Tonearm vertical axis position?

[walpurgis]

The position of the vertical pivot on tonearms seems to be generally several millimeters above the record contact point of the stylus. Surely the vertical axis should be on about the same plane as the stylus? Otherwise, if it is higher, the stylus will have a small linear back and forth movement in relation to the vinyl groove when tracking warps and all records are warped to a greater or lesser degree.

I can see why this may occur with unipivot arms as the arm would tend to 'topple' if the pivot wasn't high, but on gimbal type designs, is this desirable? (though even a unipivot could be weight 'ballasted' to lower the pivot point)

Anybody know the ins and outs of this?

[phonomac]

Hello there,
The stylus does indeed move relative to the groove if you have a warped record. Furthermore, the tracking force varies as well.

Have a read about the Dynamic Tracing System (DTS) as used on Audio Technica AT-1010, AT-1100 and AT-1120 tonearms, which are specifically designed to address these issues.

regards

Angus

[walpurgis]

Come on all you technical guys. I have a reason for asking this question and there's not a lot of info on the net.

I suspect that my personal conclusion is correct, in that ideally the pivot axis and stylus contact point should be on the same plane. What reason could there be for it to be otherwise? Are all these tonearm manufacturers wrong or do they need to build in the apparent misalignment?

[Mr Kipling]

I think it was reading about the Morsiani uni-pivot arm that the designer suggested that ideally the arm axis should be the same stylus. It seems logical to me. At the end of the day though, unless you have some adjustment at the headshell you're always going to be governed by whatever cartridge you're using.

[The Grand Wazoo]

But arranging to have some sort of adjustment at that end surely ought not to be any more difficult than at the other end? I'd have thought it just needs designing that way.

[Mr Kipling]

But arranging to have some sort of adjustment at that end surely ought not to be any more difficult than at the other end? I'd have thought it just needs designing that way.

True. I suppose you could have the adjustment at the bearing block adjusting the armtube/headshell relative to the bearing. Might impact on the overal integrity of the stucture.

[Qwin]

Geoff - you are talking about the horizontal bearing not vertical.
(A vertical bearing has a vertical axis) describes its axis orientation not the direction of travel or direction of loading.
But I get your drift.

For the stylus, the ideal position for this axis is to be centred on the same plane and some unipivot designs try to achieve this. The problem in doing this is that the majority of the mass, headshell and tonearm, is then above the axis and the whole thing is top heavy and unstable. This is why some designs have the counter balance weights slung below the armtube, in an attempt to bring the centre of gravity down and gain stability. Unipivots are most prone to this but an element of twist is still produced an a gimble style of bearing, so best avoided.

You may also notice that the bearing is at an angle. It is set to match the Offset angle of the cartridge/stylus in order to travel in unison with the cantilever movement and not induce twist here when it travels.

It is all at minute levels by the time it gets to the stylus, but cartridges are designed to pick up minute details, so I supose it matters.

[walpurgis]

Geoff - you are talking about the horizontal bearing not vertical.
(A vertical bearing has a vertical axis) describes its axis orientation not the direction of travel or direction of loading.
But I get your drift.

For the stylus, the ideal position for this axis is to be centred on the same plane and some unipivot designs try to achieve this. The problem in doing this is that the majority of the mass, headshell and tonearm, is then above the axis and the whole thing is top heavy and unstable. This is why some designs have the counter balance weights slung below the armtube, in an attempt to bring the centre of gravity down and gain stability. Unipivots are most prone to this but an element of twist is still produced an a gimble style of bearing, so best avoided.

You may also notice that the bearing is at an angle. It is set to match the Offset angle of the cartridge/stylus in order to travel in unison with the cantilever movement and not induce twist here when it travels.

It is all at minute levels by the time it gets to the stylus, but cartridges are designed to pick up minute details, so I supose it matters.

Yes, perhaps I should have made my thoughts clearer, I am referring the the bearing that allows the cartridge and arm to move up and down in relation to the record surface. It is not a vertically mounted bearing, but allows the vertical (see-saw) arm movement. There are exceptions, unipivots as I mentioned and the Connoisseuer SAU2 spring to mind.

I'm aware that some manufacturers lower the centre of gravity via underslung weight. But, that does not explain why arms are designed with the pivot out of plane to the stylus contact plane. Obviously cartridges vary in depth and there is likely to be a small variation from that.

