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Turntable speed analysis

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I'll borrow the test record from sq and record mine if that is OK Paul (I dont think my test records have a 3KHz test tone)? The deck uses a belt drive and an AC sychronus motor - the chassis is suspended but fluid damped.
 
OK - I've changed the caps back on my phono boards (no drama there). I feel awful after this mornings dental surgery though :(.
 
Maybe, but can you deduce that from these 'steady state' recordings?
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No, I think it is a matter of reason that an LP12 won't smoothly(!) wow over a period of seconds in response to stylus drag. The subchassis wobbling around under all sorts of stimuli is another matter altogether.

Paul
 
Analogue Productions test record.
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I'll post images tomorrow but the spectrum of the tone looks the same as phix's, two separated peaks. I think a query to Analogue Productions might be in order.

Paul
 
Interesting you should say that Paulthat tone just never sounded like a pure tone, it always sounds a little 'phasey' if that makes any sense.
 
I will probably be able to borrow the record that Andrew used in a week or so, which would make for an interesting comparison.

I do wonder how much of these measured artifacts are down to the test record.
 
Simon's,

The tone,

sq_3150_base.png


Markedly similar to phix's taken from the same test disc.

The wow spectrum,

sq_3150_demod.png


Since producing this I found a review that suggested the pulley/platter ratio is 40:1, implying the motor speed is 1333 rpm (which seems fast?) or 22.2Hz. Expanding the plot suggests there may be a peak at 22.2Hz, but you have to be looking for it. OTOH the plot above shows a peak at 5.56, which is near enough 0.25 of 22.2Hz. Perhaps that's a motor artifact? The other peaks, other than the ones related to eccentricity, who knows....

The polar plot,

sq_3150_polar.png


I'm not seeing much in this one yet.

Paul
 
Now YNWOAN,

The tone,

YNWOAN_3150_base.png


Everybody is running at slightly different speeds, but not by enough to worry about...

The wow spectrum,

YNWOAN_3150_demod.png


It would be interesting to know more about the mechanical arrangement of the turntable, to see whether any of the modest peaks can be explained.

And polar,

YNWOAN_3150_polar.png


Paul
 
I meant how do you make difference on the polar plot between record eccentricity, idler beat and motor shaft eccentricity? How do you recognize those factors on the plot?
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The polar plot represents a small number of revolutions of the platter, the further the line gets from the notional centre the faster the speed. The shape you see has to be interpreted. I think your original plot had 6 corners, and the idler rotates 6 times to turn the platter once. I don't know what the regular kink on the current plot represents, perhaps it is just tolerance on the platter drive surface.

I think that displaying the polar data on different timebases might be interesting, on the basis of a belt orbit, or a whole number of drive pulley revolutions. And animating the plot might show up regularities.

I do wonder how much of these measured artifacts are down to the test record.
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Undoubtedly some are, after all the test record captures the errors inherent in the lathe. But as a basis for investigating turntables I think there's potential, on a comparative basis as you upgrade or modify.

Paul
 
Many thanks Paul for all of this - very interesting indeed.

It would be interesting to know more about the mechanical arrangement of the turntable, to see whether any of the modest peaks can be explained
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Could do, but I'm not sure what the modest peaks represent?
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The Polar plot is quite a good result it seems to me, or is it? It does seem quite even and round.

I think it is interesting to compare to sq225917's as exactly the same record was used. However, the deck configurations are quite different. he has an entirely non-suspended design that uses a DC motor with an elaborate feedback circuit (SME10). Mine is suspended (though the suspension is damped) and an AC motor is used with only a large transformer between it and the mains (no other filtering). Sq has a gimbal arm and I have a unipivot - both are belt drive and the platter masses are similar.
 
Could do, but I'm not sure what the modest peaks represent?
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The plot is the spectrum of the speed variations, from 0 - 20Hz. So the peaks on the left are the eccentricity of the record at 0.55Hz and 1.1Hz. On my Linn I have an obvious peak at 4.2Hz which is the motor rpm. If I repeat the test at 45rpm using a Lingo this peak moves up to 5.6Hz.

The Polar plot is quite a good result it seems to me, or is it? It does seem quite even and round.
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I'm not sure it is very useful except with obvious problems with, say, an idler. OTOH the potential of this form of display definitely warrants more investigation. If a complete loop were timed for a complete cycle of the belt then belt flaws might be revealed, for example.

Paul
 
Hmm..interesting...there doesn't seem to be any obvious peak at 4.2Hz.

I wonder if the spikes could be the result of commutator noise - what do you think?

I wonder if the jaggediness is the notorious suspension wobble?
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Doesn't sq225917's also show this quality? His deck doesn't have a suspension.
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It would be interesting to see the results from a Nottingham Analogue as these use slow speed, low torque, AC motors.
And, of course, a Technics 1200/1210!
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So, do these results really tell us anything? I can lock up the suspension on my deck quite easily (it has bolts to do exactly that) and could record again.
 
I have got a spare arm and cartridge, I could probably rig that up on the deck too, so that it creates variable drag - the extra mass may prove an issue, but it may not - will try tomorrow.
 
Using a pure 3150Hz tone from Audacity,

The base spectrum,

Ideal_3150_base.png


The spectrum of the demodulated tone,

Ideal_3150_demod.png


and the polar plot of the demodulated tone,

Ideal_3150_polar.png


Not very exciting, as one would expect.

Paul
 
Using a 3150Hz tone modulated fairly strongly at 4Hz,

The base spectrum,

fm3150_base.png


The spectrum of the demodulated tone,

fm3150_demod.png


and the polar plot of the demodulated tone,

fm3150_polar.png


I think this shows that the process produces sensible results.

Paul
 
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