VPI TNT Turntables

Fellows,

I am no engineer (though I did stay at a Holiday Inn Express) but I have experienced how greater mass/weight has more inertia and would be much harder to change the speed than would one with less mass/weight.  For example, picture changing the speed on a playground merry-go-round that is empty, vs. one that is totally weighted down by a full load of equally distributed passengers.  It will take much more effort to slow down or speed up the heavier one.  The flywheel effect in cars and trucks is another example of the heavier, more massive one continuing to spin at a more stable rate, while the lightweight one will lose rpm more quickly.  I have noticed this too when sharpening tools on a blade sharpening wheel.  It takes much more force to affect the speed of the heavier wheel.  It seems to me that the heavier more massive turntable motor flywheel would be more stable and would be less susceptible to external forces, like fluctuations in power supply, intermittent friction, etc.  I am sure I'm wrong, please explain why. 

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.

phoenixengr,

I am going to interpret that as an agreement to what I said.  All of my examples are increasing the mass and the inertia, right?  So the vpi flywheel would be more stable, due to its increased mass?  The more massive flywheel in motion would be more likely to stay in motion, compared to just a simple motor.  We are talking about the speed stability of the motor/drive system here.  You seem to be agreeing, but believe that the two belt implementation is faulty?

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.