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-   -   DIY tensioned string suspended tonearm (http://www.diyaudio.com/forums/analogue-source/157172-diy-tensioned-string-suspended-tonearm.html)

marekst 22nd December 2009 12:52 PM

DIY tensioned string suspended tonearm
 
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To make this toy I sacrificed my nicely working “Schroeder clone” for parts and rebuilt it without magnets in the bearing (I still use magnets to correct antiskate). I don’t know if this kind of tonearm bearing was built before. With exception of the tonearm lift, some fasteners and the spring, I made all parts at home! I am not a machinist, but I have universal milling machine that I bought for $500 from Harbor Freight.

The wand hangs on a string tensioned with compressed spring. The string has few millimeters of solid contact with brass part of the wand (which allows efficient transfer of sound waves out of the wand), but the string enters and exits the brass part from the same tiny hole. So, the bearing practically has a single point of suspension and minimal stiffness when the tonearm operates in its normal range of movements. To limit vibration and horizontal movement of the string, it passes just below the wand through a hole of a diameter close to diameter of the string. The lower part of the string and the spring is submerged in silicon oil.
Feel free to criticize and ask any question publicly,
Marek

Speedysteve7 22nd December 2009 09:56 PM

Nice work - looks the biz.
How does it sound compared to conventional (or otherwise) arms?

marekst 23rd December 2009 12:14 PM

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Thanks,
I made the new tonearm to replace the first “Schroeder clone” and on preliminary listening it sounds as good as the clone, but I only heard them both with a cheap cartridge. I need to make some small changes to the string before I try it with my good cartridge, and that will take some time. I hope to make it sound as good as my second “Schroeder clone” (the mutant which I was comparing to two conventional commercial tonearms). You can read about it in my thread: Magnetically suspended DIY tonearm – post # 18.
Marek

marekst 24th December 2009 01:14 PM

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Michael, I do not know if you are still interested, but I have the answer to your old question “Do you know a better method to soak wood with oil?”
I diluted Tung oil, which is very thick, with lighter citrus oil. For pernambuco wood I used a 50/50 blend. I don’t have a strong enough vacuum pump so I used DIY (cost $10) compression chamber and applied 800kPa of pressure for a couple of days. After the treatment the wand sweated oil for a long time…

The top half of the device (picture below) is a compressed air reservoir with filter (folded rag) preventing vapors from reaching and damaging the valve located on top. To save oil, the bottom part is narrow with a wider chamber just big enough to accommodate the headshell.

One more thing, I noticed that people have problems working with Vectran. In my new tonearm, Vectran string is snaked through 5 holes 0.8mm in diameter and raps around two parts, so my snaking technique may interest you…
Marek

marekst 11th January 2010 05:05 PM

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The Vectran thread I use to make the string consists of 300 loose fibers 23 micrometers in diameter. When folded in half (600 fibers), it can easily be pulled through 0.8mm hole, but that would be too thick a string. Both thickness and vertical tension of the string determines how antiskate force changes across the LP. Unfortunately twisted string gets more twisted when tonearm moves across LP increasing slightly antiskate force when skating force decreases. Therefore, I use magnetic antiskate correction. To make a thinner thread I cut twice the required length, divide it into thinner threads, then grease one thread slightly with silicon oil to straighten and hold the fibers together. I fold the new thinner thread in halve and using thin copper wire, starting from top I pull the thread through all the holes. At the bottom end I replace the puling wire with a small brass bar to hold the thread there without making a knot. Then I make a knot on top of the assembly. The brass cylinder in the wand has three interconnected holes - the thread enters the first hole, exits through the second hole (on the other side of the cylinder) wraps around a section of the cylinder, enters into the third hole and exits through the first hole.

To absorb some of the sound waves induced in the wand by vibrating stylus, I made part of the wand on which the counterweight slides hollow, then filled it with fine metal powder (W, Cu, Pb) saturated with oil. On the other side of the bearing I used metal powder mixed with soft wax. The counterweights and tonearm wire look weird but there are good reasons for that arrangement.

Without much of customization (just adjusted effective mass) I tried the tonearm with three different cartridges: Shure M97xE, AT 440MLa and Shelter 501II. The tonearm sounds more dynamic and detailed which makes it sound a bit brighter than the Clone - more like gimbaled tonearm I had before, but better. I hope to get the sound I like by changing wood, headshell plate material and better dampening of tonearm stand - it will take months.
Cheers,
Marek

EC8010 17th January 2010 11:01 AM

Off-topic question, I know, but what's the motor and belt?

marekst 17th January 2010 02:54 PM

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The motor is from Teres. It is installed in DIY housing that doesn’t dance around. My setup allows and requires setting the belt tension before every session to match thermal expansion of the platter as my room temperature varies from day to day. The setup also allows setting the motor to lean slightly toward the platter so the belt doesn’t slip downward as I run it with very little tension. The belt is DIY made from ½” leader tape. I find VCR tape too flimsy and holographic tape too crooked and not suitable for the application. The platter is from VPI Scoutmaster, but the side is covered with copper and grounded (through the bearing) to prevent static electricity build up - plastic belt shouldn’t be used on acrylic surface.
I will be gone for a while, pile up the questions and comments,
Marek

livemusic 18th January 2010 03:45 PM

Hi Marek,
Good solid job as usual! I do not really understand what keeps your arm from falling down - kind of knot on the thread? Anyway, what is the advantage over conventional unipivot design? - your's shall have more resistance for the arm moving up and down (but in the same time it shall be less prone to lateral rocking I guess).
Thanks for wood saturating advise - I will try it.
Regards,
Michael

EC8010 19th January 2010 07:29 PM

Hello marekst, I should have recognised the leader tape - but I'm used to seeing 1/4". I like your tensioner. I'm thinking of making some variation of the Altmann turntable and was thinking of using 1/2" VCR tape, but leader is also a possibility. I will use a motor from a 5 1/4" floppy drive.

bear 21st January 2010 12:56 PM

diagram of the thing?

I don't understand the purpose or location of the spring, or how the thing is free to pivot if the string is under tension through the tonearm... but it looks impressive!

_-_-bear


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