The anti-Direct Drive topic has been discussed at length, but so far there wasn't any good evidence of "hunting and pecking" or "cogging" on the typical Direct Drive tables. Now i have found some very commendable forumers that do have the strong evidence in some DD motors
. This in a GREAT thread in another forum (link below).
Let me first comment that not all DD motors are of the same design. But many of them follow the typical Technics design.
Here's the thread. It's 22 page long and you'll either have to read all the thread (if your technical level is good) or read my summary below:http://www.pinkfishmedia.net/forum/show ... hp?t=70027SUMMARY
Many turntables are compared by using a record with a 3KHz tone and then analysing this using a computer. The actual speed at the record at every angle of the rotation is then plotted in polar fashion. With the SL-1200MK2 and Pioneer PL-71, the polar graph clearly shows speed variations that are perfectly coincident with the 'impulse' given by the motor. Extreme example is the PL-71, a highly rated Pioneer turntable which i own [great construction, great tonearm], and which my ears suspected it suffered from cogging. Now i see that i was correct:
Pioneer PL-71 speed vs angle of rotation (several rotations)
PL-71 motor design, similar in arrangement to most Technics designs
The cogging showed as a flutter component at 6Hz. This could interact with the cartridge-tonearm resonance, intermodulating the signal and reducing sound quality in the same way that sound quality is reduced when using a cartridge of too high compliance for a certain tonearm.
SL-1200MK2 (green trace)
The SP10 (i assume MK2) also showed a faint trace of cogging, but significantly smaller
than the PL-71. Platter of the PL-71 is 1.5Kg; platter of the SP10MK2 is 2.9Kg. Repeating the test increasing the PL-71 platter mass to 2.5Kg didn't show any improvement at all; it seems that a significantly higher mass would be needed and/or the control system of the SP10MK2 being much better. In my view this would mean the 10Kg-platter SP10MK3 would show no cogging effects at all, but then the motor of the SP10MK3 is much more powerful, which is not good for cogging.
For comparison, an EMT 930 idler drive, the non-plus ultra of Idler Drives, with an enourmous platter mass:
Mechanism imperfections are visible in the plot but they won't manifest as a flutter component in the critical 4-10Hz range, unlike the PL-71 example. It would manifest on a lower frequency.
Linn LP12, the flutter components are of higher frequencies, where (IMHO) won't excite the cartridge+arm resonance.
Lenco L75 flutter spectrum!! (Idler wheel, 4Kg platter.) Compare with PL-71 samples. I own both turntables...
The graph don't show that the Lenco manifested a flutter component at 30Hz which is most probably rumble from the motor (1800RPM). This can be significantly reduced with mods.CAVEAT
It is still not clear to me what is the scale of the above graphs. There is still a chance of the cogging effects shown to be too faint to be considered of importance. However, my personal experience is that i could hear "something wrong" with the rotational smoothness of my PL-71 and SL-1401 (same motor and platter mass than the SL-1200MK2), while my Lencos (L75/L78) and Thorens (TD125MK2) didn't show said effect. But this is just a personal opinion.ULTRA SHORT SUMMARY
DD motors of Technics-like motor construction, with platter weights of 3Kg or less, will cog and show a flutter component in the (IMO) dangerous zone of 4-10Hz. Add a lot of mass to a DD motor and it might approach rotational perfection. This could open a new trend for the turntable DIY people -- add mass and/or reduce power!!