Stylus-Drag..Fact or Fiction?

"Phoenix, Thank you for that explanation. Do DC motors cog at all?"

Any motor with iron pole pieces will exhibit cogging.  In a DC motor, the magnets are stationary but the rotor coils are wound on steel laminates with poles which also produces cogging.

Some of the better BLDC motors have stators with skewed poles (angled slots rather than vertical) so the rotor sees more or less a constant magnetic reluctance and produce much less cogging.

But an AC synchronous motor could still exhibit cogging, if it is poorly designed with an inadequate number of poles.

But one should look at the rpm of such motors- instead of running at 33 rpm like a direct drive, 1800 rpm is common; at that speed cogging simply isn't a thing.  It becomes a thing when the frequency of the event is slow enough that it falls within the perceptual (time) range of the human ear.

By comparison the speed variation caused by a stylus totally dominates the field.

Most of the direct drive motors (at least the better ones) are coreless so they have no cogging. An 1800 RPM motor is a 4 pole induction motor; the stator does not have the poles and gaps that an AC synch motor will have. When you turn an induction motor by hand there is no cogging because there is no permanent magnet in the rotor. The rotor is magnetized by the rotating field, but that requires a certain amount of "slip" to operate and is a function of the torque load. Because of this slip, induction motors are not truly synchronous which introduces another variable in speed control.  Induction motors are speed sensitive to voltage as well as torque, where AC synch motors are unaffected by either.  DC motor speed is affected by voltage, temp and torque load.  As stated previously by others, there’s no free lunch and there are strengths and weaknesses to each design choice.


Stylus drag is fairly constant so in most cases, it is inaudible. While there is a measurable change in speed caused by stylus drag that varies from the start of a record to the finish, it is rather small and extremely slow changing and mostly inaudible. Changes in speed with groove modulation exist in theory, but I have never seen any data that claims to measure or quantify it; it must be extremely small. Heavy platters with lots of inertia will have a positive effect on this phenomenon, because changes in groove modulation are short duration (by definition) and more inertia will reduce short term speed variations.

In my experience, the biggest change in speed of BD tables is caused by the warming of the belt and bearing oil viscosity; it is not uncommon to see speed drift of 0.2~0.3 RPM over a 45 minute playing time which is audible to those with pitch sensitive hearing and if corrected all at once, audible to just about everyone.

While coreless motors exhibit no cogging, they do show, at a much lower level, perturbations in output torque. As designed and as built are two different things. 

Is this audible? maybe. What this wonderful hobby has taught me is that pretty much everything is audible, at least in the analogue world where I live.   

Halcro, Very few of us have perfect pitch and even those that do probably still do not have the resolution to hear the degree of slow down caused by stylus drag. Unless you have a table like the Monaco that can pick up speed changes in 1/72nd of a rotation there is little you can do other than set your turntable speed while playing a record. The drag (speed) is going to change across the record and with various frequencies and modulation thus the speed is going to vary fractionally. I would love to see how a table like Kuzma's Stabi XL DC performs with the timeline. I would hazard a guess that it does very well. I know for a fact my SOTA slows down a little just by using a strobe disc. Their new drive might counter that but it has lower resolution, 1 rotation. Will that be good enough? It should certainly do better. At any rate I can not hear changes in pitch that slight so does it really matter? Again to me the most important aspect of vinyl playback is noise. Not only can you hear it but it can also drive your subwoofers crazy particularly if you use room control. I have had several records that I had to return because the lath used to cut the master was in bad shape and the rumble was obnoxious, all Rykodisc I might add. 
Today is Yom Kippur and I would like to apologize to anyone I have pissed off last year:)