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Vi consiglio di
leggere qui:
http://www.tnt-audio.com/sorgenti/garrard.html
http://www.tnt-audio.com/clinica/garrard2.html
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La modifica che ho fatto ad un altro Garrad 100: |
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La geometria di questo piatto spiegata bene : |
Dal sito:
http://www.quadibloc.com/science/freq03.htm
A Problem in
Applied Geometry
In
most record players, the tonearm pivots on one point, so that the
headshell, cartridge, and stylus swing out an arc over the record. But
the groove on the record is oriented so that motion corresponding to
sound is always precisely radial, that is, along a line going out from
the center of the record.
The
headshell on the end of a tonearm is at an angle with the path to the
pivot of the tonearm so as to compromise between making the tonearm
longer, thus reducing the angle by which the tonearm pivots to cover a
distance along the record, and increasing the distance the stylus moves
from the periphery of the record to the end of the grooves containing
sound upon it.
There are linear-tracking turntables, some using electronic feedback,
and others suspending a radially-moving tonearm on a cushion of air.
Then there was the technique illustrated in the diagram below:
I
trust that some of the readers of this page may actually remember the
Garrard Zero 100 turntable, whose tonearm used an ingenious mechanical
linkage to minimize tracking error. While the original Garrard Zero 100
turntable used traditional idler wheel drive, a later model, the Zero
100SB, which can be recognized by having a silver surround, used belt
drive. One previous attempt at a tonearm that reduced tracking error
through a linkage had been made over a decade previously by another
British firm, Burne-Jones & Company. In that tonearm, the two rods
making up the arm were obviously not parallel, converging on the
headshell; in the tonearm of the Zero 100, the pivots were put on
opposite sides so that instead of the arms visibly diverging as they
approached the headshell, they were approximately parallel.
Older consumer turntables simply used a rubber friction wheel, which
could be switched in and out as needed, to transfer motion from the
motor to the turntable. This allowed changing speeds from 33 1/3 RPM to
45 RPM and perhaps even 78 6/23 RPM.
More sophisticated turntables, intended for the advanced high-fidelity
enthusiast, used a rubber belt to transfer the motion of a spinning
drive shaft to the turntable, providing better isolation from the
vibrations of the motor, and reducing the potential for problems
resulting from the idler wheel becoming flattened while the turntable
sits idle. These normally would use synchronous motors.
Later, more advanced servomotors allowed the construction of
direct-drive turntables, with impressive specifications in terms of the
accuracy of speed and of wow and flutter. But audiophile purists were
not enthusiastic about the direct-drive turntable.
In
roughly the same era of the nineteen-seventies when the Garrard Zero 100
turntable was offered to the public, an interesting design attempting to
combine the advantages of both approaches was made available. The
Philips Direct Control turntable combined the principle of belt drive,
which helps to smooth out the vibrations inherent in electrical motors,
with electronic control of speed as found in direct-drive turntables.
Because of the separation between the motor and the turntable provided
by the belt, which was, of course, the goal of the design, the feedback
loop had to be designed so that the reaction to feedback would be based
on a long time-average of the feedback signal.
Even without measures such as using a linear-tracking tonearm or a
design such as the Zero 100, in a conventional pivoted tonearm, while
the arm may move by 20 degrees on its pivot, maximum tracking error is
usually only about 2 degrees. This is because a circle centered on the
tonearm pivot, with a radius somewhat larger than the distance from the
pivot to the spindle, can cross the grooves of the record at an angle
which is nearly a constant value, somewhere around 20 degrees. This
explains the shape of a normal tonearm, with the portion containing the
cartridge tilting inwards.
The diagram below
illustrates how one can search for an optimum tonearm shape:
the black circles
representing the grooves of the record, the blue arcs representing the
paths of the stylus for different lengths of tonearm.
The diagram below
illustrates how the Garrard Zero 100 worked.
Note that, because of where the pivots are located, the actual linkage
is an irregular quadrilateral, even though the tonearm shaft and the rod
accompanying it to correct the tonearm position are parallel, because of
the locations of the pivots. This may not just improve the appearance of
the tonearm, it may make it behave more like a conventional one as well
in other respects.
The
arcs show how the pivoting rod, moving around a smaller circle with a
different center, will pull the headshell into a position with a smaller
tilt relative to the main tonearm shaft as it moves towards the center
of the record.
