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That's not a stupid question. Presumably because the impulses that propel the platter around aren't as clean or delivered as smoothly as they might be. In the case of the 1210, I think the 'slowing under load' issue might be a red herring or at least slowing under load that causes power supply grunge to become more evident. I suspect the grunge is there, manifests itself in some way in the motor drive signal and results in the platter not turning as smoothly as it might.  The effect wouldn't impact on any measured w&f figures which tend (as I understand it) to be focussed on speed fluctuations occurring below about 250 Hz anyway.



There have been far worse sounding direct drive designs than the 1210 in the past that still turned in exemplary w&f performance. On those, I expect the motor was getting an even more crudely synthesised series of impulses. The old SL110 was one such but most of the Japanese majors made cut price DD's with decent measured speed stability & awful sound. Some of that is down to the flimsy structures the motors were fitted to but not all of it.



Of course, all of the above is conjecture. I'll be interested to read what the planned experiments reveal but I don't think that the traditional measurements of w&f will reveal much.


Its worth reading a little about the type of motors this concerns and the complexity involved in driving them cleanly & accurately.


http://en.wikipedia.org/wiki/Brushless_DC_electric_motor


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