    Classicrock Forum Resident

    Location:
    South West, UK.
    They also improve the sound of rim driven designs. The accuracy of machining of the sub platter in such designs is likely most critical. An area I feel most Rega and Project decks fall down. Decks such as Michell sound very speed stable and close to Direct Drive designs. I think in these designs the heavy platter as well as rim drive play a significant part.
     
  9. Grant

    Grant Senior Member

    Location:
    United States
    Interesting reading! Keep going, guys! I'm learning a lot. This is the kind of stuff I wish we had on this forum again.
     
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  10. missan

    missan Forum Resident

    Location:
    Stockholm
    Normally DDs have lower speed variations and W&F than BDs. This is due to that the rotating magnetic field is controlled in a much better way. It´s always using at least a servo controller, the best use a Quarts locked design. So this ratio doesn´t really mean much. The similar design can of course be used also for BDs, so many can be better than what they usually are.
     
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  11. avanti1960

    avanti1960 Forum Resident Thread Starter

    Location:
    Chicago metro, USA
    Thanks. The bearing load (on both the platter and motor bearings) is constant during running because the drive force and belt tension is constant. It is higher at startup (unless the belt slips) as you suggested because of acceleration.
    The flywheel effect of the platters does help maintain speed stability with each design, however motors and power supplies are still not perfect. A platter driven drive will still subdue / significantly minimize motor fluctuations better than a sub-platter driven design.
    I still lean to the platter drive as the best mass market design - see the U-turn orbit.
    The belt life will be less, all else equal, for designs that have higher drive ratio because of the difference in tension between the pull side and the slack side. The pull/ slack cycle constantly stretches and relaxes the belt as it moves with the pulleys.
    All this does not imply that there aren't / weren't any outstanding belt drive turntables that use sub-platters- of course there are.
    However, the physics of the platter drive system gives engineers the opportunity to further optimize the system. It is up to them to use good design expertise to take advantage of the physics.
     
  12. avanti1960

    avanti1960 Forum Resident Thread Starter

    Location:
    Chicago metro, USA
    Unfortunately I am not a fan of the direct drive systems I have owned and heard over the years. My ears must be sensitive to pitch but I can hear them occasionally hesitate and "hunt" for speed during the closed loop feedback of the quartz locked system.
    Just the other day in my favorite record store a Technics DD table warbled noticeably during a sustained note.
     
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  13. missan

    missan Forum Resident

    Location:
    Stockholm
    That is odd as the pitch variations with DDs always are less than with BDs.
     
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  14. Raunchnroll

    Raunchnroll Senior Member

    Location:
    Seattle
    The problem with these sorts of tables is they leave out lots of factors. The elasticity of drive belts vary. Bearing load is affected by the design of the bearing, the weight of a given platter and so on. Fluctuation in a direct drive motors rotational velocity are not going to equate to an equivalent fluctuation in the platter speed. Theres simply too much inertia and some of the energy imbalance goes into torsional stress and the properties of the material the platter is made of. Its all interesting though. In the end I just go with what I hear and like best!
     
  15. Yes. That pitch variation could also have been due to an off-center pressing I suppose.

    My Clearaudio 'table uses a sub-platter, and maintains a very stable speed and pitch (except when the belt got old). However, I see their highest-end turntables like the Innovation use the platter-drive so there must be something to the theory.
     
  16. Jack Flannery

    Jack Flannery Forum Resident

    Location:
    Houston, TX
    I purchased a TD150 from Vinyl Nirvana. That table was built around 1970 or so. It is far, far from shot.
     
  17. Mr.Sneis

    Mr.Sneis Forum Resident

    Location:
    Phoenix, AZ
    What are the advantages if any to the sub platter driven belt? Motors easier or cheaper to manufacture?

    None of my observations are quantifiable or qualifiable in sound or speed accuracy really but my experience with an outer platter belt table...

