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Turntable speed stabilty
Turntable speed stabilty
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Old 24th June 2017, 10:35 PM   #1
billshurv is offline billshurv  United Kingdom
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Turntable speed stabilty
Default Turntable speed stabilty

This is a new thread that has been spun off from here mechanical resonance in MMs to discuss the measurement, analysis and interpretation of turntable platter speed and stability, with the end goal of understanding it more and the fixes for it when needed. The Dr Feickert test records and apps have made measurements a lot more accessible, but the presentation and analysis part has been missing. For example Fremer always posts Feickert output on his reviews (e.g. https://www.analogplanet.com/content...200g-turntable) but you can see that they heavily filter the measurement, throwing away a lot of useful information (and the reviewer wouldn't know what to do with that information anyway).

Member Luckythedog has taken this up a couple of notches by plotting the speed on a polar chart, so you can see speed variation on a rotation by rotation basis. This immediately gives useful additional information you can use to work out where the speed changes might be coming from.

I should cover on this post why this all matters, after all we are talking subsonic effects in the most case. The reason it matters, along with record eccentricity and cart-arm resonances is that you are frequency modulating the wanted signal. Unless you are chasing a 'phat' analog sound this is something that it's worth trying to minimise.

Floor open for discussion
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Old 24th June 2017, 11:41 PM   #2
Pano is offline Pano  United States
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Turntable speed stabilty
Luckdog should post here some of his TT turntable plots. I can post FFT graphs of the same signal, for comparisons.
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Old 25th June 2017, 09:24 AM   #3
volken is offline volken  Netherlands
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Default Compare two test records

Interesting subject I did a compare between two testrecords Adjust and a DIN W@F testrecord both 3.15khz center freq. span 400 hz on the same turntable Linn LP12 with SME 5 tonearm. Both after carefull centering the record.

Linn LP 12 -SME-V Speed spectrum Adjust testrecord .png

Linn LP 12 -SME-V Speed spectrum DIN 45545 Testrecord .png
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Old 25th June 2017, 12:21 PM   #4
luckythedog is offline luckythedog  United Kingdom
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I have a polar plot for an LP12, attached below. It's from a 3rd party who provided a short 3kHz or so test file. I made a habit of collecting these test files for 'TTs of some reputation', so chances are I have examples of most of them in my archive and can reel them out as we like.

It seems a good place as any to explain the polar plot, which presents how frequency of a 3kHz or so test tone varies once around a platter revolution. So once around the plot is one platter revolution, and radius is 'instantaneous frequency' in Hz. Each radial line represents pitch variation of 0.1%, said to be about the audible threshold of pitch variation for most people. Inset is spectrum of whatever modulates the test tone.

This LP12 example is actually quite good as these things go. One can readily see mostly there's a once per platter revolution 'eccentricity', which might well be record mis-centering. Otherwise, one can see contributions around the 8-12HHz region which might well be cart-arm resonance - often this can dominate stability in a good TT, IME, as it appears here.

So in this case although the underlying TT seems good, the cart-arm resonance might well be dominant in determining pitch stability, or there might be something else TT drive-wise that contributes the 8-12Hz instability.

Chances are this TT was running slow and the tone was intended to be 3kHz I suspect.

Interesting way of looking at things?

LD
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File Type: jpg LP12.JPG (325.1 KB, 1448 views)
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Old 25th June 2017, 12:55 PM   #5
scott wurcer is offline scott wurcer  United States
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Just in case someone is interested, a bit heavy on the maths. https://www.math.ucdavis.edu/~saito/.../boashash1.pdf
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Old 25th June 2017, 03:27 PM   #6
kaputt is offline kaputt  Germany
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I have a Rohde&Schwarz UPA audio analyzer that can be used to measure wow and flutter. Please crank up some peer pressure so I finally try that feature
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Old 25th June 2017, 04:56 PM   #7
volken is offline volken  Netherlands
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Quote:
Originally Posted by luckythedog View Post
I have a polar plot for an LP12, attached below. It's from a 3rd party who provided a short 3kHz or so test file. I made a habit of collecting these test files for 'TTs of some reputation', so chances are I have examples of most of them in my archive and can reel them out as we like.

