"Man on Wire" unipivot tonearm. - diyAudio
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Old 26th August 2012, 04:11 AM   #1
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Default "Man on Wire" unipivot tonearm.

Dear gentlemen,

This is my first experiment on analog audio. After a two year break I come back with renewed energy to our beloved forum.

Having stolen my Dad's Lenco L75 a few years ago, I planned a massive plinth and a new, DIY tonearm. I posponed this too much, I know realise.

My DIY tonearm: the "Man on Wire" tonearm.

I had a few requisites that were fullfiled:

*It had to be relativelly easy to build, so I chose unipivot...
*It had to be made of the best wood available to me. Pernambuco wood is the best wood for stringed instruments' bows so I picked a broken, +/-150 year old violin bow and used it as stick. I mean, if it is good for violins and cellos, it has to be good for TTs.
*It had to be impossible to fall down, so I made the pivot point high and traversing the stick, for which function I made a bronze "T" piece (shown in diagram). The pivot needle articulates in the conical "ceiling" of the "T" bronze piece. I also attached weights to lower the gravity center.
*I imagined that the balancing long stick of the equilibrist would be a good addition to my arm, for improved balance and resistance to motion from the accidents of the microgrooves, which could be translated into added information retrieved by the stylus...
*Since I don't have tonearm wire, I braided my own with 32AWG OCC copper magnet wire.

Well, I made it and it worked at first attempt!
Now I have to experiment with different weights and compare the behaviour with and without the balancing stick.
As how does it sound: no child is uggly for the father, so I find it sounds great!

A few pictures and a cross-section diagram of the pivot:

Click the image to open in full size.

[IMG]https://picasaweb.google.com/lh/phot...eat=directlink [/IMG]

[IMG]https://picasaweb.google.com/lh/phot...eat=directlink [/IMG]

[IMG]https://picasaweb.google.com/lh/phot...eat=directlink [/IMG]

Comments and advices are very welcome.
Hope you like it.

M.

Last edited by maxlorenz; 13th September 2012 at 05:42 PM. Reason: Better picture.
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Old 26th August 2012, 02:42 PM   #2
bbftx is offline bbftx  United States
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Hi M
I like the idea of using the bow as the arm. Beautiful wood.

But, in the side view of the cartridge, it appears as if your VTA is way off. Perhaps the "bow" of the bow is making it hard to get the cartridge geometry right, as the arm appears to be angled upward at the cartridge mounting point.

Perhaps you can turn the bow so the arc is sideways, and use overhang, to get better VTA and acceptable tracking error.
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Old 26th August 2012, 03:38 PM   #3
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Thank you.
I'll study these possibilities. I removed a little wood at the tip to receive the cartridge and have it stable. Perhaps I must make it horizontal to the disc...is that what you are referring to?

I am new to vinyl. The truth is that all the technical concepts escape me: I designed it based on intuition and improvisation with the materials available (as we Latins like to do). Now I have to study the theory, as I should have done beforehand, to optimize it.
Here's another view:

[IMG]https://picasaweb.google.com/lh/phot...eat=directlink [/IMG]

It tracks wonderfully and sounds way better than the original arm. My inherited record collection lacks vinyls in good condition. I have some ondulated and some excentric records, and especially with the last, the arm does an interesting dance but keeps tracking without any reduction in audio quality.

The good thing about this pernambuco wood is that one can modifye its shape (to a certain extent) applying localized heat, then bending to the desired curve and then cooling it down rapidly, so the mechanical and elastic properties remain unchanged. I use the "burned fingers technique".

Cheers,
M.

Last edited by maxlorenz; 26th August 2012 at 03:57 PM.
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Old 26th August 2012, 04:32 PM   #4
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This is a really clever idea. There's no reason why it shouldn't work fine once it's fully adjusted.

You can test the groove tracking by playing a mono record and comparing the left and right outputs with a scope or meter. If the stylus sits perpendicular to the groove, each channel will be equal. Even better is to buy a test record, which will include both stereo and tracking tests, allowing for precise calibration.
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Old 26th August 2012, 04:47 PM   #5
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Thank you ChicagoJTW, for the kind words and advice. Interesting test. The lateral weights on the "equilibrist bar" (I don't know the specific term in English) help regulate the lateral inclination of the cartridge.
For now, the stereo image is well centered and the dynamics are good.

I expect the audio quality will improve as long as the magnet wire "burns-in". With the tinny currents involved, this must be slow.

Cheers,
M.

Last edited by maxlorenz; 26th August 2012 at 04:53 PM.
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Old 27th August 2012, 08:58 PM   #6
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OK. I found an "issue" or potencial problem. The equilibrist bar has downwards curvature on both sides. Even with the little inclination when putting the arm at work, the weights on the extremes of the bar fall behind the axis of the pivot, so they act as dampers of the effective weight or mass applied on the stylus, depending on the heigth it reaches: thicker LPs could get more weight than thin discs. I think this can be not desirable as a feature, in which case I will build a bar that keeps the weight always on axis with the pivot and whose weight does not sum to the weight of the tonearm sections.

Photo of the simple pivot system:

[IMG]https://picasaweb.google.com/lh/phot...eat=directlink [/IMG]
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Old 28th August 2012, 04:39 PM   #7
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Very cool, it obviously works so it is mostly a matter of refining the design as your understanding increases with experience.

Once in a while I feel motivated to tinker, this project looks quite inspiring. I have a nanook uni-pivot arm I have been playing with off and on..
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Old 28th August 2012, 06:45 PM   #8
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Thank you KevinKr.

I've been conducting a series of mental experiments, because I don't have time to go to the soldering workshop, and now I feel that the best arrangement for the equilibrist bar is one cylindrical bar, soldered on top of the dome for the pivot, with long weights pending like "ear-rings" on the extremes of the bar arms, with free rotation, independent of the tonearm inclination, which will allow them to remain practically on axis with the pivot. The weights should be placed very low, to produce a lower gravity center. This arrangement will provide the best balance and stability.

Apart that, I am worried by the cartridge effective mass concept v/s cartridge inertia...but I'll have to let this for later.

For those interested, broken pernambuco violin/viola/cello bows can be purchased inexpensivelly on eBay. Just search for "old violin viola celo bow" and contact the sellers for specific requisites

Cheers,
M.
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Old 29th August 2012, 06:28 AM   #9
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Default perhaps of interest...

http://jeffsplace.me/wordpress/?p=838

http://www.lencoheaven.net/forum/index.php?topic=2278.0
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Last edited by moray james; 29th August 2012 at 06:46 AM.
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Old 29th August 2012, 06:52 AM   #10
freddi is offline freddi  United States
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its quite cool -- I remember 30 years ago mucking with Mayware & JH unipivots. Heldentenors and near heldentenor such as Rosvaenge & Wittrisch are cool too on vinyl transfers. Good preamp there - are the capacitors stock?
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