I'm aware of various tonearm design types, but want to know if there's a specific reason for the plane differences or is it just for expediency, as the effective error in geometry is small.

[Qwin]

I thought I had covered it Geoff:
"The problem in doing this is that the majority of the mass, headshell and tonearm, is then above the axis and the whole thing is top heavy and unstable."

The designers try to keep everything as well balanced for the bearings as possible. There is some trade off with what is ideal for the cartridge. But equaly an unstable arm will be detrimental to cartridge performance any way.

There are many compromises in tonearm design and different approaches taken by manufacturers to try and minimise their effects. An equally important aspect is the centre of gravity when viewed from above and where it lies in relation to the tone arm centre line. Some put the stylus on this centre line and put up with the majority of the cartridge weight being on one side and inducing twist. Others centre the cartridge mass and put up with the stylus being off centre so to speak. With an "S" shaped arm they have to balance the bend in the arm against the cartridge/headshell to try and achieve a balanced load for the bearing.

http://www.jkwynn.co.uk/Project_Images/TechArm/TechArm_001.jpg

There are many different soloutions to a variety of conflicting constraints. Where you gain in one repect you usually lose out somewhwere else.
Thats what makes Tonearm design such a challange. Belive me, if lowerring the horizontal axis to the record playing surface plane was a wonder solution with no downside, every tone arm would be built that way. ;)

[John Gordon]

Qwin

There are many different soloutions to a variety of conflicting constraints. Where you gain in one repect you usually lose out somewhwere else. Never was a truer word said...


if lowerring the horizontal axis to the record playing surface plane was a wonder solution with no downside, every tone arm would be built that way.
Not necessarily.

There are good reasons against having the bearing for vertical movement at stylus level: it makes things more complicated and therefore costs more; it compromises on space for other assemblies - counterweight, lifter, bias; it reduces the amount of space under the bearing so that decks with shallow platters/high arm boards are difficult to accommodate; it requires a bit of thought rather than just copying an existing simple design.

However, there are good reasons for doing it: it improves tracking, surface noise and imaging.

If the bearing is not at stylus level then any force which has a resultant acting to pull against the arm horizontally - groove friction, groove modulation, dust particles, warps - will also have a vector acting to rotate the arm upwards. This obviously compromises tracking as VTF varies, surface noises are accentuated and prolonged, and imaging suffers due to variations in bias and vertical modulations.

You might think the advantages of a lowered bearing would outweigh the costs and effort to incorporate it. Why more very expensive arms don't incorporate it, I don't know. Laziness? Maximising profit even further?

The lateral balance issue is also complicated. It can be useful to load bearings in ways which help reduce resonances, and a conventional arm that is laterally balanced may still be prone to tracking and cartridge compatibility issues if the headshell doesn't move vertically in the plane of the cantilever, or the counterweight in a plane parallel to it.

There's more to it than meets the eye.
John

[Qwin]

hi John - you pretty much enforced the point I was trying to make, that there are a huge number of constraints and variables at play and that Tonearm design is a very complicated business.

I tried to make the point that, what is an ideal state of conditions for the cartridge to perform at its best may not be best for the bearings to do same. I wasn't proposing lowering the axis, just making the point there are good reasons not to. Its not as straight forward as manufacturers cutting corners, being lazzy or trying to maximise profits. Lowering the horizontal axis will solve some issues and is easily achieved, trouble is it raises a whole lot more, so you end up chasing your tail. Pretty much in your corner again.

This is why all tonearms are a compromise and different designers have different views on where the emphasis should lie. Leads to some very contrasting and interesting soloutions to tracking that groove. :D

[walpurgis]

If the bearing is not at stylus level then any force which has a resultant acting to pull against the arm horizontally - groove friction, groove modulation, dust particles, warps - will also have a vector acting to rotate the arm upwards.

This is precisely what I'm getting at. I don't need lectures on general arm design, I just want an answer to that point! Never mind any design implications or cost, why are arms not made with the stylus and arm bearing in the same plane (allowing for variations in cartridge depth)? That was what I asked. All I really wanted to know was if there is a specific reason why having both in the same plane seems to be avoided. Am I missing something?

