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Old 12th September 2013, 07:39 AM   #121
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Default Hello sq225917

I think you meant to write 300 rpm and 250 rpm.

Sincerely,

Ralf
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Old 12th September 2013, 01:48 PM   #122
Pyramid is offline Pyramid  United States
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Default Turntable speed control

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Originally Posted by benb View Post
I've thought a bit about turntable speed sensing (other than the methods discussed so far).

To get better "high-frequency" resolution (better than 0.555...Hz at 33 1/3 RPM), there must be measurements many more times than once per revolution. Many platters offer this naturally with the strobe patterns on the side (or underneath, as in the Dual 1229). I calculate the 33 1/3, 60Hz band as having the highest resolution with 216 dots or squares (120Hz for neon light output divided by 0.555...Hz), or 432 transitions per revolution. I'm thinking this should provide adequate resolution (even a lightweight platter should provide a reasonably steady speed for 1/432 of a revolution from its flywheel effect). These transitions can be picked up with light using the light focused from an LED or even a small solid-state laser (think hand-held laser pointer), and the light picked up with a phototransistor or (for better timing and more accurate transitions) a photodiode.

Even if the spacings of these marks are not perfect, or the transitions are not exactly 50 percent duty cycle, these can be measured with a microcontroller to accuracy of a microsecond (or substantially less, depending on hardware timers available). If you put such a platter on a large, heavily weighted platter with a low-friction bearing that when spun does not slow down much (I recall a DIYaudio post with links to a two-part Youtube video of a platter spun fast that took around 15 minutes to slow to a stop), you can measure the exact time of each transition and store it for all 432 transitions (one might want to add an extra "index" transition to make it easy for the software to find the start). This (the 432 transition times) can be stored in the microcontroller's non-volatile (FLASH) memory, and compared to the transitions generated by an operating turntable, giving an accurate speed compared to a reference at the rate of 432 times per second. With careful math, one could measure the transitions of several revolutions and compensate for the decaying speed.

Another idea, using a platter with a flat cylindrical rim, put a circumference of magnetic tape on the outside of the rim, and have a tape head placed close enough to the rim to pick up a tone recorded on the tape. A higher frequency tone would require the tape head be closer to pick it up. The tape would have the wow and flutter of the recorder used to record the tone on the tape, but this could be compensated for by reading the timings of the tone's zero crossings while "calibrating" the platter on a heavy turning platter as above.

Benb- I think you would be going in the wrong direction with your method. The two limiting factors in this detection scheme (and the one used by my device) are the ambiguity of the "event" detection itself and in your case, the accuracy of the positioning of the spots. In my case, I'm able to resolve the detection of a once per rev event with < 0.001" accuracy in order to give 3 decimal places of accuracy (37.7" circ/0.001" or 1 in ~37000) which would seem to be approaching the limit of mechanical detection means and visual verification. By multiplying the number of events by 432 you automatically increase the error factor by 432x since the event window is now 0.087" and the resolution will be .087/.001 or 1 in 87, ie < 1%. This is assuming the spots are perfectly placed at even intervals which is most likely not achievable either on printed media or via machining on the platter. One advantage of using only one event per rev, is it removes the problem of accurate placement: If securely fastened, it can only occur at exactly the same point each rev, the limiting factor now singularly being the ambiguity of the detection mechanism. I also needed a solution that would work on any table, not just the ones with machining on the edges, and something that would work during disc play.

There still seems to be some confusion about what function the tach provides. As many have commented, it cannot (and does not attempt to) provide wow and flutter information, something the software promoted by SQ225917 does a much better job at. However, my concern is belt drive TTs and the reduction of wow and flutter via SPEED control of the drive motor is problematic at best (vibration control being another matter altogether). I built the tach to accurately measure results when testing a new PSU design I've been working with. One thing I discovered was no matter how accurate and stable the PSU, a belt drive table will drift off speed over time, for any number of reasons as previously posted. The logical conclusion I came to was to use the tach to correct this long term drift while allowing the PSU to do its job on short term stability and the platter to do its best on per rev stability. I was hoping to get useful feedback and critique of my design that might help improve on what I am trying to accomplish, but with the exception of a very few, I've encountered nothing close to what I had hoped would be the spirit of DIY audio enthusiasts. Sour grapes makes for lousy wine I suppose.....

