Tables That Feature Bearing Friction

Dgarretson,

I addition to the practical difficulties that Dertonarm pointed out there are some good theoretical reasons against using a flywheel at the motor. In all motors motion is produced by magnetic attraction. When there is no drag the
magnetic fields in the stator and rotor will be perfectly aligned. When drag is applied the magnetic fields pull apart and the attraction creates torque. This is a very desirable characteristic in that torque is almost instantly created in response to a load perturbation. I say almost instantly because the rotational inertia in a motor delays the delivery of torque. Any delay in torque delivery adversely affects the motors ability maintain constant speed because the corrective torque is being applied after the fact. The longer the delay the worse the problem becomes.

Belts, particularly stretchy ones, have the same effect. They filter cogging, but also delay delivery of torque. Filtering cogging is a good thing, but it comes at a price. In many cases the fix is worse than the problem. Much better to just start out with a motor that has low cogging.

In addition to the theoretical arguments there is plenty of empirical data that indicates that low inertia motors tend to sound better. That has been my experience.

BTW: Certus turntables use an eddy current brake. The braking force is much greater than what a belt drive mechanism can keep up with.

Chris

Dertonarm, the heavy platter, slip coupling approach focuses on only the issue of motor cogging. This approach does not deal with the issue of stylus drag, or any other variability in drag. Contrary to popular beliefs platter mass changes how stylus drag affects speed but does not correct it. A massive platter will reduce the magnitude of the variation but extends it over a longer period of time. A light platter will conversely allow a larger speed variation but it enables more rapid recovery. Heavy vs. light platters exhibit different sounding degradations but they are still degradations.

Intimate coupling of the motor to the platter is the only way to effectively deal with stylus drag. But intimate coupling also makes problems from cogging worse. So in the end a compromise between the two is needed. A DC motor needs less isolation than an AC motor so the compromises will and should be different. Personal preferences also will dictate the ideal compromise. For example idlers with AC motors have poor isolation from cogging, but more intimate coupling. The result is excellent rhythm and timing but finesse and low level detail are sacrificed. Some like the idler compromise and others don't.

If you start with a very low cogging motor then a better compromise can be achieved.

Dertonarm, we are talking about physics. The issue of stylus drag has been hotly debated before. It is a fact that any drag, regardless of how small will slow a platters rotation. A large platter mass spreads the variation over a longer period of time, but does not and cannot eliminate it. It's basic physics. Any amount of energy added or removed to the system will directly affect speed.

Now what can be debated is the audibility of such a small effect. Logically it seems quite implausible that a force as tiny as stylus drag could be audible. The audibility of the things being discussed here certainly are in the realm of opinion and theory.

Regardless of the theory there is a great deal of evidence that techniques that target stylus drag (like bearing friction and intimate coupling) produce positive, audible results. This would suggest that the theory of audible stylus drag is correct, but it certainly falls short of proof.

Thankfully, turntable design is not pure physics. A good design also includes compromises, tastes, experimentation and even some guesswork. Otherwise turntables would all look and sound the same. Really boring...

Dertonarm, I completely agree with you that the job of a turntable motor is only to bring the platter up to speed and then keep it from slowing down. Where I disagree is that platter mass alone is the the cure all for speed stability. A large platter mass does not negate the importance of motor quality or the quality of the drive mechanism.

In this thread there is a lot of discussion about physics. Of course what is happening in a turntable is about physics. But I posit that good turntable design cannot be purely about physics. There are several reasons that this is the case:

1) Nobody fully understands exactly what the physics are. There are a lot of very subtle things going on in a turntable that to date are not fully explained. There are plenty of theories and opinions, but not a lot of facts.

2) Good turntable design is all about producing good sound. Unless the relationship between sound and physics is fully understood then physics alone cannot be an effective method for designing a good sounding turntable. Harmonic distortion in an amplifier is physics. But building an amp with super low distortion often does not produce good sound.

3) Design is also about compromises. There are compromises to control costs that apply to any turntable design commercial or otherwise. A good designer will make the compromises that deliver the best value for the money and effort expended. In addition to cost compromises there are compromises related to balancing of conflicting technical goals. Coupling vs isolation, damping vs rigidity, light vs massive. Focusing on only one objective usually delivers poor results.

4) Like it or not good sound reproduction is a subjective pursuit. There is no such thing as an ultimate sound system nor is there or will there be an ultimate turntable. Everybody has their own set of musical priorities and no design will be a fit for everyone.

Back to platter mass I have done a lot of experimenting with various platters. I find that to my ears heavier platters do sound notably better. I also have found that a heavier platter makes that quality of the motor and drive system less critical. But even with a 70 pound platter subtle changes like belt material and even the motor pulley composition are still easily heard and are musically significant. Less subtle changes like rim or direct drive are even more obvious.

Yep, expensive is the hated word for the day. Expensive applies to both commercial and one off, all out assaults. It's just that the threshold for a one off can be much higher. Maybe a solid gold platter would sound really good...

For me expensive mostly relates to time. There are a lot of ideas that have merit. But it is not possible to experiment with all of them. I am sure that I have discarded some good ideas because I didn't have enough time. Part of the art of design is guessing the most profitable ideas to explore. Nobody gets them all right, but one of the important skills in this endeavor is the intuition to get it right more often than not.

*** Well - the physical phenomenons regarding the turntable CAN and ARE fully understood. Unfortunately and apparently not by the majority of turntable designers. ***

Some of the phenomenons are well understood but many are not. Please explain why pulley material would be audible when used with a 70 lbs. platter. I don't have a clue what physics are involved for this case. I also am quite doubtful that even with the best equipment that this effect could be measured. I am not suggesting that this is magic. Something logical and scientific is going on, I just don't know what it is. I am sure that plenty of folks can come up with theories about why, but theories are not that same as really understanding the physics.

Oh, and yes this is a real effect that many folks have heard. Please lets not get into the subjective vs objective debates...

###
- possible highest platter weight
- possible to use high inertia for self-stabilized speed
At least the last two reasons can not be used with idler or dd drives. ###

Wow you are dead wrong about this. The Rockport Sirius is direct drive and uses a 62 lbs. platter. The Certus DD turntable uses a 60 - 75 lbs platter. The Verus rim drive motor has been used (with excellent results) with platters up to 70 lbs. There is no reason practical or otherwise that limits direct or idler motors to light weight platters. The Certus motor is more than capable of driving a platter well in excess of 100 lbs.

BTW: we have done direct comparisons using the exact same 75 lbs. platter with belt and direct and rim drive. They all sounded considerably different and the belt drive was the clear loser.

Your heavy platter, slipping string design is one of many possible approaches to turntable design. But

- it is not the only correct way
- it is not the only concept that "agrees with physics"
- it is not an idea that cannot be improved on
- it is not without compromise
- in my experience the slipping string part of the design
is inferior
- the heavy platter part I agree with, but others can make
credible arguments for a light and responsive platter.

I have have no problem with disagreement. In fact it can be a lot of fun and enlightening. But the constant demeaning of dissenting opinions has gotten old. I am done...