Since this change takes place in only one direction, although the
tonearm resembles a conventional tonearm, in having a headshell tilted
at a similar angle, it does differ, as the larger arc shows, by having
no overhang, since the overhang of a conventional tonearm not only
decreases the change of angle, relative to the grooves, over the range
of the tonearm, it also causes that change to reverse direction within
the record, so that the headshell can be aligned for correct tracking at
two points. That minimizes tracking error for the conventional tonearm,
with maximum tracking error in one direction at the beginning and end of
the record, and maximum error in the other direction in the middle, but
it would prevent a correction factor that continuously increases from
the beginning of the record to the end from being correct.
A
patent, British patent 1,342,739, was eventually received for this
tonearm design in 1974, but by that time Garrard had become part of
Plessey, which became the assignee of the patent. It was also patented
in Canada, Germany, and Switzerland, but not the United States. In the
patent, it is noted that the residual tracking error of this tonearm
design is limited to an angle of 1.5 minutes.
The
patent gives the four dimensions of the quadrilateral shown in the
diagram; the distance from the tonearm pivot to the spindle, which
matches that from the pivot to the stylus directly under the center of
the main bearing, is 7.5 inches; the distance between the two pivots on
the headshell is 1.5 inches, the distance along the rod controlling the
angle of the headshell is 6.75 inches, and the distance between the main
tonearm pivot on the plinth of the turntable to the auxilliary pivot of
the control rod is 0.627 inches.
The
angle between the 1.5-inch line between the two pivots on the headshell,
and the direction from the tonearm pivot to the auxilliary pivot
relative to the line between the tonearm pivot and the spindle are left
as exercises for the reader, but they can be determined by aligning a
calculation of how this arm moves with one of tracking angle errors for
a conventional tonearm of 7.5 inch radius without overhang.
Recently, another pivoted tangential tonearm has become available, the
Thales tonearm. It is very different in appearance from either the
Burne-Jones tonearm or the Zero 100 tonearm, as it involves motion in
the third dimension as well as motion parallel to the plane of the
record being played.
A
1941 paper in the Journal of the Society of Motion Picture Engineers
by the American engineer H. G. Baerwald worked out the conditions for
optimal conventional tonearm design; one important thing to note is that
it needs to be taken into account that the effects of tracking error are
more serious for the inner grooves of the record, since the undulations
in the groove representing the sound waves are closer together there.
One approximation that can be used is to weight tracking error by the
reciprocal of distance from the spindle.
An
earlier paper by Erik Löfgren, cited in Baerwald's paper, proposed two
alignments, the first being the same one as recommended in the paper by
Baerwald which analyzed the question of tonearm design more thoroughly.
Because there are additional problems with reproduction of the inner
grooves of the record, in addition to the effects of tracking error
being more severe there, a paper by Stephenson proposed a third
arrangment more heavily favoring the inner grooves. And the turntable
manufacturer Thorens has pursued an alignment standard of its own.
The
Zero 100 was one form of articulated tonearm, aimed at reducing tracking
error. But another class of articulated tonearm, including the
Dynavector, the Vestigal, and the LOCI, was aimed at reducing the
consequences of warped records through movements in the vertical
direction. The last of the three is not to be confused with the first
Wang Labs electronic calculator, which operated on numbers in
logarithmic form.
Of
course, one could always take a brute-force approach to the problem to
provide perfect tracking, rather than simply a much closer approximation
by using a well-chosen parallelogram:
A
pivoted platform, containing pullies which constrain a rod coming from
the tonearm to always pass through the point corresponding to the
spindle in a scaled-down analog of the turntable, would ensure that this
rod follows the required tracking angle, which could then be transmitted
to the headshell by a plain parallelogram.
Because the linkage between the rod and the freely-pivoted platform is
not a simple pivot, but instead a constraint to linear motion over its
central point, one has the same mechanical problems as are involved in
building a linear tracking tonearm directly; it is very difficult to
make a linkage of this sort without excessive friction. This is why
mechanical linear-tracking tonearms have to rely on such expedients as
air suspension to reduce friction.
Note also that instead of the rod from the tonearm resting on two
pullies on a rotating platform, it might be constrained between pullies
on its right and left sides, on a frame that is not only pivoted to
allow it to rotate back and forth as the tonearm moves along the record,
but also to allow it to tilt up and down, so that the tonearm may be
lifted. Also, a cam centered on the main pivot of the tonearm could be
designed so as to control the rotating platform, instead of relying on
force from the rod to turn it.
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