Turntable speed analysis part II

[John Channing]

Can someone explain to me what the various features in the plot correlate to or point me to a post where I can find this to save me trawling through 40 odd pages? I assume a perfect result would be a circle of radius 1 centred on (0,0). Thanks.

 

[Paul R]

Further from the centre is faster. One orbit of the trace is one turn of the turntable. There are five (IIRC) rotations plotted. The normalisation procedure (to make it centre on 0,0 and extend to 1) isn't perfect. A perfect result is indeed a circle radius 1. There's an example somewhere up thread.

For reference an 0.1% modulation depth tone looks like,

The two traces represent two modulating frequencies, one locked to the nominal rotation one not.

Paul

 

[pure sound]

This idea has been used on the Bauer DPS

And on the Verdier.

 

[John Channing]

And on the Verdier.

So, I was thinking about this and given the three variables:

• motor torque ripple
• platter moment of inertia
• viscous drag

And the objective of producing the minimum angular velocity fluctuations in the platter speed, who can write down the appropriate equation and by holding two of them constant solve it for viscous drag?

 

[John Channing]

At the moment I'm not willing to support it, so it seems best to keep it in house.

Why not just open source it and let the community support it? Unless you think it might be commercially valuable?

 

[flavio81]

That is interesting, I had always wondered why multi-motor designs like the Voyd worked and that could be the answer - if you align the peaks and troughs from different motors, you get a smoother total torque.

I'd say it's easier to use just one motor and more poles; or a motor with no cogging effect like the one in a certain line of vintage DD turntables (hush hush... it's a Sony).

 

[John Channing]

I'd say it's easier to use just one motor and more poles;

I did think about that and these motors are pretty small and there will be a limit to how many poles you can cram in. I suspect 24 poles is about the limit that practically can be fitted into a small turntable motor, so with 3 as the Voyd deployed you could in theory get the smoothness of a 72 pole motor.

In theory, you should be able to produce a perfectly smooth multi pole synchronous motor by carefully shaping the stator, rotor and adjusting the current (i.e. not a pure sine wave). I believe the original Pinnk Link tried the last one, introducing a small amount of third harmonic to smooth cogging.

 

[Paul R]

Why not just open source it and let the community support it? Unless you think it might be commercially valuable?

I've thought about it, we'll see. I think the being asked for support bit doesn't go away.

so with 3 as the Voyd deployed you could in theory get the smoothness of a 72 pole motor.

ISTM that if they are synchronous motors on the same supply then they all step in sync and reduced cogging isn't an obvious virtue of the arrangement.

I think the PLL arrangement of the Technics DDs or the small DC motor with brushes are the most promising approaches. The latter obviously being much more accessible.

There is some thought afoot to be able to get a resaonably precise reading of turntable speed without the added complication of a test record with its intrinsic errors and the added drag from the stylus. Then some clever chap might be able to model a belt drive in terms of the platter inertia, bearing drag, belt springiness and motor 'stiffness'. Although empiricism might be more productive...

Paul

 

[audioflyer]

I did think about that and these motors are pretty small and there will be a limit to how many poles you can cram in. I suspect 24 poles is about the limit that practically can be fitted into a small turntable motor, so with 3 as the Voyd deployed you could in theory get the smoothness of a 72 pole motor.

In theory, you should be able to produce a perfectly smooth multi pole synchronous motor by carefully shaping the stator, rotor and adjusting the current (i.e. not a pure sine wave). I believe the original Pinnk Link tried the last one, introducing a small amount of third harmonic to smooth cogging.

Have a look here.

Voyd; This turntable uses three synchronous motor.
http://www.pinkfishmedia.net/forum/showpost.php?p=1502368&postcount=272

Phonosophie No.3; This turntable uses a stepper motor.
http://www.pinkfishmedia.net/forum/showpost.php?p=1503278&postcount=297

Kuzmaesque with Geddon with LP12 synchronous motor.
http://www.pinkfishmedia.net/forum/showpost.php?p=1662193&postcount=690

 

[John Channing]

ISTM that if they are synchronous motors on the same supply then they all step in sync and reduced cogging isn't an obvious virtue of the arrangement.

You are correct. I had visualised physically rotating the motors relative to each other would push the cogging out of sync, but it won't.

 

[sq225917]

I wish someone with a souped up Technics DD would post a sample, they do seem conspicuous by their absence. I have a sneaking suspicion that they won't rival the Phonosphie and Kuzma...

 

[Stuart Mason]

I'd be happy to post a sample from mine, but it will be at least another month before renovations are finished and I can set it up again.

 

[sq225917]

Awesome.

 

[pure sound]

I must try to get a better test record & resubmit the Voyd & an SP10. I think the old HFS69 record I used had various problems not least eccentricity.

 

[YNWOAN]

And on the Verdier.

Is that right, I didn't know that.

 

[John Channing]

Have a look here.

Voyd; This turntable uses three synchronous motor.
http://www.pinkfishmedia.net/forum/showpost.php?p=1502368&postcount=272

Phonosophie No.3; This turntable uses a stepper motor.
http://www.pinkfishmedia.net/forum/showpost.php?p=1503278&postcount=297

Kuzmaesque with Geddon with LP12 synchronous motor.
http://www.pinkfishmedia.net/forum/showpost.php?p=1662193&postcount=690

The Voyd looks surprisingly poor.

The Phonosophie P3 looks practically perfect, Julian Vereker was clearly onto something when he chose to use his Aro on it. Just a shame it looks like a cheap veneered MDF fruit box. ;-)

Kuzmaesque looks pretty good, maybe you need to see how it looks with the test record used for the Phonosophie P3?

 

[sq225917]

I guess so, the one I use isn't well centred and reads the same on the three decks it has been used on in this thread.

 

[Paul R]

I think Audioflyer is using a copy of the same test record as sq. We can tell by the distinct wobble on one of the revolutions.

Paul

 

[John Channing]

I think Audioflyer is using a copy of the same test record as sq. We can tell by the distinct wobble on one of the revolutions.

It would be nice to collate all of the turntable measurement into an easy to view table (including things like test record) so it is easier to make comparisons.

 

[John Channing]

I am sure it is not, unless the motor happens to perform significantly worse under light load.

What happens is this: a belt drive is primarily a reactive (resonant) system formed by the inertia of the platter bouncing on the compliance of the belt. This system is lightly damped (only by the belt's losses) and thus of fairly high Q. It is also quite non-linear. So speed is expected to be modulated at the drive system resonance, its lower harmonics, the suspension system resonance (if any), and any motor breakthrough. Most of these are high Q. Adding damping in the shape of drag (i.e. a speed dependent force, such as viscosity, and not just a static force!) knocks down these modulation peaks.

I ran some simulations of this years ago, but it taught me that one needs quite a lot of viscosity if it is applied at the main bearing. An eddy current brake at the platter's perimeter seems to be more efficient. Oh, and when active controlled can be used for dynamic speed control, within limits of system bandwidth.

I think Werner's description of the dynamics sounds very plausible. Rather than viscous damping, The Roksan solution to this is:

"The motor, an AC synchronous type, is mounted in such a way as to allow it to move towards the platter in response to changing torque demands, again addressing the problem of the motor driving the chassis. Roksan believe that the platter should control the speed, the motor only adding a 'top-up' to keep the thing spinning. The problem is that when a synchronous motor is put under load it can cog or stall, with the Xerxes this situation causes the motor to move towards the platter to reduce drag and so keep the motor operating at optimum torque at all times. It also means that the belt is kept at constant tension rather than being tugged-and-released a situation that means the belt stores then releases energy in an uncontrolled manner."