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. |
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.
Provided the controller is properly adjusted, it will rotate at the correct average speed. This is the beauty of a synchronous motor. In other words, I would be surprised if a properly designed and adjusted DD TT using a synchronous motor failed the timeline test. This assumes a stable reference frequency, quartz.
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. 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.
For a DD drive, it has a reasonably high inertia platter but this was not enough to "push thru" these load changes.
The only way for the motor to slow down, on a continuous basis, if it's field is rotating at the correct speed, is for a massive retardation torque to be applied. This would be extremely violent as the rotor rode backwards over the poles.
Wrap a properly designed servo around this type of motor and the phase angle change with load is reduced. |
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. |
HF Dover
You appear to have invented a whole new type of motor. One where the rotor follows the rotating field with zero phase lag.
Please explain to us all how such a motor produces any torque.
Also one which "Instantly" draws more current and supplies more torque.
Please explain to us how it senses the need for more torque.
Caution needs to be exercised when quoting manufacturers advertisements. |
Regarding the eddy current brake as used in the Beat TT
Go to teresaudio.com and follow the link...micro precise speed technology.
The Certus motor.
As I said earlier, this is an elegant design. |
Halcro
For consistency, as you have done, it may be advisable for people posting videos to show say 1 minute of stylus raised followed by a time with it lowered playing some robust music. Something like the lovely piece you played.
While having the stylus raised presents no load, it would give us at least a guide of how the TT would perform when tracking very lightly modulated grooves followed by heavily modulated ones. |
Lewm
8 feet to the wall, that is a tough test.
Don_c55
I understand your skepticism, but would argue that these micro speed changes are indeed audible.
Sure we have tape machine speed instability along with eccentricity problems. That said, speed changes due to stylus drag, which is where this thread has kinda coalesced, are fundamentally different in nature. This because the music itself causes it. This results in a subtle smearing and bending of the notes. A general softening of the soundscape. The effect can be " nice " but it is not what was laid down on the record. |
Halcro
Agree it would be nice to have consistency in the test methods.
HF Dover
Phase lag is a result of drag, not a cause of it. |
Halcro
Ok.., what is the music this time?
Nice
Lewm
I missed your report on the MK3 with the Timeline. Are you able to repeat them here?
Thanks |
Don
I don't think that any of us are claiming perfection. Well maybe one of us is. But we are, most of us, on an impossible quest for same.
Religious? Maybe a substitute, but it is a whole lot of fun.
Given the manyfold problems of vinyl playback, it is a miracle that anything resembling music is possible at all. So maybe a Devine presence is at work. |
While I agree that it would be best for every test to use the same LP, this is likely an unrealistic requirement since it presupposes that every tester has this disc.
Futher " The sum total of the error at the end of the test would be truely comparable between turntables"
This would not necessarily be an informative result.
Say a TT that isn't speed stable, which has adjustment is set for correct speed at moderate groove modulation. When playing heavy grooves it may under speed when playing light grooves it may over speed. The average speed over the test could be correct with the laser back on its starting point at the end, apparently passing the test.
The no load followed by some, ideally standard, heavy grooves is an arduous test. If it passes this it is likely that it would pass real world playback.
This provided the laser is observed under no load conditions and this is the control, (TT adjusted if possible, necessary, to zero drift)
Also if a TT passes this test it would be reasonable to assume that it would be ok with light grooves.
But, as I implied with the Goldmund comments, we are not seeing how the TT performs at a note by note level. |
Actusreus.
Assuming
1)the stylus drag is constant throughout the LP
2) the TT slows due to stylus drag,
3) speed control is open loop ( maybe)
Then it will naturally increase in speed as it tracks towards the centre grooves.
This is a torque moment effect. On the outer grooves, the drag is acting on a larger radius so it requires more torque from the motor to counteract this. |
Actusreus.
Re my last post and tracking progressively towards the center grooves.
For clarity, I should have said it progressively becomes less slow as the torque demand decreases.
Lew
The servos we use at my work can have response times in the micro sec range. The limiting factor is the frequency response of the actuator they are controlling, so in practice, slower than this would suggest.
The goldmund feedback is very fast as observed on the scope current draw plot. Much faster than 0.3 sec . As I posted earlier, it was responding note by note. A startling observation.
