As I subscribe to Alex Korf's blog I received his latest post titled "Turntable Drives. Part IV, Direct Drive Introduction".
So far I don't agree with his findings posted. He states that he has a non contact way of measuring the speed 5000 times per second and than shows a graph with a "top quality 1970's Japanese DD turntable" that shows speed fluctuations from 32.4 to 34 RPM!
The other fault that he talks about is flux leakage effecting the cartridge. I've had several DD tables before settling in on my Victor (JVC) TT71 motor drive unit. Using LOMC cartridges I would think that I'd be aware of any flux leakage effecting the sound quality of my turntables.
Has anyone else seen this and have any opinion? His latest blog entry has me questioning his expertise in these matters.
BillWojo
So far I don't agree with his findings posted. He states that he has a non contact way of measuring the speed 5000 times per second and than shows a graph with a "top quality 1970's Japanese DD turntable" that shows speed fluctuations from 32.4 to 34 RPM!
The other fault that he talks about is flux leakage effecting the cartridge. I've had several DD tables before settling in on my Victor (JVC) TT71 motor drive unit. Using LOMC cartridges I would think that I'd be aware of any flux leakage effecting the sound quality of my turntables.
Has anyone else seen this and have any opinion? His latest blog entry has me questioning his expertise in these matters.
BillWojo
The huge speed fluctuations are a bit of a head scratcher for sure. Typically, wow & flutter would be .02-.03% depending on the weighting, for a decent quartz deck.
jeff
jeff
It makes me question his methodology as well. There is no time associated with that graph, were those fluctuations happening in milli-seconds or over a long time period. Can't be milli-seconds or it would show up as W & F, besides, it would take an enormous amount of energy to accelerate and decelerate a platter in a very short time period. Not happening. If it's over a long time period this would be noticed and would have been measured and reported years ago.
Remember that ANY company that builds rubber band drive TT has a vested interest in killing off DD tables, the British press tried to help it's industry for years with all kind of printed nonsense.
Heck, we don't even know what table it is and where it came from. Maybe it has all kinds of issues.
The thing that bothers me is that Alex Korf would even print this when a gut-check tells you that something is very wrong in the measurements. It's going to be taken as gospel by all those that have a love for rubber band drive tables and ignite a firestorm.
I also question the flux leakage as well. Not saying that there isn't any but I imagine it's so low at the cartridge area to be inconsequential.
I'm starting to wonder if he has alliterative motives, is he developing a rubber band drive table? I usually wait in anticipation of his latest blog, they have been very interesting and his scientific approach is welcome. It's just that his latest post reads more like a British press release.
BillWojo
Remember that ANY company that builds rubber band drive TT has a vested interest in killing off DD tables, the British press tried to help it's industry for years with all kind of printed nonsense.
Heck, we don't even know what table it is and where it came from. Maybe it has all kinds of issues.
The thing that bothers me is that Alex Korf would even print this when a gut-check tells you that something is very wrong in the measurements. It's going to be taken as gospel by all those that have a love for rubber band drive tables and ignite a firestorm.
I also question the flux leakage as well. Not saying that there isn't any but I imagine it's so low at the cartridge area to be inconsequential.
I'm starting to wonder if he has alliterative motives, is he developing a rubber band drive table? I usually wait in anticipation of his latest blog, they have been very interesting and his scientific approach is welcome. It's just that his latest post reads more like a British press release.
BillWojo
I have had issue with the measurement methodology of Korf previously. I am a qualified electronics tech and metrologist with over 40 years of experience under my belt repairing/testing and calibrating sensitive electronic test equipment, so I'm not coming at this as a DIYer.
The issues i have with graph are
a) There is no uncertainty data. Without uncertainty info you can not make a call on the measurement. An example here measuring a stack of paper with a ruler or a micrometer.
b) There is no detail on what type of sensor is used or how it's mounted. Is the mount exactly the same on both TT's. How many measurements were taken and what is the tolerance of the sensor.
c) There is no clarification over what time period the speed fluctuated, was it 1 revolution or was is 1ms.
I machined a new platter for my SP10mk2 with greater inertia. The measurements showed clearly that overall W&F remained constant. What changed was instantaneous speed improved and drift was slightly worse. These measurements were performed 5 times with less than 2 counts difference between all 5 samples. The increase in inertia had slowed the PLL servo. So there is something in what he posted, it's just not clearly presented.
