My unipivot tonearm

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Almost for a half year I have been searching for a worth second-hand tonearm, navigating on ebay, on audio market websites... Nothing, or too expensive or too many bidders. So I decided to build one by myself: unipivot, knive-bearing, or ball-bearing? I choosed the first one as my father has the other two types of tonearm.
Oh, it was very hard to design and in the same time to be able to realize it, while you have only lathe and miller machine (not CNC): every idea is good but not always makeable!
Using lathe and miller macchine of my company (after working hour, of course!), I made all the parts.
Here are some photos of my tonearm.
 

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Yes Pixpop, this tonearm lenght is set to 12" (the effective lenght is circa 340mm), you can see the numbers in the picture that indicates different parts (without the support and the entire holder block):
1- the first part of the body is in carbon fiber, and it is from a fishing rod spare part. At first I wanted to use arrow as the body, but there is no sizes for this purpose (diameters are too small).
2 - the second part of the body is in steel, hardened and tempered then grinded, well finished.
3 - the screw, or the pivot of this tonearm, has the end sharpened. It had also been hardened to prevent wear, as it supports all the mass of the tonearm.
4 - the 'V' part is in steel. It balances the lateral rotation, as for a unipivot tonearm is a big problem.
5 - two Ø3mm shaft that connect the counterweight with the V part.
6 - counterweight shaft. In alluminium.
7 - the brass counterweight. Around 200 gram.

The support of the screw was a big problem since the begining of the project. It should be absolutely sharp! Not round ended. This problem brought me to the hardness test machine. It a Vicker's hardness test machine with use of diamond cone. Before heat treating the support, I imprinted the cone form on it, so as result a very sharp and hard cone form!

The headshell is worked with miller machine (router is for wood), this one is in steel, but i'm not very satisfied about this choice. May be later i'll make one of alluminium.
 

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Eric,

polishing is one of the matter that care a lot, but as you have noticed it is still not as reflective as a mirror... Every part has been grinded for hours (the polishing took so much time that i almost became mad!) and sand sprayed. May be a chrome coating should bring it shining!! haha

Or using a render software.... hehe
 

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Ah! I see WMINCY...
I thought the headshell was aluminium.

The reason I asked is because, I love to polish aluminium myself, I got a metal polishing kit from ebay & you can get a beautiful mirror finish with a little effort! Works with plastic bits too!

POINT OF INTEREST; I've noticed a lot of counterbalances which are de-coupled with rubber grommets etc. I can't help wondering whether this is a good or a bad thing? Could this just be creating an 'energy storage area?'

Has anybody examined the merits or drawbacks of a pivoted [swinging] counterbalance?

Anyway WMINCY it’s a wonderful arm. Keep up the good work :)

Eric
 
The V shaped counterweight mounting increased the arm effective mass considerably. If one moved the V shaped counterweight mounting aft of the pivot, one would decrease the effective mass and use the counterweight as an lateral balancer also. One can also tune the unipivot arm by the arm effectivemass because the amount of friction and the pivot interfacing materials at the pivot affect the sound. One can also counterweight the arm by a ballon with a lighter am effective mass also. Counterweight located aft of pivotal point is a good thing in tonearm.
 
Ex-Moderator
Joined 2003
A friend of mine had a similar idea and floated a ball in goo as a before-pivot counterweight. I told him at the time it would have even more effective mass than the SME weight he was replacing... (Quite apart from what it did to the restoring force of the knife-edges.)
 
Just been reading this thread -- Nice arm

It occured to me to ask my usual silly noob question :-

If one could theoretically hang a cartridge from a proverbial sky hook would the mass of the cartridge body alone be sufficient to enable the stylus to track large excursions? and if not how does one calculate the minimum rotational inertia an arm should have?
 
nghiep, thanks for your suggestion. You and my father had the same idea, and soon i'll bring on with a new one.

In these days we put the tonearm in testing, to assist its performance.
We tried with 3 cartridges, all are low compliance type: Sumiko Bluepoint, Micro Benz Silver and Grado Sonata Wood. The first two, high output, sounded with deep powerful bass and many details (playing with Wham! 45 rpm), Micro Benz prevailed in bass responce, Sumiko Bluepoint was more in sustain. Both are high quality cartridge! Superb sound! In term of cost/performance.

Today we tried the Grado. New in the box, haven't run in. It sounded slower, warmer, more "analog" and digged more information. Hearing Carol Kidds, her voice was sweet and involved the entire atmosphere. Pavarroti's voice never fatigued.
 
Hi, wmincy
I think all three cartridges have medium compliances. I also think your design providing more stabilization than aft pivot counterweight. Your counterweight controls lateral movements between the cartridge and pivot rather than aft of pivot point. The normal formula for tonearm effective mass may not work on your design. Your tonearm effective mass may be lighter than appeared. Your unique design threw conventional wisdom out of the window. That's one creative idea.
 
Attached with the turntable(a modified Gyro back in 1992 where the sub-chassis was my design back in 1982 in one of my diy turntable) with the unipivot+bluepoint(in function) and the SyrinxPU3+Benz micro silver(as a reference).Stability is the key factor of this arm,and I love it This combination does not shy before the Syrinx PU 3.Anyway there are areas need to be reconsidered and rethought for further perfection.And his second and third (my modified version---Wmincy's father) versions will be coming up soon.
 

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