Last edited by Pyramid; 12th September 2013 at 01:51 PM.
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Old 12th September 2013, 02:01 PM   #123
Pyramid is offline Pyramid  United States
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Default VPI turntable motors

Quote:
Originally Posted by Straight Tracker View Post
I think you meant to write 300 rpm and 250 rpm.

Sincerely,

Ralf
If the 240V/50Hz motors rotate at 250 RPM (or 500 RPM?) then the pulley ratio would also have to be altered to give 33.33 and 45 RPM as well, correct? Using only the large pulley on the motor spindle, the ratio of line freq to platter RPM is then 1.333333:1 no matter what the configuration?
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Old 12th September 2013, 10:42 PM   #124
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Quote:
Originally Posted by Pyramid View Post
If the 240V/50Hz motors rotate at 250 RPM (or 500 RPM?) then the pulley ratio would also have to be altered to give 33.33 and 45 RPM as well, correct? Using only the large pulley on the motor spindle, the ratio of line freq to platter RPM is then 1.333333:1 no matter what the configuration?
For an AC synchronous motor, speed in rpm is always 60 times line frequency in Hertz divided by the number of pole pairs. VPI have used 12 pole and 24 pole motors.
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Old 13th September 2013, 10:36 AM   #125
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Pyramid. I don't think there's sour grapes on anyone's part, we all just have different experience and different ideas on what will work to make a stable TT. It's natural that the concepts we are invested in hold importance for us and that all posters, yourself included don't like to see those views being belittled. That's just human nature. There are very few people out there who are willing to look at every problem with an open eye and manage not to let their view of what might be possible, and beneficial, be framed by their own preconceptions and assumptions of what 'might' work.

The thing to do is to build your system, measure it by all means possible and then present the results to confound your critics and move the knowledge base onwards for those parties who are interested enough and open minded enough not to worry about having their views tested.

I'm very happy with my set-up with high viscous drag fronted by a large transformer and a simple phase shift capacitor who's value has been selected for least vibration on my specific motor, rather than just running a nominal value. It works for me. But I can't help wondering if it could be improved, so I've just finished building Meldano's TT-PSU and I'll investigate how will this works with it's voltage, frequency and phase shift being adjustable in the digital domain. if I can get it to work as well as my current solution then i'll start looking at how it can be improved by feeding in a signal other than a perfect sine wave.

Don't worry about the lack of positive response, this is one area of audio design where there really are very few people doing any serious investigation beyond rehashing old solutions in less expensive packages.
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Old 13th September 2013, 12:40 PM   #126
Pyramid is offline Pyramid  United States
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Default Stick a fork in me, I'm done.

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Originally Posted by sq225917 View Post
The thing to do is to build your system, measure it by all means possible and then present the results to confound your critics and move the knowledge base onwards for those parties who are interested enough and open minded enough not to worry about having their views tested.
I appreciate your comments. I think to some extent, I have been attempting to do just what you recommeded. The problem is, I really don't need the frustration. I retired from successfully running my business for 20+ years and I think my time would be better spent on the golf course or with my family. I think I'd be happier just designing and building my projects for myself and keeping my mouth shut. I guess that makes this my final post.

I had a look at the Meldano design, looks very well built. I don't read German, but I think I got the gist of it. I hadn't thought of making the AC level adjustable from the front panel, but it wouldn't take much to add digital pots to both outputs (currently have 10T pots). Everything else looks similar in capabilities to my design: Dual DDS chips, 0.01 RPM adjustable freq, 1 adjustable phase, etc. I found an off the shelf extrusion to house it, 2.5"H x 3"W x 7" long.

Good luck with your project.
Attached Images
File Type: jpg PSU1.jpg (137.3 KB, 214 views)
File Type: jpg PSU2.jpg (152.9 KB, 207 views)
File Type: jpg PSU3.jpg (136.8 KB, 203 views)
File Type: jpg PSU4.jpg (178.1 KB, 203 views)
File Type: jpg PSU5.jpg (91.5 KB, 201 views)
File Type: jpg PSU6.jpg (102.2 KB, 47 views)
File Type: jpg PSU7.jpg (121.8 KB, 56 views)
File Type: jpg PSU8.jpg (93.7 KB, 66 views)
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Old 15th September 2013, 07:27 AM   #127
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Wow that looks neat, you should offer it for a group buy, people would tear your arm off to get one.
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