I remember when I was being trained in metrology my mentor said to me if you put 1 foot in a bucket of boiling water and the other in a bucket of dry ice your average temperature is fine, but you will lose both feet. The point is without detail of the measurement methodology the result is questionable. In statistical analysis it's not what you leave in the data but what is left out that influences the result.
WRT stray magnetic flux this is potentially something to investigate. I know the SP10mk2 has a metal structure that the magnet is mounted in. This metal can will reduce stray magnetic fields. I have not spent any time to investigate this effect. I will say my SP10mk2 spanked both my previous LP12 and Rega RP8.
Lastly every single motor bar none has torque ripple. Some are better than others and have very low ripple. Cogging is associated with cored motors as the pole passes the core it's attracted to the core and released which causes the cogging. The tradeoff is coreless motors have less torque than cored motors. The Technics SP02 motor used in the later Neumann lathes were cored motors.
The issues i have with graph are
a) There is no uncertainty data. Without uncertainty info you can not make a call on the measurement. An example here measuring a stack of paper with a ruler or a micrometer.
b) There is no detail on what type of sensor is used or how it's mounted. Is the mount exactly the same on both TT's. How many measurements were taken and what is the tolerance of the sensor.
c) There is no clarification over what time period the speed fluctuated, was it 1 revolution or was is 1ms.
I machined a new platter for my SP10mk2 with greater inertia. The measurements showed clearly that overall W&F remained constant. What changed was instantaneous speed improved and drift was slightly worse. These measurements were performed 5 times with less than 2 counts difference between all 5 samples. The increase in inertia had slowed the PLL servo. So there is something in what he posted, it's just not clearly presented.
I remember when I was being trained in metrology my mentor said to me if you put 1 foot in a bucket of boiling water and the other in a bucket of dry ice your average temperature is fine, but you will lose both feet. The point is without detail of the measurement methodology the result is questionable. In statistical analysis it's not what you leave in the data but what is left out that influences the result.
WRT stray magnetic flux this is potentially something to investigate. I know the SP10mk2 has a metal structure that the magnet is mounted in. This metal can will reduce stray magnetic fields. I have not spent any time to investigate this effect. I will say my SP10mk2 spanked both my previous LP12 and Rega RP8.
Lastly every single motor bar none has torque ripple. Some are better than others and have very low ripple. Cogging is associated with cored motors as the pole passes the core it's attracted to the core and released which causes the cogging. The tradeoff is coreless motors have less torque than cored motors. The Technics SP02 motor used in the later Neumann lathes were cored motors.
I guess the diagram he shows is a histogram. Claiming to have 5000 speed measurements per second means that he is measuring an momentary speed every 200 usec. Without knowing how he is measuring and what the accuracy is, the graph at least for me is meaningless.
Flux leakage: What flux is leaking?? If the strong magnet with e.g. eight sections like in the DUAL motors is used, the field changes eight times per revolution. This will induce a signal with approx 2.2 Hz in a magnetic cartridge (.55 Hz x 4 as two adjacend magnet poles are of different polarity creating 4 full waves per revolution). And the coils change their current with identical frequency. As this signal is magnetically induced (and not mechanically via the stylus) it does not create modulations of audio frequencies (if the field is not so strong that it saturates the magnetic path in the cartridge. If this would be the case, the cartridge would already start ratteling).
Bottom line, if there is flux leakage, it should not harm - induced frequencies are too low and will get filtered out by most phono pre-amps.
Flux leakage: What flux is leaking?? If the strong magnet with e.g. eight sections like in the DUAL motors is used, the field changes eight times per revolution. This will induce a signal with approx 2.2 Hz in a magnetic cartridge (.55 Hz x 4 as two adjacend magnet poles are of different polarity creating 4 full waves per revolution). And the coils change their current with identical frequency. As this signal is magnetically induced (and not mechanically via the stylus) it does not create modulations of audio frequencies (if the field is not so strong that it saturates the magnetic path in the cartridge. If this would be the case, the cartridge would already start ratteling).
Bottom line, if there is flux leakage, it should not harm - induced frequencies are too low and will get filtered out by most phono pre-amps.
