Turntable Speed: What matters?
- Thread starter PeterA
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Rob, the Radikal that I installed is a power supply upgrade and a DC motor replacement. Certainly is a major improvement on my old Valhalla power supply.
Micro, its has been many years since I heard a SP10Mk2. I do remember that at the time everyone was going away from Technics DD's ( including the SP10's) and their ilk due to the "cogging" factor. I never heard a Goldmund, so I cannot say about that TT.
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I don't know if your motor is brushless or not but its susceptible to cogging, what keeps it in check is the radikal PSU. fwiw, the motor in the DD Technics SP-10 III is the same unit used for cutting lathes (sp-02) its got to be dead nuts accurate.
Davey, I never heard or read about any one complaining about a "cogging" SP10mk2. People left them mainly because the standard plinth was resonant and made it sound poor, and most people built them in inadequate structures or plinths. Or simply because more fashionable
or better overall turntables showed in the market.Yes Rob, but that is one of the reasons why Linn and others use a belt drive. Isolates the motor from the platter. The accuracy of the motor is generally not in question with a DD, it's the direct connection to the platter that was always thought to be problematical.
Davey, I never heard or read about any one complaining about a "cogging" SP10mk2. People left them mainly because the standard plinth was resonant and made it sound poor, and most people built them in inadequate structures or plinths. Or simply because more fashionable
I agree with microstrip!!
I agree with microstrip!!
Have you guys ever heard what I am referring to in "cogging"? If so, maybe you can give us an example.
Cogging exists, but it isn't always noticeable unless there is another turntable in the room that doesn't cog. It sounds a lot like digital jitter from a cheap CD player, if the turntable is a truly inferior one. More often, such comparisons are made with much better turntables, of course. By that, I mean A-B comparisons. In those cases, the apparent symptom is a very subtle smearing of the mids and highs that presents itself like a veil over the music. In a large symphonic work, background instruments are more likely to be diminished, where their presence is more prominent from a turntable that adequately addresses the issue. Also, dynamics are affected. Take away the better turntable, and the smearing turntable becomes acceptable again to most people after a few sides are played. We are talking about subtleties, after all. Still, it is an unacceptable condition for the guy who spent a lot of money on analog playback. Maybe he doesn't know it yet, as that seems to be the case most of the time.
In my experience, the turntables that cog most are those which do not regenerate power, but employ a feedback scheme that corrects often. Bad motors cog, too. High-end turntables aren't impervious to bad motor choices by their designers, unfortunately. I would surmise that motor selection is one of the more difficult aspects of turntable design, and you guys would be surprised at who in the industry takes the cheap path with the motor.
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Cogging exists, but it isn't always noticeable unless there is another turntable in the room that doesn't cog. It sounds a lot like digital jitter from a cheap CD player, if the turntable is a truly inferior one. More often, such comparisons are made with much better turntables, of course. By that, I mean A-B comparisons. In those cases, the apparent symptom is a very subtle smearing of the mids and highs that presents itself like a veil over the music. In a large symphonic work, background instruments are more likely to be diminished, where their presence is more prominent from a turntable that adequately addresses the issue. Also, dynamics are affected. Take away the better turntable, and the smearing turntable becomes acceptable again to most people after a few sides are played. We are talking about subtleties, after all. Still, it is an unacceptable condition for the guy who spent a lot of money on analog playback. Maybe he doesn't know it yet, as that seems to be the case most of the time.
In my experience, the turntables that cog most are those which do not regenerate power, but employ a feedback scheme that corrects often. Bad motors cog, too. High-end turntables aren't impervious to bad motor choices by their designers, unfortunately. I would surmise that motor selection is one of the more difficult aspects of turntable design, and you guys would be surprised at who in the industry takes the cheap path with the motor.
+1
Great summary.
Cogging exists, but it isn't always noticeable unless there is another turntable in the room that doesn't cog. It sounds a lot like digital jitter from a cheap CD player, if the turntable is a truly inferior one. More often, such comparisons are made with much better turntables, of course. By that, I mean A-B comparisons. In those cases, the apparent symptom is a very subtle smearing of the mids and highs that presents itself like a veil over the music. In a large symphonic work, background instruments are more likely to be diminished, where their presence is more prominent from a turntable that adequately addresses the issue. Also, dynamics are affected. Take away the better turntable, and the smearing turntable becomes acceptable again to most people after a few sides are played. We are talking about subtleties, after all. Still, it is an unacceptable condition for the guy who spent a lot of money on analog playback. Maybe he doesn't know it yet, as that seems to be the case most of the time.
In my experience, the turntables that cog most are those which do not regenerate power, but employ a feedback scheme that corrects often. Bad motors cog, too. High-end turntables aren't impervious to bad motor choices by their designers, unfortunately. I would surmise that motor selection is one of the more difficult aspects of turntable design, and you guys would be surprised at who in the industry takes the cheap path with the motor.
As defined to Wiki:
Cogging torque of electrical motors is the torque due to the interaction between the permanent magnets of the rotor and the stator slots of a Permanent Magnet (PM) machine. It is also known as detent or 'no-current' torque. This torque is position dependent and its periodicity per revolution depends on the number of magnetic poles and the number of teeth on the stator. Cogging torque is an undesirable component for the operation of such a motor. It is especially prominent at lower speeds, with the symptom of jerkiness. Cogging torque results in torque as well as speed ripple; however, at high speed the motor moment of inertia filters out the effect of cogging torque.
