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Old 6th June 2007, 07:47 AM   #71
YNWOAN is offline YNWOAN  United Kingdom
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If you look at your two diagrams it is not ouright stability that is the major issue but more an issue of changing geometry.

If you consider the lower diagram; as the cartridgre tracks warps the arm will move around a pivot with a radius the length of the supporting bearing - this will wildly alter the relationship between the stylus and the groove.

The effect is much smaller in the top diagram because the distance (in the horizontal plane)) between the Stylus and the pivot point is much smalller.

Ideally, the centre line of the mass of the counterweight should be in the same horizontal plane as the tip of the stylus to reduce the effect of tracking weight altering when warps are tracked.
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Old 6th June 2007, 10:16 AM   #72
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If you look at a Ladegaard arm, the pivot point is more like the lower of Nanook's 2 drawings. However, the pivot point, and the stylus are at the same height. The centre of mass of the counterwieght is below both of these.

I have made a tonearm with vertical bearings something like the Ladegaard desigh, but with the bearing head from a VHS machine as the horizontal bearing. The vertical bearing is made of a knife blade resting on 2 edges of Ali on the head drum.

The 'Headshell' is a peice of metal on the bottom of the arm tube, just big enough to provide a flat surface to mount the cart on. In this way, the record surface, pivot pint and counterwight are level.

When the counterweight was above the pivot point, the VTF would vary wildly- if the arm was up high, it would stay there, and if it was low, the VTF was very high (IIRC- could be the other way round). With the C.O.M. of the weight level with the pivot, the VTF was far more stable. I have not tried with the C.O.M. below the pivot point.

james
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Old 6th June 2007, 11:23 AM   #73
jamikl is offline jamikl  Australia
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All the reading I've done, and I've done a lot over the past few days - including patents, indicates that the stylus tip, the pivot point, the counterweight should be in a straight line although it seems that centre of gravity may be allowed to be partly below this line but not far enough to create any pendulum effect which the 2nd drawing certainly would.
When viewed from the front the stylus - cartridge assembley has a slight tilt or lean - is this usually arranged at the headshell or with a shim under the cartridge or where?
jamikl
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Old 6th June 2007, 11:46 AM   #74
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That sounds about right.

Re the lean: Do you mean a front-rear lean (Vertical Tracking angle) or a side-to-side lean (Azimuth)? VTA, if adjustable, is usually adjustable by raising or lowering the mount of the tonearm. Azimuth is done wither with thin shims under one of the cartridge screws, or by twisting the headshell- if possible.

james
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Old 6th June 2007, 11:57 AM   #75
jamikl is offline jamikl  Australia
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Azimuth was the one I wanted, thanks James. What should this angle be?
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Old 6th June 2007, 01:36 PM   #76
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The cartridge body should be parallel with the record. Therefore, the stylus should be totally vertical in the groove.

James
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Old 6th June 2007, 05:14 PM   #77
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yes, Nanook, sounds strange...
and apparently introduces some form of assimetry that leds me to think (maybe wrongly) that the tonearm lever effect is somewhat distorted...

But not so out of the blue if I´m understand well what I can see of the Wheaton Triplanar ( http://www.triplanar.com/), where the armtube is partitioned in such a way so record surface and bearings are at same level...
But, yes, in that example pivot point and center of gravity (weight) are at same level.
That´s a very complex one example, and has very little in common with this experiment of mine.

I was doin´some adjustments and listening since my last posting.
Thank you very much for all the advice provided meanwhile!
(Nanook, Ynwoan, Jrewillug, Jamikl et al)

In the scheme that I have adopted, putting bearings at record level only causes severe mistracking, loss of bass and ¨hollowness¨ in the sound.
I understand now that this happens because center of gravity is too high.

And all those nasty things dissapeared when arm longitudinal axis and vertical movement (bearings) axis intersect each other (as in a ¨cross¨ figure) .

Now the cart/arm combo tracks extremely well -in fact, the cheap and old Shure m70 -spherical stylus- that i´m using for this experiment is sounding like a much more sophisticated cart/stylus!-
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Old 7th June 2007, 12:24 AM   #78
Nanook is offline Nanook  Canada
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Default TNWoan...and others

re: changing geometry, YES. But also controlling a "hung" mass is easier than a "sprung" mass. Same holds true regarding using bearings. Try controlling an arm such as the 2nd one in my sketch vs the 1st one.

re: centreline of the counterweight mass in"line" with stylus tip--YES, or slightly below.

The counterweight can be below.

Implementing a unipivot is much much easier, as is the azimuth adjustment, when an anit-skating device is used.
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