It seems a good place as any to explain the polar plot, which presents how frequency of a 3kHz or so test tone varies once around a platter revolution. So once around the plot is one platter revolution, and radius is 'instantaneous frequency' in Hz. Each radial line represents pitch variation of 0.1%, said to be about the audible threshold of pitch variation for most people. Inset is spectrum of whatever modulates the test tone.

This LP12 example is actually quite good as these things go. One can readily see mostly there's a once per platter revolution 'eccentricity', which might well be record mis-centering. Otherwise, one can see contributions around the 8-12HHz region which might well be cart-arm resonance - often this can dominate stability in a good TT, IME, as it appears here.

So in this case although the underlying TT seems good, the cart-arm resonance might well be dominant in determining pitch stability, or there might be something else TT drive-wise that contributes the 8-12Hz instability.

Chances are this TT was running slow and the tone was intended to be 3kHz I suspect.

Interesting way of looking at things?

LD

Yes it is which testrecord was used for this measurement ?
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Old 25th June 2017, 05:28 PM   #8
luckythedog is offline luckythedog  United Kingdom
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Originally Posted by volken View Post
Yes it is which testrecord was used for this measurement ?
It works with any test record that provides a continuous sine tone at about 3kHz or 3.15kHz, and just needs a short recording of 10s or so in any lossless format at any reasonable level.

So, over a few years, I collected together recordings of test sample tones that people have posted on forums, but usually I don't know and it doesn't matter what the actual test record was.

I don't mind running a few examples here, if anyone likes?

The software is a set of VB modules I wrote a few years back for myself to diagnose and improve pitch stability on my rigs. I never got round to making a proper user interface, so I still have to run them them manually.

Meantime, here's another good example from the archive. This is from a Micro Seiki SX-777. Red and Blue traces are for 2 consecutive revolutions.

LD
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File Type: jpg SX-777.jpg (338.6 KB, 1410 views)
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Old 25th June 2017, 05:40 PM   #9
luckythedog is offline luckythedog  United Kingdom
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Quote:
Originally Posted by volken View Post
Interesting subject I did a compare between two testrecords Adjust and a DIN W@F testrecord both 3.15khz center freq. span 400 hz on the same turntable Linn LP12 with SME 5 tonearm. Both after carefull centering the record.

Attachment 622759

Attachment 622760
Thanks, Volken - that's interesting.

I venture the difference might be due to different warp/vertical run out, or to different stylus-groove friction between the test records?

Stylus-groove friction isn't steady, it varies with time in a 1/f flicker noise like profile about an average value IME. This variation causes variation in skate force in an offset arm, and in downforce due to VTA on all arms. This is a stimulus that can cause the headshell to move which is a source of FM or pitch variation. What happens next depends on the cart-arm resonant system, and how much damping there is around.

Could you post short recordings, of the test tones please - about 10s or so ?

LD

Last edited by luckythedog; 25th June 2017 at 05:42 PM.
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Old 25th June 2017, 07:23 PM   #10
volken is offline volken  Netherlands
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Quote:
Originally Posted by luckythedog View Post
Thanks, Volken - that's interesting.

I venture the difference might be due to different warp/vertical run out, or to different stylus-groove friction between the test records?

Stylus-groove friction isn't steady, it varies with time in a 1/f flicker noise like profile about an average value IME. This variation causes variation in skate force in an offset arm, and in downforce due to VTA on all arms. This is a stimulus that can cause the headshell to move which is a source of FM or pitch variation. What happens next depends on the cart-arm resonant system, and how much damping there is around.

Could you post short recordings, of the test tones please - about 10s or so ?

LD
These two speed spectrum measurements shows that the test record is very important , and indeed it shows the tonearm/cart freq. and the 100hz motor vibration .

Pitty enough I have no records from these I use the spectrum measurements for revision work before and after .

Here a EMT 930


EMT 930 st ser.8828 Speed spectrum after revision 2.png
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