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One of those techie concerns of an electric motor, especially a DD table.
Cogging is a bad thing in any turntable motor, regardless of how it drives the platter. With belt-drive, it could be even more annoying and noticeable than with DD. But the fact is that in most any modern turntable of reputable design, motor cogging should not be an issue, even if it's happening.
bimasta, If you hear no issues with the turntables you own, then how do you know the motor is guilty of cogging? Is it merely because they are iron core motors?
To reduce or even eliminate cogging, try using a variostat between the wall outlet and the turntable. For turn-on, use full 120v (or your local wall current). Once the platter reaches speed, reduce the voltage to just above the minimum required to keep the motor spinning the platter. I don't know how effective this will be with a quartz-controlled dd table, but for AC synchronous motors, particularly VPI motors. It's a LOT cheaper than a VPI SDS or other motor controller and just as effective.
The facts are that almost all motors/drives cog to some extent. Its caused by the finite number of torque pulses delivered to the platter. Its similar to the smoothness of a car with a 4 cylinder motor vs a car with a 6, 8 12 or 16 cylinder engine or even better an electric motor. Generally the more cylinders/poles, the smaller the torque pulses and the smaller the resulting variation in speed of the platter/car that are felt as vibration or lack of vibration or smoothness. There are lots of ways to reduce it given a number of poles/cylinders. Some 4 cylinder engines (BMW) are a lot smoother than others. So if the cogging is below an audible level, its not a problem. Usually it shows up on piano music. Wow and Flutter are also forms of speed variation that are specified. Typically they are lower in frequency than the cogging caused by a direct drive. In a TT with a belt, the belt and the inertia of the platter act as a mechanical filter to reduce it. The electronic drive to a direct drive TT can be designed to reduce the cogging also--as the above poster mentioned, reducing the voltage after startup is a simple way to do this for AC synchronous motors as well as steppers. An oil bearing support versus low friction ball bearings is another thing that can help. As with all things, its a balance and most generalizations have their qualifications. Execution and fine grain detail design is required.
The facts are that almost all motors/drives cog to some extent. Its caused by the finite number of torque pulses delivered to the platter. Its similar to the smoothness of a car with a 4 cylinder motor vs a car with a 6, 8 12 or 16 cylinder engine or even better an electric motor. Generally the more cylinders/poles, the smaller the torque pulses and the smaller the resulting variation in speed of the platter/car that are felt as vibration or lack of vibration or smoothness.
This is not accurate. Cogging is caused by the change in variable reluctance as the rotating permanent magnet rotor passes over the metal pole pieces and air gaps between them. The drive signal is not "pulsing", it is a continuously smooth sine wave and the rotor synchronizes to the rotating field; if there were no metal pole pieces in the stator (coreless motor), there would be no cogging.
Adding more poles to a motor does not reduce cogging, the attraction between the rotor and the stator poles remains the same. It may feel "finer" in a 300 RPM motor vs 600 RPM, because there are twice as many poles, but the motor spins at half the speed so the vibrations will be identical if the motors are the same power rating. This was measured and discussed here:
The metal poles of the stator concentrate the magnetic field developed by the stator coils. The higher the drive signal, the higher the magnetic field and the stronger the attraction to the rotor and the higher the cogging. The vibration in an AC synch motor is directly proportional to the power consumption, not the number of poles. Reducing the drive voltage will reduce vibration, but it also reduces torque, this is only a band-aid and can start to have a negative effect on the sound.
Skewing the angle of the stator gaps can greatly reduce cogging as the rotor is never completely aligned with the poles or the gaps. Most 3 phase BLDC motors are constructed this way. When driven as 3 phase AC synch motors, they exhibit little or no cogging and are as smooth as coreless motors.
The best way to demonstrate cogging is to spin the platter by hand, apply stylus to vinyl, and notice the sound. Then engage the motor. If the sound deteriorates in clarity or shows increased high frequency content, you have demonstrated cogging. Obviously engaging the motor will improve pitch consistency.
If you can't demonstrate cogging, it may be because of bearing noise - the bearing is so noisy that it swamps cogging noise. An air bearing TT is capable of resolving even tiny amounts of cogging, which I can hear clearly with a 1.8W multiphase motor driven by an adjustable, precision supply. For the significance of this, read Phoenix above.
bimasta, If you hear no issues with the turntables you own, then how do you know the motor is guilty of cogging? Is it merely because they are iron core motors?@lewm I just noticed your intelligent question. Honestly I don’t know; I guess because they don’t mention cogging, and usually non-cogging is advertised as a feature. Maybe you, or others, know: my DDs are Denon DP-80, and Micro Seiki DQX-1000.
Thanks for prompting me to think deeper, and sorry for not answering sooner.
Oh, and you never answered my question about the FR MC-201. So I guess we’re even... if anyone’s keeping count....
bimasta, I've never owned or even heard a FR MC201 cartridge, or any other FR cartridge, for that matter. Does that answer your question, which I have forgotten? (Sorry for not having come forward sooner.)
The Denon DP80 does have a 3-phase AC synchronous motor, or so I was told when mine was in for restoration. Probably its an iron core type but built and driven to minimize cogging. Bill Thalmann thought its motor and servo control mechanism were advanced over the contemporary Technics systems. Denon published graphs to document its smooth running in their advertising literature. (Imagine a modern company doing the same; well I guess one or two have done.) I don't know much about the M-S DQX-1000, except based on my reading at Vintage Knob, it is an updated version of the DDX-1000, distinguished from the latter by its use of quartz-referenced speed control.
Thanks for the info Lewm, and sorry about the FR question — I confused you with another Audiogon member whom I follow closely.
The DQX uses a different motor entirely than the DDX — it's made by Victor — I don't know if it's the same as used in Victor's TOTL DDs. And apart from the quartz-control (which can be defeated in favor manual control, so it offers both) it also provides 40% more torque... also according to Vintage Knob. Sorry for the 'ad-speak'; I like mine, though the DP80 is a rung above in performance.
None of this resolves the cogging question however.