How important is low W & F performance anyway?
I recently completed work on a direct drive motor controller for a turntable mfr with IMHO, rather impressive results (0.004% 2 sigma method, 0.002% RMS). In measuring other tables actual performance (vs published specs) I was shocked at the rave reviews two tables received that have rather lousy measured performance (but impressive specs). It made me wonder whether the goal of ultra low W&F performance was really necessary? I trust the measurements as they were verified by several methods and software tools and they correlated rather closely, yet the reviewers almost universally praise these tables. It made me wonder if the reviewers even know what they are hearing or listening for and not to put to fine a point on it, does it even matter?
this video, assembling a Linn TT in the Bedrock Plinth (it’s not a plinth, merely a frame). https://www.youtube.com/watch?v=SqP4eHBSlI4&t=2s watch and listen at 5:15, see the platter and shaft drop like a stone, AND listen, a clunk, what the hell is that?
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I’ve seen this rationale applied before in the past and it is counter-intuitive to me. The concept is if you load the drive/platter with torque (either through an eddy brake or thick bearing lubrication), then small dynamic changes in torque will be "swamped" by the static torque load and will therefore go unnoticed. If the speed is stable with whatever static load is placed on it, the system is in balance and dynamic loads will still disturb that balance proportionate to the amplitude of the dynamic load, not in ratio of the dynamic to static load. The only way this makes sense is if the torque load/rotor angle is not linear, which it is not-torque is proportional to the sine of the rotor/field angle, but the relationship is very nearly linear at the lower end of the torque curve. So unless you are applying a static load that is very near the stall torque (close to 90° rotor/field angle where the slope starts to flatten) it’s difficult to see how this could have any benefit. I suspect the improvement in sound that some report with thicker bearing oils is due to mechanical damping and not to speed stability. |
@phoenixengr. I tend to agree with you. Viscous pre-loading of the drive can be beneficial as it damps the bearing and by extension the platter. But perhaps more importantly, it damps any tendency of motor to oscillate about the synchronous speed when presented with a sudden change in load, torque demand. Cheers. |
With regard to the Garrard 301, it has always been my understanding one of the purposes of the eddy brake was simply to load the motor and push the operation of the motor higher up the torque curve. One would assume that it would increase the resistance to stylus drag, but obviously does not eliminate speed variations. |