Question is what is "high" and "slow" speed?
So as you intimated, cogging can affect most any turntable design, not just DD. And as I remember, the number of motor poles plays a large role in cogging effects, though that's not a constant as the Continuum using a 3 phase/8 pole motor and others use 12 and 24 poles. http://www.continuumaudiolabs.com/caliburnmotor.html IIRC there's also a tradeoff between poles and torque?
A far bigger problem is the choice of whether the table is AC or DC driven and how the designer implements that design. Older DC tables were far more subject to speed fluctuations than AC driven tables. A lot of "hunting for the right speed" occurs during the revolution and not enough drag to smooth out speed fluctuations. Perhaps is the biggest thing not being talked about: tables with "toy" platters and motors. As Bill Firebaugh demonstrated, you need a small amount of drag in the drive path to smooth out speed fluctuations. The same thing also goes for massive platters/adding intertia (after all records are cut on a lathe with a 300 lbs direct drive motor so dd motors can't be all that bad) and employing flywheels, etc. in the drive path.
I do know the design of the new VPI motor addresses cogging effects.
Oh and has anyone seen those motors in the Technics DD tables? (know Mark has.) The same ones are used in their tape decks that are known for their tape path speed stability.
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How does it prevent the cogging effect? The review in the latest Tone Publications says the same thing, but I don't think he goes into more detail.
Is the VPI high or low torque, and do you think this matters? The SL-1200 MK2 that used to be popular as an entry level TT was low torque; you could slow/stop it with a finger and some pressure on the platter. From what I'm reading the SP-10 are very high torque and can't be stopped so easily.
I am no motor expert, but I think "high speed" is usually 1000 rpm or more if I remember my power class from ages ago.
Sizing the pulleys so the motor spins much faster than the platter helps. A belt helps by smoothing the ripples (torque shock) from the motor. High magnet/stator (pole) counts also help by reducing the lag as the current is passed from one stator to another. There are also some clever motor design tricks (electrical and mechanical) that can reduce cogging but I have barely a hand-waving idea of how they work.
Sizing the pulleys so the motor spins much faster than the platter helps. A belt helps by smoothing the ripples (torque shock) from the motor. High magnet/stator (pole) counts also help by reducing the lag as the current is passed from one stator to another. There are also some clever motor design tricks (electrical and mechanical) that can reduce cogging but I have barely a hand-waving idea of how they work.
Yes, but a stretchy belt that helps smooth cogging, also smears the sound by doing so. It's not an ideal solution.
I would consider the VPI motor to be very low torque. Anybody seen of one their motors outside of the fancy motor enclosure? They are dinky. I replaced the 600 RPM with the 300 RPM motor on my TNT and was shocked at the size of the motor when they sent it to me. They are mounted in large enclosures which gives the appearance of a giant motor (kind of like a padded bra gives the appearance of...), but once you get a peek inside it's a tiny motor. I believe this is common for many belt drive tables. As for the SP-10, you aren't going to stop the platter with your pinky. Before you can take your finger off of the power button, the SP-10 is already up to speed. Technics used to advertise that if you could mount 50 arms to the SP-10 and all of them were tracking at 15 grams, you still wouldn't affect the speed. You can imagine how one arm tracking at normal levels is not going to affect how the SP-10 turns. And none of this would matter if the SP-10 didn't sound great, but I think it does.
you must have not owned an LP12, roksan xerxes, or SOTA star/cosmos - they all have dinky low torque motors and I believe its by design. I have a Mk V TNT w/ 300 rpm motor inside a 20lb machined housing, anything looks dinky next to but it does the job. the platter's rotating mass (23lbs) has the inertia to the maintain excellent speed stability and its up in less than 5 secs. it doesn't have the torque of my denon but its among the best of all the belt drives i've owned.
the strongest belt-drive motor i've encountered to date was in the Versa Dynamics 1.0, it used a high resistance ceramic bearing lubed with heavy grease, the motor required to turn it was this torque monster mounted on elastomeric bushings.
This is one of the reasons I don't like this device, and measure speed in mine differently - basically, by first asserting test-instrument frequency-measurement accuracy using a test test CD's 1kHz tone, then adjusting tt speed until I get 1kHz output from the amps with a test LP. This still suffers from speed variations within the test LP itself (due to potential cutting lathe speed variations), and/or the tt's motor; therefore, I eventually adjust for a range from 999 to 1001Hz (sometimes readings are outside this range), and then revert back to the KAB strobe - and it appears that my speed as measured by the KAB is overall dead steady - apparently, the SDS speed controller does its job. Recently, I am relying more on the KAB for final speed adjustment than ever before, and it looks to me it's good enough.
The other reasons I don't like the Timeline are: 1) unknown mass, affecting speed in indeterminate ways when removed and replaced by one's own clamp/weight of different mass and rotational properties; 2) it takes a very long time to make a decision
I owned a SOTA Star Sapphire for many years. Have you seen the actual motor of your VPI outside of the 20 lb machined housing?
I just added a TimeLine video of my old direct drive Denon DP-45F from 1980. YouTube muted the audio as Led Zep IV is copyrighted. Kind of interesting to compare the results of the Denon to the SME, my first and current tables, respectively. They don't sound alike either. I am now working on a video of a friend's Technics SP10 MK2a and will upload it shortly.
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