VPI TNT Turntables

"Analogluvr- The one upgrade I would not do without is the flywheel, whether you have one or two motors. "

What was the biggest improvement you perceived when using a flywheel?

"Melm-  Quite similar to the improvement gained by using a speed controller. More of the same. "

That's interesting.  A 12 lb 3" flywheel has less than 4% of the inertia of a 20 lb 12" platter; it's difficult to see how that would have much of an effect on speed stability.  Maybe it's the additional isolation of having 2 belts, but then you double down on belt creep.  Any thoughts as to what is causing the change in perceived sound?

I'm not questioning whether it makes a difference, everyone would seem to agree that it does.  I'm interested in finding out WHY it makes a difference.

The reason posited by most is the increased inertia that a flywheel provides improves stability, but the math doesn't support that assertion.  For a solid disk of even density, the formula for rotational inertia is Ir=(m*r^2)/2 where m is mass and r is the radius (note that there is no speed component in the formula).  So for a 20 lb, 12" platter the result is 360 and for a 12lb 3" flywheel it is 13.5 or ~27x smaller.  Now, adding a periphery ring to the platter would definitely help more as the mass is concentrated at the largest diameter (although the mass is smaller compared to the platter).  I just don't see where a smaller, lighter flywheel has that much of an impact at least on inertia, angular momentum or kinetic energy.

FYI, a properly designed speed controller will provide excellent instantaneous speed stability, but long term speed accuracy is improved with feedback.

Constant motor speed does not equate to constant platter speed however.

Would you rather have constant motor speed with constantly changing platter speed, or motor speed that changes to keep the platter speed constant?

Well, from the formula, there are only 2 parameters that effect inertia, mass and diameter.  Diameter will have a greater impact because inertia increases with the square of the radius.  If your flywheel is 5" instead of 3", it changes the additional inertia from <4% to ~20% of the platter.  Maybe that's enough to make a difference?  The periphery ring is probably the better of the two as all of its mass is concentrated at the largest radius.

As far as the flywheel and stylus drag goes, it won't isolate the platter at all;  if anything it will very slightly isolate the motor from the incredibly small amount of additional torque, the platter will still slow because of the stylus drag.  With 2 belts, I would suspect that belt creep is higher than with a single belt, so speed instability caused by stylus drag would potentially be worse.

I agree that flywheels are a benefit, provided they are constructed properly for the task at hand.  Usually, the flywheel is larger in both diameter and mass than the object it is trying to stabilize.   Increasing mass does help, increasing radius helps much more.   In the case of the VPI flywheel, it is both smaller and lighter than the platter.  I would guess 20% increase of inertia improves stability by 20% as well, all other things being equal. If that explains the improvement in SQ, then mystery solved. If 20% improvement is acceptable for the added cost, then it's a worthwhile investment.

I agree that anything that improves the speed stability of the platter is a good thing. I don't know that adding another pulley (flywheel) and another belt is "faulty", but it bears investigation. All belt drive systems suffer from belt creep which affects speed accuracy and stability. In theory, having two belt drives in series would make belt creep worse (vs 2 belts in parallel around 1 pulley and 1 platter which should make things better), but I haven't measured it.

sbank-

Understood.  My initial comments had nothing to do with speed controllers, I was curious about why a putatively undersized flywheel seems to make such a difference in SQ.  Somehow, the thread got pulled in the direction of arguing over the physics of speed control.  I don't mind sharing my knowledge of that subject, except with people who refuse to learn anything from the exchange (it's difficult to make people understand something when they already know so much).

The Hurst AC motors will always be a major limiting factor on the VPI tables.  I own a number of the Hurst motors from the 49mm series up to the 60mm T series and the one consistency is they all vibrate, some much worse than others (within the same series).  It looks like VPI just changed from the 59mm series to the cheaper and smaller 49mm on some of their tables (Classic and Scout, possibly others). 

I ditched my SAMA with the Hurst 5W 59mm series on my Scout table and replaced it with a 3 phase BLDC motor with a custom enclosure and controller.  Best thing I ever did for this table.  Zero burn on the belt, 33/45/78 RPM with the same pulley, reversible rotation for idler drive, lots of torque and the smoothest, quietest motor I've found for a turntable.

http://www.diyaudio.com/forums/analogue-source/288730-3-phase-bldc-motor-turntable-use.html

Actually, I have a problem with Hurst motors, but thanks for your unbiased opinion.

BTW, the motor in your TNT table is not a VPI motor;  it is sold by Hurst and manufactured in China.

I bought the motor overseas;  it has a 4mm shaft (the Hurst motor is 3/16") so I had to have pulleys made as well.  The PSU is also a prototype and very different design than the Falcon or Eagle, although operation is the same and it works with the tachometer.

Right now, I have no interest in developing it commercially.  I built it to investigate whether it would work better than the Hurst motors, and it turned out better than I could have expected, so mission accomplished, but also, mission complete.