Lewm,
That is a good point about belt contact with the platter. My SME 10 has about one inch of exposed/free belt before and after the motor pulley leaving the vast majority of the belt in contact with the platter. This is in great contrast to some other designs which have a motor on a remote platform 9" or more away from the record platter. Tables like the Walker, the Raven and the Micro Seiki all have a lot of exposed/free belt which is why some owners have switched to thread, I think.
I have not tried the Timeline on my SME or read of anyone else doing so, but the SME tables have a reputation for having pretty constant speed stability and accuracy.
I started this thread a while ago, and I appreciate all of the contributions to it. I have learned a lot. Thank you to all those who have participated. |
Hi Peterayer,
I had mentioned in the NVS thread I had tested two and both were off. Using the same TimeLine it was found one was too fast and another was too slow. |
Hello Dev,
Yes, I had read that now deleted NVS thread, but I forgot about your particular post. Do you remember how far off the speed was for each of the tables?
According to my KAB strobe, my SME is 0.18% fast, as determined by the fact that four (4) "33" numbers drift out of the red strobe light during a 1 minute test. This was while a record was playing. Unfortunately, my SME motor controller does not have a speed adjustment, so I change belts after about one year and that reduces the error.
I would guess that if you found one SME table too fast and the other too slow that the belts were probably stretched to different degrees. Were these models with adjustable speed on their controllers? If so, did you try to adjust the speed on the controllers to get a more accurate speed? Also, did you listen to each to see if you could hear a difference? |
The concept of "belt creep" and what to do about it is not mine intellectually. This phenomenon was first described to me and to anyone else who read it by Mark Kelly. Mark played at designing a bd tt to combat it. The Artemis is a less complex solution compared to Mark's, IIRC. But your SME is well designed, Peter, if it places the motor pulley as close as possible to the platter's edge. Notts recommend this also. That also is a way to maximize the contact between platter and belt. You BD guys should look for posts by Doug Deacon on "tape drive". Doug uses a specific kind of recording tape which he then treats chemically to make one side rough, so it has traction on the platter. Conceptually, this should be better than string, IMO. Reality is sometimes different from concept, however. But Doug claims excellent results with his Galibier turntable. |
Hi Ralph, Belt drive turntables differ from one other in belt compliance and platter moment of inertia, so their oscillation wow patterns differ. That explains in part why belt drive turntables sound so different from one another in pace and rhythm, in steadiness of pitch, and in solidity of bass (bass notes last longer, so they require a longer sustain of turntable pitch accuracy to sound straight and massively impactive, rather than warbly, wobbly, and weak).
Can the belt drive designer reduce or damp the unwanted oscillations between platter and belt? In principle yes, but in practice it's tricky to execute. In principle, what's required is simply the addition of some resistive damping. This would damp the reactive LC tank circuit of belt and platter so it would no longer oscillate.
A common form of resistive damping is friction. Thus, if a knowledgeable turntable designer wants better speed constancy, he might well consider intentionally adding some friction to his rotating platter. It's worth noting that some Swiss and German engineers are so justifiably proud of their ability to produce nearly frictionless bearings that they cannot bring themselves to make turntables with high friction. As a result, the Thorens turntables exhibit some of the most spectacularly low friction bearings on the planet, and will spin seemingly forever (with the belt removed); but, at the same time they also exhibit some of the worst audible wow, in part because there is, as a matter of engineering pride, almost no resistive damping for the oscillating reactive tank circuit.
What might be useful ways to introduce friction? The fit and/or finish between platter spindle and well could be made poor, instead of smooth and polished. But this would be causing friction via crude irregularities, like two meshing mountain ranges, rubbing each other. The crude irregularities of these two mountain ranges rubbing together would cause unwanted speed irregularities (snags and letting gos), as well as unwanted vibrational rumble (the earthquake rumble of each letting go after each snag). So that tactic is out.
One useable tactic is to introduce a viscous fluid in the bearing, which provides friction in a liquid hence smooth form. This can be especially effective if the spindle is made in a larger than normal diameter, so that the viscous fluid has a larger moment arm (more leverage) with which to work its resistive magic (as in the Linn Sondek). The use of viscous fluid for resistively damping platter rotation can also be enhanced by various helical screw kinds of arrangements, which force the fluid to do extra work in opposing the rotation of the platter (as in the turntables from Max Townshend). It's no accident that these two brands have the best reputation among belt drive turntables for pace and rhythm, solid bass, and master-tape-like clarity. It's because both these designs recognize that belt drive, far from being a simple Hail Mary solution, brings with it new problems that must be addressed, and that overcoming the problem of speed constancy requires at least the addition of a fourth element, resistive damping, to the three usual elements of a belt drive turntable. That's a quote from Peter Moncrief's incisive article about turntable speed control. As also described in his article....is the fact that a 'stretchy' rubber belt can iron out the 'pulses' of a poled motor whereas if a non-compliant tight thread were employed........we're back to transferring that cogging into the platter? |