[Qwin]

Sorry if I was lecturing, the thing is you seem to be over simplifying and looking at one item in isolation. Your specific question has been answered.
You keep repeating why arn't there arms that have the bearing axis on the record plane, there are several that do. Also extremes in the oposite camp 6" above the record. I've given my reason why I don't think it is practical, so nuff said. :)

[walpurgis]

you seem to be over simplifying and looking at one item in isolation. Your specific question has been answered.


Precisely, I was "looking at one item in isolation" and the question has not been answered.

The issue has been skated around, but no direct answer has been put forth. I'll assume that there is no reason why the stylus and pivot should not be in the same plane.

I've asked this specifically as I'm intending to build a pickup arm (correct name really, tonearms belong on gramophones). I've had a design in mind for some time and it's very different to the run of the mill. My design includes bearing axis and stylus in the same plane. I merely wanted to know if there were good engineering reasons why this is not universally used. I cannot think of one.

[Qwin]

Good luck with the project, wouldn't mind seeing the result.

Like I said its been done many times before.

http://www.excelstereo.com/Triplanar/large_photo.jpg

[Mr Kipling]

Isn't it simply a question of following convention and the convention being to have the horizontal bearing axis in-line with the armtube? Which means you're never going to get the bearing and stylus point in the same plane. Every arm I have ever seen - apart from the odd one or two - has been made this way. I don't really think manufacturers generally give it much thought.

[walpurgis]

Isn't it simply a question of following convention and the convention being to have the horizontal bearing axis in-line with the armtube?

That seems to be the case.

My thinking often goes against convention.

[Mr Kipling]

Will it be a uni-pivot? Ever since I got my first Hadcock I've been intrigued by uni-pivots.
I have one or two ideas of my own to help with stability that I've never seen on commercial designs.

[NRG]

The Kuzma Stogi S is a great example of how to get great stability with the pivot point on the same plane as the stylus. Not only does it use counter weights at or below the pivot point theres substantial mass in the shape of a bell type housing below the pivot point ensuring stability.

[walpurgis]

Will it be a uni-pivot? Ever since I got my first Hadcock I've been intrigued by uni-pivots.
I have one or two ideas of my own to help with stability that I've never seen on commercial designs.

Yes!

The bearing will be different to anything I've seen tried elsewhere (if it works). I have some evaluation to do yet.

I'm aiming to keep the bearing axis approximately in plane with the stylus tip. Which was why I'd been asking my question. The arm will be of low to medium mass and stabilised by keeping as much weight as possible underslung. I'm hoping to use little or no metal in the construction. The bearing will be directly fluid damped. Not saying too much, as I don't want to give the game away.

I already have all the materials and need to 'soak test' my design thoughts. I seldom go wrong when building something, because I think it all out, over and over and visualise things in 3D in my head. Works for me.

[John Gordon]

Geoff, Stephen,
There are many arms with bearings at stylus level. I used it when I designed my first arm over 30 years ago. It is a good idea. Do some research - eg SME, Graham, Triplanar, AT, etc.

Don't use it if you want a simpler arm, or one easier (or perhaps, but not always, cheaper) to make - eg Schick.

Good engineering depends on the given design specification and budget.

John

[Mr Kipling]

But how many is "many"? Surely the vast majority of designs simpley don't take it into account.

[John Gordon]

Mr Kipling
The majority of arms are/were quantity items made down to a price. Most were made in Japan by companies like Jelco to a customer spec which would be based on cost. Therefore most arms are similar and simple, with what has become a conventional armtube/bearing/counterweight design. It is the conventional way to make a budget arm because it is simple and cheap. It is also used in some very expensive arms...

I have tried in the distant past to design a competitive budget arm, but it is not easy unless you have enough capital to tool up for high volume, like Rega did with their arm.

As manufacturing budgets rise and/or manufacturing quantities rise, there is more scope for incorporating more expensive features, recently, the preferred option being better wire, or fancier finish, but usually something that adds perceived value at minimum cost.

A manufacturer like SME, not producing budget arms, but with relatively high volume of more upmarket arms has chosen to incorporate better design features, one of which is low set bearings in the V, IV.

So, how many is many? Granted, probably not that many in absolute terms, but of the better arms, a fair proportion. Other arms do not use lowered bearings for various reasons, the two main ones I would guess are cost and the saving on design time, not that they haven't considered it (at least I would hope so).
John