Any eccentricity between the rotor, stator, bearing and platter in a DD turntable will manifest as speed variation (wow). If Alex has disassembled the motor then it's possible he didn't pay enough attention to alignment upon reassembly. This effect is additional to record eccentricity. On Technics TT's the clearances between parts (spigots etc) are greater than the alignment tolerance necessary to meet wow/flutter specs. Ask me how I know.
It's possible that Technics (and others) used more precise alignment/assembly techniques than can be achieved merely from the spigots. Be aware of this before you pull a DD TT apart.
It's possible that Technics (and others) used more precise alignment/assembly techniques than can be achieved merely from the spigots. Be aware of this before you pull a DD TT apart.
Reopening this because discussion is IMO relevant.
His posts regarding Direct Drive are a let-down for me, otherwise I considered the "Korf Blog" a rush of fresh news; it has been quite a long time since somebody published new experiments on turntable aspects and published it.
There "Turntable Drives. Part V, Direct Drive Torque Ripple" post has an even more worrying claim:
"Back in 1970s, generating a clean synchronised sine current was very hard. Most manufacturers used a simple trapezoidal ("on/off") commutation instead"
I find unbeleivable how somebody who takes a lot of time researching other stuff can make such a claim.
I can't find any 1970s or 1980s half-decent DD motor which used "on-off" commutation.
The Sony BSL and the Hitachi Uni-torque motors, as well as most Pioneer and late-generation Technics DD motors, all of them have something in common: They use linear hall sensors... those sensors sense the change in magnetic field produced by the poles and the output of these sensors is a sine wave ... and this very sine wave is then amplified and fed to the corresponding coil(s)... this makes the motor rotate. All the aforementioned motors work in that way.
The Dual EDS-1000 motor works in a similar way: hall sensors generate the sine waves that drive the coils.
And the early 1970s technics motor (as used in the SP10, SL1100, SL1200MK1, MK2, SL10 and others) uses coils (instead of hall sensors) but the operating principle is the same: the coils generate a current proportional to the magnetic field -which will be a sine wave- and this is amplified and used to drive the motor.
It is amazing that Mr. Korf would make such a claim, which means he hasn't taken the time to research how these motors work. Surprising for a guy that apparently does a lot of (interesting, useful) reseach on tonearms, headshells, etc.
Perhaps he thinks Hall sensors give an "on-off" output? This is only true of a certain kind of hall sensors.; for example the ones used in inexpensive CPI cooling fans. Not the kind used in DD turntables, which are linear hall sensor.
Additionally, claiming "Back in 1970s, generating a clean synchronised sine current was very hard" is just plain wrong. The Thorens TD125 motor controller (1969) does generate a two sine wave outputs placed exactly 90° apart... using techniques that already existed since the invention of vacuum tubes...
Otherwsie, as I said, his blog posts are very interesitng, even exciting.
His posts regarding Direct Drive are a let-down for me, otherwise I considered the "Korf Blog" a rush of fresh news; it has been quite a long time since somebody published new experiments on turntable aspects and published it.
There "Turntable Drives. Part V, Direct Drive Torque Ripple" post has an even more worrying claim:
"Back in 1970s, generating a clean synchronised sine current was very hard. Most manufacturers used a simple trapezoidal ("on/off") commutation instead"
I find unbeleivable how somebody who takes a lot of time researching other stuff can make such a claim.
I can't find any 1970s or 1980s half-decent DD motor which used "on-off" commutation.
The Sony BSL and the Hitachi Uni-torque motors, as well as most Pioneer and late-generation Technics DD motors, all of them have something in common: They use linear hall sensors... those sensors sense the change in magnetic field produced by the poles and the output of these sensors is a sine wave ... and this very sine wave is then amplified and fed to the corresponding coil(s)... this makes the motor rotate. All the aforementioned motors work in that way.
The Dual EDS-1000 motor works in a similar way: hall sensors generate the sine waves that drive the coils.
And the early 1970s technics motor (as used in the SP10, SL1100, SL1200MK1, MK2, SL10 and others) uses coils (instead of hall sensors) but the operating principle is the same: the coils generate a current proportional to the magnetic field -which will be a sine wave- and this is amplified and used to drive the motor.
