sexta-feira, 13 de fevereiro de 2009

The Turntable


Welcome to the shiny world of Vinyl !

After coming ac
ross some DIY efforts at building turntables, I too decided to give it a try… For the turntable, I went for an all aluminium construction and a decidedly “mechanical” look.

The frame consists of 2 aluminium sheets strongly bolted together, and the platter is a “sandwich” of aluminium-acrylate-aluminium epoxyed toge
ther for some vibration dampening.


As for the drive I went for what I believe to be an original path.

A bipolar stepper motor drives the platter through a toothed belt (to prevent slippage) at a 3:1 ratio. However, steppers are not known for a smooth motion which is absolutely critical here.

Here, it is driven by a special purpose Toshiba chip that generates two “pseudo” sinewaves 90 degrees apart (aka microstepping), to drive the stepper coils, thus strongly reducing any vibrations.



A Picaxe 28X1 controller running at 20Mhz provides the timing inputs to that chip with the required precision, both at 33 and 45 rpm.


The Tangential Tonearm

But the tonearm is the "piece de resistance"...


During the early 80's, tangential tonearms were all the rage, mostly for top of the range models. At the time, I admired their sophistication but I was clueless at how they worked. Nowadays, thanks to the web, few secrets remain. As a personal challenge, I decided to make one myself, based on the workings of the Denon Linatrack.

In it, the cartridge was suspended from a very short arm that could rotate horizontally from a carriage assembly that would move linearly along the record radius.
While following the record groove, the needle would pull the arm from its straightahead position, an optical switch would detect the motion and command a small motor to pull the carriage assembly along using a wire guide, thus restoring perpendicularity of arm and cartridge. So, even a small tracking error leads to an immediate correction.


My tonearm follows the same principle, although with a different mechanical layout. Here, the carriage is attached to a toothed rubber belt pulled by a stepper. A photodarlington is used to detect the cartridge movement and 2 micro-switches signal the limits of the carriage travel along a linear guide.


User input is provided by a 4-way joystick, and the following features are supported: Auto-stop at the end of play, Auto start for LP's and Singles, Auto-Repeat, Up/Down, Linear movement and finally Stop.

Due to the number of I/O lines needed, I opted again for a 28X1 controller. The extra lines also allowed me to control the stepper using halfstepping, providing a bit of a smoother action from the stepper.


All the mechanical bits are attached to an aluminium frame for rigidity and everything is partly enclosed with an acrylic cover. The tonearm itself is a somewhat complex part and for ease of prototyping, it was drawn using Google Sketchup and 3D printed by the folks at http://www.shapeways.com/



The End Result

So, how does it sound ?

All in all, it sounds reasonably good, although it ain't ready for commercial release and reviews. There are some noise sources that remain and are harder to suppress. That's where the consumer electronics giants spent most of their R&D budget - the small important details !

From the turntable itself, there's a few noises coming from the type of bearing I've used for the platter spindle, and only redesigning it will bring a complete cure. The stepper used for the platter should have more torque and that means that startup to 45rpm can be tricky sometimes.

And lastly, I forgot to provide for some kind of tension adjustment for the drive belt, which on ocasion brings some "light" warble when playing. However, a simple fix is already in the works.

The tonearm works incredibly well and reliably. Only a very muffled sound can be heard on quieter parts, when the arm assembly moves. Guess that driving the stepper using microstepping would take care of that. That's a more complex circuit maybe for the future.

Electrical noise and its interference on the signal lines became a bit of an issue towards the end of the build. Inexperience made me overlook it initially, and later on I had to reroute and shield (with aluminium foil) some of the lines. Also, some more grounding was needed.

It took me about 10 months from concept to finish. I had to learn a lot to make it work but it was hugely rewarding when it all came together, and on top of that it even sounded good !

Downloads

As I don't intend to start the next Sony (or Samsung !), I make available some of the information needed if you want to try your own version. The Eagle file schematic for the arm controller can be found here, and the necessary software (Picaxe Basic) in here.

Have a try and let me know of the results !


video

47 comentários:

  1. Estoy enamorado ;)

    No seriously. This is really cool. How about using an infrared LED or laser to enable it to seek/skip the tracks? That was always a dream of mine...

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  2. Nice!

    Would like to try something similar but I don't think I have the skills.

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  3. Impressive project. As a similar point of reference I offer the Altmann DIY turntable - Altmann addresses many of your possible sound quality issues in his Mother of Tone Series.

    http://www.altmann.haan.de/turntable/

    http://www.mother-of-tone.com/index.htm

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  4. Chuck, the Altmann was my initial inspiration and where I got the idea for using a stepper. However, I wasn't aware of mother-of-tone which offers some food for thought... Thanks for the tip.

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  5. It is a very interesting project that you have engineered. I understand that this is an early attempt and that you plan to refine/ change/ imporove the construction. A turntable is in fact an object (platter, plinth) to which a sensor (cartridge) is connected.
    The objective of any turntable designer is to eliminate even the smallest vibration which occurs in the turntable and is not coming from the record. The signal in the groove of the record has to be separated from all other parts, moving or not moving.
    One possibility to give mechanical vibrations no chance, is to detach the motor completely.
    This goes for the drive motor of the platter as well as any other motor of lift and arm, for example in the tangential arm you designed.
    Queston is: what does the toothed belt add to the retreived signal.
    A second one is to give the platter enough mass so it cannot resonate.
    Furthermore the spindle (bearing, bearing housing) should be of the highest quality.
    The electronic solutions you devised are in essence correct. The only question is: are these refined enough. Do not forget that the groove of a record contains signals engraved in micrometers!!!
    Of course I can not estimate the result you have achieved other than by listening to the sound of the video clip you did sent me.
    Good luck when refining the design.
    Best regards,
    Rudolf (Sound Fountain).

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  6. this is so cool! a DIY turntable! turntables looks really borring actually but with your turntable.. it looks wicked! great project!

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  7. 每次看完你的文章,總是回味許久,要經常發表喔。.........................

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  8. 當一個人內心能容納兩樣相互衝突的東西,這個人便開始變得有價值了。............................................................

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  9. 認識自己,是發現妳的真性格、掌握妳的命運、創照你前程的根源。............................................................

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  10. 愛情是一種發明,需要不斷改良。只是,這種發明和其他發明不一樣,它沒有專利權,隨時會被人搶走。.................................................................

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  11. 生存乃是不斷地在內心與靈魂交戰;寫作是坐著審判自己。. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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