unipivot vs blade as bearing

[c3conv]

I saw some thread with regards to the DIY unipivot and bearing type using blades and these really got my juices going. My question will be
1. what are the pros and cons of these two types?
2. Which will you find better?
3. If remember right using a unipivot arms are choosy when it comes to cartridge, what is your comment.
4. Is using VHS head drum for horizontal bearing can be considered as one of the best approach?
5. How do these arms solve its anti skate?
Thanks for helping out

 

[jrevillug]

I have no experience of the unipivot. However, I did investigate making one. The advantages are that it is very simple to make the bearing - some people have used the nib of a ball-point pen as the bearing. However, it seems that they are very hard to get stable - they wobble about all over the place, and are very sensitive to the type and position of the tonearm wire, which acts as a spring.

The VHS head drum makes it very easy, but make sure that the bearings in the one you get are quiet and have no play in them. More important than a long 'spin-down' time is that they shouldn't be hard to turn when you just start moving it.

I used the VHS head drum and knife edge bearing on my DIY tonearm, and I like what it does, but I haven't really heard anything commercial to compare it to.

However, I am building a Ladegaard tonearm at the moment to eliminate the tracking error that radial arms suffer from.

For anti-skate you can run a peice of thread around the edge of the drum, over a hook and hang a weight on the end. Play with the weight to get the best result.

The most difficult part is getting the cartridge aligned properly - you need to make sure that your offset angle and effective length are about right - there is a spreadsheet covering the Baerwald alignment formulae, but I can't remember where I got it. A web search should find it.

James

 

[sreten]

Hi,

The secret, if there is one, of making a unipivot is getting the CofG
much lower than the pivot point and possibly making the effective
mass somewhat assymetric, horizontal being higher by a low heavy
(relatively) bar across the arm at the pivot which also increases
the torsional inertia - all the above reduce frontal "wobble".

AFAIK a tiny point is not required, 4 ball bearing in a triangular
pyramid arrangement should work well with a bit of damping.

🙂/sreten.

 

[EC8010]

The secret, if there is one, of making a unipivot is getting the C of G much lower than the pivot point.

I disagree. Think of the arm as a pendulum with a fixed mass. As you move the C of G closer to the pivot you shorten the pendulum and raise the frequency of oscillation. Now, more cycles of oscillation in one second means more energy can be extracted, in other words, fluid damping is more effective. We only need the C of G to be just below the pivot (so that it doesn't fall off), then we have best damping. And yes, I have made an arm in this way and it works very well http://www.diyaudio.com/forums/showthread.php?s=&threadid=10171&highlight=.

I'd say the more important thing with a unipivot is to put the pivot at stylus height.

 

[sreten]

I disagree. Think of the arm as a pendulum with a fixed mass. As you move the C of G closer to the pivot you shorten the pendulum and raise the frequency of oscillation. Now, more cycles of oscillation in one second means more energy can be extracted, in other words, fluid damping is more effective. We only need the C of G to be just below the pivot (so that it doesn't fall off), then we have best damping. And yes, I have made an arm in this way and it works very well http://www.diyaudio.com/forums/showthread.php?s=&threadid=10171&highlight=.

I'd say the more important thing with a unipivot is to put the pivot at stylus height.

Hi,

Well we see things differently. I should not have said "secret".

What I said I think is the way to do it, but other approaches
are possible and I did not intend to suggest they do not work.

I do not think having low rotational inertia and thus a high wobble
frequency is the best approach, the point being that any wobble is
unwanted and minimising it in the first place seems a valid option.

The pivot point being at stylus height seems not that critical to me.

🙂/sreten.

 

[Nanook]

hello folks, I'll "weigh in" here...

sreten...seems we frequent similar pages and topics...are you my long lost "twin"?😉 .I may no jack about electronics, but I do understand mechanical things pretty well.

First, EC8010, your unipivot is quite beautifully done. The headshell and arm tube are things of beauty and you should be proud of them (I hope you are).

I've built a few tonearms, non of which are what I would call "stellar, but are refinements to Charles Altmann's DIY tonearm. They have all sounded great.

My considerations in terms of centre of gravity, pivot location, etc are pretty "straight"(sorry for the pun) forward. I locate the pivot on the "top" portion of the arm tube (above the centre of the arm tube). This places it above the plane of the record when playing. I hang the rear weigh so that it is slightly below the plane of the record, to ensure some stability. Because the CofG is lower than the pivot it is quite stable.

An addition could be to hang some sort of "outrigger" weight at or near the headshell. That can provide some lateral stability and help with adjusting azmuth as well (somebody else's idea , Dynaco? Hafler?). it increase the inertia required to upset the headshell while playing.

attached is a picture of a re-plinth of a Sugden Conny, with my tonearm on it. It looks crude but makes some pretty beautiful noise. 12" effective length...

stew

 

[EC8010]

Hello Sreten,

well, we'll agree to disagree about C of G. The reason I think pivot height should be at stylus height is to cope with eccentric LPs. (All LPs are eccentric once the cutting lathe dynamically varies groove pitch to extend playing time by coarsening groove pitch during loud bits and tightening during quiet.) If the stylus is below pivot height an eccentric record provides a force that twists the arm and increases stereo crosstalk.

Hello Nanook,

yes, I'm pleased with my arm; making that headshell was a lot of work. Yes, I seem to remember outrigger weights either side of the headshell too. However, having found that even a finger lift rings, I'm not wild about deliberately adding something resonant. Where's your picture?

 

[Nanook]

oops, sorry about the picture....

it should be attached to this one or in the post🙂

 

[sreten]

Hello Sreten,

well, we'll agree to disagree about C of G. The reason I think pivot height should be at stylus height is to cope with eccentric LPs. (All LPs are eccentric once the cutting lathe dynamically varies groove pitch to extend playing time by coarsening groove pitch during loud bits and tightening during quiet.) If the stylus is below pivot height an eccentric record provides a force that twists the arm and increases stereo crosstalk.

Hi,

That does make sense in the context you are encouraging it to wobble
in the first place. FWIW it seems reasonable to consider all LP's to be
eccentric to some degree, this is usually far more than any variation
in groove pitch, the arm continually goes in and out, and gradually
moves towards the centre, a major issue for parallel arms.

Another arrangement the original poster could consider is as in the
"Well Tempered Arm", ingenious, the bias arrangement is clever .....

🙂/sreten.