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On 18/12/2011, George Brock-Nannestad wrote:

> From: Patent Tactics, George Brock-Nannestad
> 
> 
> Hello,
> 
> Eric Jacobs has given a very full answer to the S- and straight arm
> question.
> 
> However, there is one further aspect of the tangential tracking arm
> question and pivoted arm question that has to be kept in mind. The
> original derivations of the advantages and disadvantages of the
> various pivoted arm geometries was based on mono only, and
> coarse-groove dimensions. Percy Wilson pointed out a special form of
> time distortion: the overhang in his elaborated example meant a
> gradually slower reproduction because in effect the needle tip moved
> along the groove during play. However, he calculated that on one
> revolution at 78rpm the delay only amounted to 1/400,000 second. That
> is not discernible to a listener. However, in the stereo case, and in
> particular with line contact or elliptical stylii, the situation is
> different, and the time scale between the two channels shifts. With a
> slower speed and higher recorded frequencies there are definite time
> delay shifts between channels at the high frequencies.
> 
I think this would be negligible compared to the effect of not being
precisely equidistant from the speakers.

> A tangential tracking arm will only alleviate the later problem if the
> line contact or elliptical stylus moves in a true radius from edge to
> centre, and if the edges of the stylus lie on that radius. A
> tangential arm is also pivoted, but a mechanism ensures that its
> angular variation is held within small tolerances; this is obtained by
> moving the carriage the pivot is sitting on.
> 
> A different problem that is related to the problem of offset arms when
> used for scratching is that when the stylus is pulled, it tries to
> straighten the link from stylus tip to pivot. This link consists of
> three parts: the cantilever carrying the stylus, the elastomer or thin
> wire bearing for the cantilever in the cartridge and the tonearm. The
> straightening causes the cantilever to assume a "zero" position that
> is at an angle within the cartridge. Cartridges have individual
> sensitivities to having a "bias" in the centre position; some magnetic
> cartridges will cause distortion, because they "bottom" earlier in one
> channel than the other.
> 
> However, almost all cartridge-tonearm combinations, irrespective of
> pivoted or tangential, have the same "straightening" problem in the
> vertical plane. In order to alleviate this, the pivot for the vertical
> movement of the tonearm ought to be placed where a line through the
> cantilever extended backwards hits a vertical line from the pivot. And
> this has to be the case when the stylus pressure is the intended one.
> The only tangential tracking arm that has a chance of getting this
> relationship correct is the Revox. I have never checked whether this
> is the case in practice.
> 

Regards
-- 
Don Cox
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