Other TT manufacturers would, I suspect taylor the response time to suit their design philosophy. |
Dover,
Less than 0.3 seconds servo response came from observing current draw modulation in the kHz range, in sync with the music being played. The test was done with the system muted and live to check if it was some sort of acoustical feedback phenomenon.
There is only one possible cause of this.....
The platters speed was being modulated note by note. The controller / motor assembly was responding to this and " attempting" to stabilise the speed. This shows a controller frequency response in the kHz range. A speed that is not unusual in the industrial field.
Suggest a better test piece for you would be " Final Fantasy" by Nobuo Uematsu. |
Don.
re your post to Lew. "Nobody has used the timeline.... and detected any audible sonic changes."
Firstly. The time line shows average speed per revolution. A TT that passes the timeline test has a "consistent average" correct speed, no more than this. We cannot see what is happening at a micro time interval level, but we can infer.
I would be surprised if anyone could detect the adjustment of this average speed back to say 33 1/3 if the TT in question showed slowing due to stylus drag. We are talking small changes in absolute frequency with this adjustment. The owners of these TT's cannot however adjust the TT's dynamic speed stability since this is intrinsic in how the TT was built and designed. This includes its power supply and controller. Of course there will be no audible changes since these owners are making no changes to the TT build, design itself.
However there is, I believe, at least one person posting here who has direct experience with the audible effects of an improvement in resistance to stylus drag and dynamic speed stability.
That poster is........you.
You reported a "huge" improvement in the VPI when using the SDS controller.
This device makes the motor run more consistently. What you heard was a platter rotating more accurately at a micro speed level. It is quite possible that its average speed with a load, did not change, so no difference would be seen with a Timeline. But you correctly reported the positive changes.
It would make logical sense that a motor which runs more consistently would be better able to respond to and minimise speed changes due to a dynamic load, such as that caused by stylus drag. So it would further be logical that the VPI would show greater resistance to speed sag when lowering the stylus (no load, with load) and we would likely see an improvement if this test was done with the Timeline.
So we can say that you are a champion of a design that reduces speed changes due to stylus drag.
|
Lew
A crude but effective test for RFI which is perhaps worth trying.
If you have a portable AM radio. Tune it off station and move it around your room listening for increased static. Maybe something there that is not at first apparent. |
Lew
Apart from static issues, which I'm sure you have considered, are there any transmitting devices near by? Something with more output than a cell phone?
VHF, RT, radio ham neighbor? |
Peterayer.
Yep, I went back and had a look at the posts, 05-29-12. on "Turntable Speed Accuracy" thread
Heavy platter and thread drive. |
Lew
I know that it is a leap but, we have experienced problems with servo feedback systems in our, hydraulic industry.
This where shielding has been inadequate. ( yes I thought of Halcro's birthday suit) while simultaneously being exposed to RFI.
A famous one was where the Cats hydraulic lifted stage in London would move all by itself. Problem was traced to poor shielding letting in transmissions from the ubiquitous London cabs outside.
When the obvious causes are exhausted we need to explore the path less travelled.
Good luck |
Halcro
I have no doubt that they are better au-naturel.
It makes sense to eliminate this material from around the motor. Imagine the fields set up in it due to the power supply and the rotating magnets.
Fields acting on sensitive analogue circuits.
It would likely be better still to move the PCB's away from the motor.
I remember you talking about this?
Removing the shield though does open the very remote potential to outside RFI issues. But I suspect, as earlier, a very big leap. |
Lew.
Good points raised. Perhaps I should have said "little doubt" Experience has shown that doing away with shielding, provided their is no problematic external RFI, is "almost always" better.
Don't kick it too hard. |
Lew
Bill T advises that electonics of that era be kept powered continuously to improve longevity.
From memory it was something to do with the substrate used in IC's of the time.
Others have noticed sonic benefits when doing this regardless of the vintage. Maybe worthwhile for reliability as well ?
Could imagine reliability benefits simply by avoiding the heat cycling switching on and off causes.
Good luck. |
Peterayer.
Further data for the list.
Way back on the "Turntable speed accuracy" thread, Dover posted results of stylus drag on his Final Audio TT.
From memory, this came out as 2mm movement per rev on a 400mm radius, when stylus lowered. Don't recall a video being posted, but this is still worthwhile info. |
