Turnable database with TimeLine

Thanks for the great post, Richard. I think of a 3-phase AC synchronous motor is a cousin to a DC motor. In fact, the Victor TT101 does make +/-12VDC in its PS to supply its motor. Likewise, I thought the SP10s used DC motors. But the Denon uses a 3-phase AC motor. Whether or not all of that is correct is not my point or question. What I do wonder about are the "modern" DD turntables, such as the NVS, the Beat, and the new VPI, among others. Their adverts all claim that they've avoided the nasty [sic] effects of a servo by using a 3-phase AC synchronous motor that holds speed by virtue of the mechanism you've just described, sans any servo. At least 2 of those 3 mentioned use fairly massive platters, so it seems to me they are able to give up the servo mechanism by virtue of the mass effect afforded by their platters, much as is the case for some of the better belt-drives. But still, the platter must first slow enough to trigger the speed up commanded by the AC synchronicity. Do you have any thoughts on this?
Thanks.

01-03-14: Richardkrebs
Many years ago I owned a Goldmund Studio. In an attempt to improve it, I built a larger power supply. I scoped the supply output while playing a record. To my amazement I could see the supply output voltage being modulated by the music I was playing. There is only one conclusion that one can draw from this finding. ... The platter speed itself was being modulated by the music in the form of stylus drag. This was occurring even at quite high frequencies. It is likely that the Goldmund would do well in the time line test, since it's average speed would be close to the mark. But what was happening on a micro level was a completely different story.

I thought this thread was to be a database of timeline checked turntables - not a guessing session.
I heard that modified Goldmund mentioned above on numerous occasions and it got slaughtered by a stock standard unmodified Kenwood L07D. It was shown to be so bad in terms of musical timing that the owner promptly went and bought a Technics SP10. So either the mods were deleterious to the performance, or the Goldmund is not stable. Whether it would pass the timeline test is speculation. If it does then clearly the timeline test is not an indicator of whether a turntable will preserve the musical timing as recorded.

There is only one conclusion that one can draw from this finding. ... The platter speed itself was being modulated by the music in the form of stylus drag

Since you failed to measure the platter speed, then your conclusion is simply a guess.

01-03-14: Richardkrebs
Yes the Goldmund used a JVC motor.
Like, I suspect most DD designs, it is a synchronous motor. The rotor ( platter) is compelled to follow the rotating field, back slightly in phase.

I'll explain how the Goldmund motor really works.
Firstly Brushless DC motors require an electronic controller to continually switch the phase to the windings to keep the motor turning.
Secondly, the Goldmund Studio uses a coreless DC servo JVC motor. The JVC motor has a frequency generator which generates a frequency when the motor is running. A phase comparator then compares that motor generated frequency to a reference frequency generated separately by the Quartz Crystal. Any difference detected is then fed back to the servo controller to correct the speed. The motor will not run or hold speed without the servo running.

In a nutshell the speed accuracy of the Goldmund will be dependent on the quality of the electronic controller and the quality of the servo.

Note that Goldmund themselves then added lead mass to the platter to increase the inertia. They claimed that this was required to smooth out any speed irregularities. Clearly Goldmund themselves did not believe that properly designed servos are adequate in of themselves for accurate and consistent speed when they designed the Goldmund Studio. The Goldmund Reference TT was belt drive, high inertia.

Brinkmann allude to the issues of servos in their white papers. They have gone for a low torque minimally invasive ( S L O W ) servo action in the interests of sound quality.

It is interesting to note that in listening tests comparing the Goldmund Studio and the Kenwood L07D side by side, the Kenwood has better timing, despite the servos not kicking in until a speed error threshold of +-3% is reached. The Kenwood L07D relies on inertia and back emf within the motor to hold the speed.

01-03-14: Richardkrebs
The rotor lags in phase slightly behind the rotating field. If it didn't it would produce no torque. Increase the load and this phase angle increases and the motor draws more current. But the motor then continues rotating at the same speed. .

If the phase angle increases and the motor draws more current, this means that the motor has lost speed. The servo kicks in and brings the speed back up.

01-03-14: Richardkrebs
This is what I observed on the scope. The motor was responding to stylus drag, literally note by note. It was not showing a problem, it was showing the motor working properly and the relative enormity of stylus drag.

The conclusions from your testing are wrong. What you are measuring on the scope is the solution that Goldmund has provided to record playback. You are measuring the sum total of the quality of their controller, their motor, the inertia built in to the platter and the quality of engineering and design for THAT particular turntable. You have not measured the platter speed.
If you had scoped the power supply of a Garrard idler, Micro Seiki or EMT you would get different results.

01-03-14: Richardkrebs
For a DD drive, it has a reasonably high inertia platter but this was not enough to "push thru" these load changes. .

The Goldmund Studio does not have a high inertia platter. The Goldmund Studio platter weighs about 3kg. Both the Technics SP10mkIII (11kg) and Kenwood L07D (7.7kg with stabiliser) DD’s ( comparable products in terms of market positioning ) have significantly higher inertia - 2-3 times higher.
Of course the Micro’s are 10-15kg, EMT 927 5kg distributed to a 16” platter to achieve an equivalent 50kg, and my Final Audio has 22kg. These are what I would describe as high inertia TT’s.

01-03-14: Richardkrebs
Wrap a properly designed servo around this type of motor and the phase angle change with load is reduced. .

There is no consensus on what is a “properly designed servo” for Direct Drive turntables.
The Technics SP10 servos use algortihms to estimate predicted errors and employ rapid response times (limited by the technology of the day). The servo action includes error and overshoot.
The Victor/JVC decks use an averaging process to provide a smooth transition when servos call for speed correction.
The Kenwood L07D uses no servo, and relies on inertia and back emf unless the speed error is quite large, at which point the servos kick in and additional torque is applied.
The Brinkmann uses very slow servos for soft recovery.

In reality the L07D and Brinkmann DD's are closer in conception in maintaining accurate speed behaviour to a high inertia, non servo AC synchronous motor driven solution like the Micro or Final than they are to the Technics SP10. I also note that Brinkmann claim that 15kg is the minimum platter weight required for adequate speed stability.

Lew
The Beat appears to uses electromagnetic drag to "pre load" the motor. Moving the rotor back in phase relative to the rotating field. I am not sure about the other TTs mentioned. The theory being that this artificial torque demand is significantly higher than any seen due to stylus drag. It seems like an elegant solution. The question is of course how does it sound? I do know that we are very sensitive to micro pitch changes and that stylus drag is large. The choice of open loop or closed loop speed control in ones TT is a personal one and we all make that choice with open ears and hopefully open minds.

Very interesting thread Peter. Thanks for getting it going. I'll volunteer my Grand Prix Monaco to be a test subject, as soon as I can barrow Albert's timeline...

The Grand Prix claims highly accurate speed control (from their web site):

"DSP Signal Processing / Active Feedback Loop Technology:
Our speed control features a CPU utilizing a Digital Signal Processor that interrogates and maintains the speed of the platter thousands of times each revolution and thereby ensures frequency accuracy and distortion free playback that is simply not possible with conventional types of drive systems. A highly accurate test procedure has demonstrated the speed error to be an extremely low .002%! Speed accuracy equals frequency accuracy, which means there is virtually no distortion on playback."

Peter, one thought on your SME, have you ever cleaned the motor pulley (and sub platter) and then flipped the belt over? I used to do this with my SME 20/2 every 9-12 months and it did seem to sound better...(that is unless you used a new belt for the test)...then I'd replace the belt every 18-24 months...