It is amazing that Mr. Korf would make such a claim, which means he hasn't taken the time to research how these motors work. Surprising for a guy that apparently does a lot of (interesting, useful) reseach on tonearms, headshells, etc.
Perhaps he thinks Hall sensors give an "on-off" output? This is only true of a certain kind of hall sensors.; for example the ones used in inexpensive CPI cooling fans. Not the kind used in DD turntables, which are linear hall sensor.
Additionally, claiming "Back in 1970s, generating a clean synchronised sine current was very hard" is just plain wrong. The Thorens TD125 motor controller (1969) does generate a two sine wave outputs placed exactly 90° apart... using techniques that already existed since the invention of vacuum tubes...
Otherwsie, as I said, his blog posts are very interesitng, even exciting.
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The coils operate as a type of variable reluctance pickup where a ferrous mask generates the commutation signals which are used as the drive signal. For the SP-10MKII/2A/3 the signal is trapezoidal, while for the 1200MK2, SP-15, etc., it’s square.
Whether the drive is sine or trapezoidal is totally down to the topology of the motor. The back EMF waveform of the motor on his blog is clearly trapezoidal and even if driven with a sinewave, harmonics will be created because of the mismatch between BEMF and drive signal. The SP10mk2 motor has perfect sinusoidal BEMF and the drive phases I have measured show very clean signals, which is why the SP10mk2 is such a great TT. I'm sure all of the highend Jap DD TT's had the same attention paid to motor engineering to ensure sinusoidal BEMF.
It was not impossible to engineer a DD drive that was sinusoidal but it cost a lot more than the much more simple trapezoidal drive. I suspect this is the reason that lowend consumer DD TT's had such a bad rap. I can only talk to Marantz as I was a bench tech for them in the early 1980's and the lowend DD's were average at best, but the highend TT1000 was superb.
In engineering terms a trapezoidal motor is a BLDC and sinusoidal is PMSM the differences are in how the stator and magnet poles are designed.
In a PLL DD this is only the feedback and it doesn't matter what signal is generated, sinusoidal, trapezoidal or squarewave. The input to the feedback amplifier will condition the signal, in the SP10mk2 case it's a schmitt trigger. In very simple terms the electronics compares this feedback signal to the reference to maintain speed.
The Victor TT81 and SP10mk2 also have sensors in the motor to tell the electronics where the magnet poles are in relation to the stator. This is so the electronics can vary the drive signal depending where the magnet poles are in relation to the stator coils, this reduces torque ripple as the magnet passes the stator slot.
In the SP10mk2 there are 3 sensors in the stator and a copper ring on the FG rotor that are used for rotor position. The electronics can then vary drive between the phases. Victor call this this their Bidirectional Servo. Basically the drive circuit can push/pull each magnet pole.
One phase pushes the magnet pole and the next phase pulls the magnet pole. If drive current is not varied there will be more torque in the middle where push and pull are strongest and least where only and push or pull are used nearest the coil winding.
One phase pushes the magnet pole and the next phase pulls the magnet pole. If drive current is not varied there will be more torque in the middle where push and pull are strongest and least where only and push or pull are used nearest the coil winding.
Let's clean up the ambiguity here.
Firstly, I've not seen any Victor/JVC literature mention bi-directional servo control. What they do mention is "positive and negative servo control" (from the TT-101 SM, but is the same in others):
Seemingly "bi-directional" is just a colloquial. Today we find this an odd thing to point out, as all PLL controlled 'tables have "positive and negative" servo control, but when these units came out circa 1976/1977 it was something new. The comparison was obviously previous F/V servo 'tables, not other PLL 'tables.
Note that in either case the subject is "servo". They're not talking about motor drive - positive and negative drive is simply full-wave, and "bi-directional" in this context really doesn't make any sense.
In regard to Technics and the SP-10 type motor specifically, the drive waveform is derived directly from the copper mask that the three position "sensors" (variable-reluctance pickups) read. In addition to shaping the drive signal, and by the very nature thereof, their signal also controls commutation. This is the same function linear hall elements perform in other motors, as @flavio81 pointed out. I've never seen a motor use hall sensors to implement an FG.
There is no provision for the drive electronics to "vary the drive phases" or "vary the drive between phases". The waveform and phase relationship is set by the copper mask and drive voltage control is applied across all phases in unison.
