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Old 28th May 2006, 08:05 AM   #1
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Default Questions regarding tone arm materials and designs

Currently I'm nutting out ideas for my DIY TT, There seem to be many options with tonearms and I would like a few opinions and clarification.

First up is there any reason an arm would not work where the balance weight were below the pivot point, I am thinking of a unipivot arm here with the point central in a cutout in the arm and supported on a pin that rests on a cross piece, so the counterwight can be below deck height. hard to explain but the upshot would be that the weight would also counter side rocking and vertical at the same time. If anyone understands what I mean, (I really think I need to draw it so it can be seen) and has built such an arm I'd be intersted to know if it worked OK.

Next what are the differences in sound between arms made of aluminium and wood, I have the gear to make either so it really comes down to the sonics.

Third another option I was looking at was using knife bearings for the vertical plane in a more conventional arm, do these perform better or worse than ball races, on the surface to my niave understanding I feel they may be better so long as they can be located properly, however I'm probably missing something here.

Fourth is it desirable for the horizontal movement of the arm to be as free of friction as possible or would some friction be preferable to stop the arm being tossed around horizontally too easily by record defects etc, I wonder about this because parallel trackers must have a fair degree of resistence in this plane no matter how well built.

Fifth the pivot point for the arm, what is the ideal location, ie level with the stylus tip, above it?

Lots of questions I know and I probably should have made several posts but any answers would be appreciated.
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Old 28th May 2006, 08:46 AM   #2
phn is offline phn  Sweden
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This should answer some of your question, RS Laboratory tonearm. Maybe give you some ideas as well.
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Old 28th May 2006, 10:01 AM   #3
AndrewT is offline AndrewT  Scotland
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Hi,
you posed a number of questions and I can give only a few pointers.

There are at least some arms and probably many that have a low mounted counterweight to reduce the arm Centre of Gravity height to closer to, or even at, the recording surface height. I would suggest this is a good idea. Something to do with modulation of tracking force over record surface undulations.

I have used the SME 3009/11improved Fixed which majored on low inertia and low friction. It did it's job well within limitataions. It's main criticism was the use of knife bearings in the vertical plane and the low restoring force/stiffness in the upwards direction from large vertical accelerations. It only had mass to assist it. In the downward direction it has the stiffness of the knife edge and it's support structure. This very asymetrical stiffness regime probably led to some kinds of distortion that became more apparent as stiffer cartridges became more popular. Certainly it was never designed to carry the very stiff MC cartidges.

Arm pivot MUST be located at record surface height. Totally at odds with the previous link. Let's argue this one to death!!

Arm material is of little concern PROVIDED it has the following chracteristics:- light weight, high stiffness, adequate damping (inherent or added)

And thrown it for for discussion. Counterweight using very dense material tungsten, osmium, depleted uranium, any other contenders?

Notice, I use stifness repeatedly not often strength. Appreciate the difference and use the knowledge wisely.
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Old 30th May 2006, 09:47 AM   #4
sreten is offline sreten  United Kingdom
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Hi,

for a homemade unipivot the "Well Tempered Arm" I'd regard as essential research.

http://www.vinylengine.com/library.p...ell%20Tempered

Just built a beefier armtube and headshell, the armtube - carbon fibre Kite rod ?

/sreten.
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Old 30th May 2006, 11:53 AM   #5
AMV8 is offline AMV8  United Kingdom
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Hi

I use various arms. My favourites are the Townshend rock and the Hadcock. I find that both work with high end mc and mm cartridges. However the designers adopted opposite approaches to designing an arm.

The Townshend arm is high mass and very rigid; with the arm, bearing structure and counterweight carrier all rigidly connected. It uses massive and high quality horizontal bearing and high quality knife edge bearing for the vertical plane. It is a heavy arm with heavy counterweight.

Th Hadcock arm is low mass and lightly made from aluminium tube. The arm and housing and counterweight holder are disconnected from each other by plastic vibration damping. ( These fit around the aluminium tube where if fits into the central housing. The uni-pivot bearing is a vertical pin resting in a bearing race within the central housing.

I find that both work well. Both get good reviews and my personnal independant tester who has little interest or knowledge of hi fi ( my wife!!! ) confirms the that both are the best sounding of the many arms that I have used.

If I were making an arm I would go for the hadcock design as it is so simple in comparison to a much more complex arm with a central bearing structure.

I hope this helps.
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Old 30th May 2006, 08:30 PM   #6
EC8010 is offline EC8010  United Kingdom
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I've always used aluminium. Unipivot pick-up arm

Wood seems attractive until you consider the screening and clamping issues. A composite of balsa with a hardwood skin might work well, but you will need to add a screen over the wires as they pass down the arm.
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Old 31st May 2006, 11:44 AM   #7
phn is offline phn  Sweden
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I have no workshop. By that I mean that making a tonearm isn't really an option for me. But IF I would make one, it would be something like this. The photo is from a DIY hi-fi gathering in October 2000 in Japan. I wish I had more.
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Old 2nd June 2006, 11:53 AM   #8
EC8010 is offline EC8010  United Kingdom
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Default Bizarre!

It's a pity there don't seem to be any English words to explain why that arm is the way it is. Frankly, it looks like a nightmare. It seems to have some weird sort of cantilevered vertical pivot to allow the cartridge to rise and fall without changing its VTA, but with the disadvantage of hugely increased vertical friction and changing arm length. Its effective mass must be enormous (all that hardware near the cartridge), and it has weights on thin supports that must ring like tuning forks.
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Old 3rd June 2006, 09:08 PM   #9
vta is offline vta  United States
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Regarding counterweight. Hanging the weight via a unipivot type bearing can work, But contact point for the weights pivot point must be below the arms pivot point. Do a searh for a picture of the Roksan Tabriz Zi arm.

Having the arms pivot point at the same verticle plane as lp/stylus interface is desirable as it will keep the pivot to stylus distance constant over warps (but there have been some pretty good arms that don't adhere to this).

Horizontal motion (all pivot points) should be free and lack resistance, but this can be added with a damping system. You could even damp the pivot point directly as I am concidering by having pivot point rest in an oversize pivot cup filled with damping fluid.

www.vinylengine.com is a good resource and starting point as it has a pretty good listing of al the main arms that are and have been available and the specifications (measurements). you quickly see that 80 percent of them share the same specs *** most others!

Adam
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Old 11th June 2006, 10:12 AM   #10
too299 is offline too299  Singapore
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how about this


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