    I can belt a thorens table in a few seconds flat, first times the charm. When I had to do the same on a marantz tt15s1 it took several attempts, cursing, and several minutes to install the belt.

    I also couldn't stand seeing the belt slip up and down on the platter, it would click slightly as it did so as the belt was not a flat band but round with a finishing seam.

    In my mind though, the table was probably more the exception than the rule.
     
    Last edited: Oct 15, 2016
  18. avanti1960

    avanti1960 Forum Resident Thread Starter

    Location:
    Chicago metro, USA
    advantages- smaller belt and a smaller plinth footprint- you can put the motor under the platter. in a platter drive turntable the motor mounts outside of the diameter of the platter. the u-turn orbit was able to do a platter drive cheaply though.
    yeah, the belt on my VPI is tricky to put back on when i lube the platter bearing. it does not slip on the rim though- the platter has a machined groove for it to fit in. execution i guess.
     
  19. avanti1960

    avanti1960 Forum Resident Thread Starter

    Location:
    Chicago metro, USA
    agreed, but consider it as an "all else being equal" kind of situation. all else being equal, a platter drive is superior to a sub-plater drive. it gives the designer the best chance for success.
     
  20. Raunchnroll

    Raunchnroll Senior Member

    Location:
    Seattle
    Yeah thats likely (and generally) 'true.' I notice that ultra expensive TTs are usually platter driven. Of course, the fact that a TT is expen$ive is no guarantee of its performance or owner satisfaction!
     
  21. The FRiNgE

    The FRiNgE Forum Resident

    Very true! The comparison of sub-platter to circumference drive is meaningless with the high end. All high end TT's have ultra-heavy platters which must be circumference driven (Platter driven) and have so much mass that their flywheel inertia is more than sufficient to overcome any motor vibration or debris or imperfections of the belt or drive pulley.

    A well designed lower mass platter system can benefit from sub-platter drive, as the inertia of the platter is beneficial to lowered flutter. This is a matter of good engineering based on physics.

    The fact the massive high end tables have to be circumference driven, does not indicate the sub-platter method is inferior, because it isn't. If buying a low to mid-priced turntable, go for the quality of manufacture, not on how the platter is driven.
     
    Last edited: Oct 18, 2016
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  22. avanti1960

    avanti1960 Forum Resident Thread Starter

    Location:
    Chicago metro, USA
    All else being equal, a platter drive turntable is superior to a sub-platter drive turntable by a significant margin regardless of platter mass or price point.
    The U-turn Orbit (for example) uses the platter drive design and by default has the advantages of lower bearing load, longer belt life and improved speed stability when compared to a sub-platter design.
    The Orbit's "stock" went way up in my opinion once the results of the engineering drive belt analysis. Smart engineering.
    Based on this analysis, I am glad I own a platter driven turntable, the results are evident in my listening and ownership experiences and I would never purchase a turntable without it.
     
  23. Your experience is the direct opposite of mine. No pun intended. But hey, whatever works & let's you enjoy the music is what counts.
     
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  24. P2CH

    P2CH Well-Known Member

    Is the power of the motor calculated into the design of a TT? What I mean is, my MMF isn't the fastest starting platter, but is there a reason for this other than taking in the consideration of the weight it has to get moving?

    If the motor was more powerful, it would seem to put more stretch on the belt as it attempts to turn the platter from a dead stop. Is that slower start-up an actual design benefit or just a cost cutting value in terms of a lower cost motor?

    If I were to be putting a TT together, without all of the numbers the OP addressed and lack of engineering knowledge, I'd simply toss any old motor into it.
     
  25. avanti1960

    avanti1960 Forum Resident Thread Starter

    Location:
    Chicago metro, USA
    Power of the motor is a consideration in the design of a belt drive system. The higher the gear ratio, the more power required from the motor to accelerate the system (platter) to operating speed. Whether it is acceptable is up to the user but the designer should have established a benchmark "time to speed" specification that most users would consider acceptable, if not BIC "Best in Class".
     
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