The drive waveform is trapezoidal, BTW.
Firstly, I've not seen any Victor/JVC literature mention bi-directional servo control. What they do mention is "positive and negative servo control" (from the TT-101 SM, but is the same in others):
Seemingly "bi-directional" is just a colloquial. Today we find this an odd thing to point out, as all PLL controlled 'tables have "positive and negative" servo control, but when these units came out circa 1976/1977 it was something new. The comparison was obviously previous F/V servo 'tables, not other PLL 'tables.
Note that in either case the subject is "servo". They're not talking about motor drive - positive and negative drive is simply full-wave, and "bi-directional" in this context really doesn't make any sense.
In regard to Technics and the SP-10 type motor specifically, the drive waveform is derived directly from the copper mask that the three position "sensors" (variable-reluctance pickups) read. In addition to shaping the drive signal, and by the very nature thereof, their signal also controls commutation. This is the same function linear hall elements perform in other motors, as @flavio81 pointed out. I've never seen a motor use hall sensors to implement an FG.
There is no provision for the drive electronics to "vary the drive phases" or "vary the drive between phases". The waveform and phase relationship is set by the copper mask and drive voltage control is applied across all phases in unison.
The drive waveform is trapezoidal, BTW.
Thanks for the input, JP.
As for Alex Korf (bless him, i must say i'm a fan of his articles on tonearms), perhaps he should take a look at AC direct drive motors, like the ones used by Denon, Sony and others.
These AC motors, at least the early Denon and Sony motors, have copper (or similar material) rotors and should give exceptionally smooth rotation. They are not controlled by frequency variation but by voltage (or current) variation.
My take is that the reason Technics used DC motors was to easily achieve the huge torques needed for instant cueing. Which is an important feature if you want to do a radio/broadcast turntable and thus compete with the earlier idler drive broadcast turntables. Which do feature instant cueing and enormous motors.
A motor with magnets in the rotor (and iron cores in the stator) will probably be more powerfull than a non-ferrous-rotor (or "eddy current rotor" IIRC) AC motor of similar size.
As for Alex Korf (bless him, i must say i'm a fan of his articles on tonearms), perhaps he should take a look at AC direct drive motors, like the ones used by Denon, Sony and others.
These AC motors, at least the early Denon and Sony motors, have copper (or similar material) rotors and should give exceptionally smooth rotation. They are not controlled by frequency variation but by voltage (or current) variation.
My take is that the reason Technics used DC motors was to easily achieve the huge torques needed for instant cueing. Which is an important feature if you want to do a radio/broadcast turntable and thus compete with the earlier idler drive broadcast turntables. Which do feature instant cueing and enormous motors.
A motor with magnets in the rotor (and iron cores in the stator) will probably be more powerfull than a non-ferrous-rotor (or "eddy current rotor" IIRC) AC motor of similar size.
All electrical motors are AC, even DC brushed motors. It is only when you look at them on the power supply side that some of them seem DC.
Drive signal and commutation method are the factors that typically distinguish AC motors from DC motors from what I've seen. There is appreciable gray area with some of these designs. The SP-10 type is one that seems rather unique - trapezoidal waveform but not trapezoidal commutation (6-step) and a sinusoidal BEMF.
The Denon are indeed AC delta - they include a mains frequency selections switch to change the phase shift to the motor. The only Sony I've worked on was a TTS-8000 but that was typical hall commutation. I've a PS-X9 that I've not gotten in to yet but I believe that motor is coreless also with hall commutation.
I've not seen any OG DD that is controlled by frequency variation.
The Denon are indeed AC delta - they include a mains frequency selections switch to change the phase shift to the motor. The only Sony I've worked on was a TTS-8000 but that was typical hall commutation. I've a PS-X9 that I've not gotten in to yet but I believe that motor is coreless also with hall commutation.
I've not seen any OG DD that is controlled by frequency variation.
Hi @JP
For Sony DD turntables, Sony went through different motors...
At the beginning, an AC motor (example: PS-2250, TTS-2500, TTS-4000), almost the identical AC motor configuration as the earlier Denon DD turntables. Mains frequency, controlled by voltage variation. As mentioned, Denon did the same, also i've found the Toshiba SR-410 and SR-510 use exactly the same scheme too.
Then some 70 models had a very strange DC motor (they called it "saturable inductor" motor and "brush and slotless" motor), which HAS brushes, two brushes. It's very strange and i haven't really found the patent nor detailed schematics, but for example it has multiple capacitors inside the stator assembly, what seems to be one capacitor per slot. The PS-4750 has this design. (Curiosly the TOTL PS-8750 is an AC motor instead)
The late 70s models have what Sony calls the "BSL" motor (which they mean "brushless and slottless" indead of "brush and slotless" for the prior motor), this is a DC motor with two sets of "butterfly-looking" coreless coils where linear hall elements are the source of the coil driving signal. The PS-X50, 60, 70, 75 and others use this kind of motor, as well as many Sony DD turntables.
Your TT is the PS-X9, it was the absolute top of the line, something to make Sony lovers drool. It would be interesting to see which kind of motor is inside it, perhaps Sony would choose an AC motor again, just like it did with the PS-8750?
I've read that the PS-8750 was the reference TT for the "Boston Audio Society" (BAS) for a long time.
Neither have I, however i've seen mentioned somewhere that there was some DD turntable using an AC motor driven by the mains frequency. Which means a huge number of poles of course, I understand 216 poles for 60Hz and 33.3333RPM. Who knows, maybe somebody did it. Some Fisher DD turntables have the rotor at the edge of the platter and boast 120 poles, perhaps those TTs are frequency controlled (?)
For Sony DD turntables, Sony went through different motors...
At the beginning, an AC motor (example: PS-2250, TTS-2500, TTS-4000), almost the identical AC motor configuration as the earlier Denon DD turntables. Mains frequency, controlled by voltage variation. As mentioned, Denon did the same, also i've found the Toshiba SR-410 and SR-510 use exactly the same scheme too.
Then some 70 models had a very strange DC motor (they called it "saturable inductor" motor and "brush and slotless" motor), which HAS brushes, two brushes. It's very strange and i haven't really found the patent nor detailed schematics, but for example it has multiple capacitors inside the stator assembly, what seems to be one capacitor per slot. The PS-4750 has this design. (Curiosly the TOTL PS-8750 is an AC motor instead)
The late 70s models have what Sony calls the "BSL" motor (which they mean "brushless and slottless" indead of "brush and slotless" for the prior motor), this is a DC motor with two sets of "butterfly-looking" coreless coils where linear hall elements are the source of the coil driving signal. The PS-X50, 60, 70, 75 and others use this kind of motor, as well as many Sony DD turntables.
Your TT is the PS-X9, it was the absolute top of the line, something to make Sony lovers drool. It would be interesting to see which kind of motor is inside it, perhaps Sony would choose an AC motor again, just like it did with the PS-8750?
I've read that the PS-8750 was the reference TT for the "Boston Audio Society" (BAS) for a long time.
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I've not seen any OG DD that is controlled by frequency variation.Neither have I, however i've seen mentioned somewhere that there was some DD turntable using an AC motor driven by the mains frequency. Which means a huge number of poles of course, I understand 216 poles for 60Hz and 33.3333RPM. Who knows, maybe somebody did it. Some Fisher DD turntables have the rotor at the edge of the platter and boast 120 poles, perhaps those TTs are frequency controlled (?)
Hi, Although Korf have produced some very helpful bogs & charts. Remember they are a business, trying to drum up sales. It's hard out there for a small company, trying to break into the international market.
I have 3 decks, all of very good quality, a D/D - Idler wheel & belt driven. My arm is high end & I run a re-tipped Accurhase AC-2 MC cart. Using the same arm/ cart & phono amp. I can not tell the difference between the 3 at my listening chair. Obviously close up I can hear the Idler drive, due to it's very nature.
I am totally against going back to the 70's method of comparing meaningless specs & saying this is good & that is bad.
Trust your ears
Cheers
I have 3 decks, all of very good quality, a D/D - Idler wheel & belt driven. My arm is high end & I run a re-tipped Accurhase AC-2 MC cart. Using the same arm/ cart & phono amp. I can not tell the difference between the 3 at my listening chair. Obviously close up I can hear the Idler drive, due to it's very nature.
I am totally against going back to the 70's method of comparing meaningless specs & saying this is good & that is bad.
Trust your ears
Cheers