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Old 17th April 2004, 11:25 PM   #21
sreten is offline sreten  United Kingdom
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lets be clear on this :

Although I find parallel tracking arms interesting I have
no interest whatsoever in replacing my Rega tonearm.

Its mounted on a suspended subchassis turntable.

However as a why not ? if not ? and thats just wrong ?
person I find the the basic principles of alternative
approaches interesting, especially if they are dodgy.

Here I'm firstly interested if its possible, and secondarily
if possible what are the compromises involved. I feel if
its possible unless a serious disadvantage is revealed
that it would be viable approach for arm builders who
accept the performance trade-offs.

sreten.
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Old 18th April 2004, 09:50 AM   #22
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Hi Sreten,

The system you propose in the drawing is not stable. It only works if the moving magnetic system is perfect vertical. But if it turns a little there will be a horizontal force on the upper pole of the moving part applying a force on the tone arm, either up or down, depending on the direction of the turning.

Regarding the force possible, a flux density of 0.1 T shouldnít be a problem with standard ceramic magnets, i.e. those from magnetic door locks.

Regarding eddy-current braking, you can put a strip of copper at the inside of the upper stationary pole. But anyway the speeds involved are too less to let an eddy-current brake be effective IMHO.

Cheers
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Old 18th April 2004, 11:44 AM   #23
sreten is offline sreten  United Kingdom
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Quote:
Originally posted by Pjotr

The system you propose in the drawing is not stable. It only works if the moving magnetic system is perfect vertical. But if it turns a little there will be a horizontal force on the upper pole of the moving part applying a force on the tone arm, either up or down, depending on the direction of the turning.
Cheers
If you read the text with the original diagram this is discussed,
some optimisation is possible, whilst on the subject :

Quote:
Originally posted by dice45
Iwould not be happy concerning the hanging architecture, i prefer to have my vertical pivot axis as close to the records surface as possible; warps and pressing bubbles have the fewest effect on effective stylus speed then.
I agree there would be a motion vector along the groove.
However using double v blocks and a knife edge vertical
bearing as previously mentioned would address this if it is
a concern. both approaches have merits / disadvantages.

sreten.
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Old 18th April 2004, 12:02 PM   #24
sreten is offline sreten  United Kingdom
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Quote:
Originally posted by dice45
Maybe it's me only who is incompatible to the sonic effects caused by a small magnet or a piece of iron located in close proximity to a signal-conducting wire (or worse: around that wire, add to that list: faintest trace of magnetic attractability in any passive or active component except Xformers).
Certainly magnetic effects cannot be ignored.

And a DC coupled phono pre amplifier would not be a good idea.

Asumming the phono preamp has a sensible subsonic
filter I can't see any major problems with the magnetics.

Though I would presume any vibrations in the tonearm wires
would produce a signal, so some care is needed here, twisted
pairs at least I would of thought.

sreten.
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Old 18th April 2004, 02:00 PM   #25
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Quote:
Originally posted by sreten

If you read the text with the original diagram this is discussed,
Oops missed that point. Maybe something along the lines of the attached pic may get rid of it. But I donít think it will do completely and such a thing is not easy to make. You need a skilled instrument maker for it with the appropriate tools and machines.

Cheers
Attached Images
File Type: gif mag_bearing.gif (3.5 KB, 327 views)
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Old 18th April 2004, 02:33 PM   #26
sreten is offline sreten  United Kingdom
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nice diagram !

But it misses some of the other points, namely the deliberate
\o/ bottom interface to "increase stiffness ?", the top section
is suggested as a possibilty in the text, the text also suggests
the top gap is much larger than the bottom.

The hanging design makes the structure easy to make, the idea
is the instabilility is deliberately exploited to reduce the increase
of tracking force over warps due to the hanging design.

sreten.
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Old 18th April 2004, 05:10 PM   #27
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Hi Sreten,

I tried to make the air gap as small as possible, just to minimise play and as such increase stiffness. I didnít intend to suggest any further construction details. The hanging construction seems to me an obvious way. You need to put the counterweight somewhere and anyway you canít put is through the magnets

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Old 19th April 2004, 02:33 AM   #28
hifiZen is offline hifiZen  Canada
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Has anyone considered active servo control of the magnetic suspension to improve the stiffness? An actively driven coil setup with optical (laser) feedback system could concievably maintain extremely high precision positioning.

Specifically, I'm thinking about the laser heads and servo systems used for CD/DVD players, as I have some experience with these. Off the top of my head, the focus accuracy needs to be something like 5 microns. Focus and tracking of a DVD laser head has a frequency response of roughly 2kHz bandwidth, due to the low mass of the lens and positioning coils. Obviously this kind of response would not be possible with the mass of a tonearm carrier, but if a very tiny gap could be maintained between the carrier and it's rails, then one could rely on the air film for additional stiffness and damping, just as with an air bearing system...

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Old 19th April 2004, 11:28 AM   #29
sreten is offline sreten  United Kingdom
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Hi hifiZen,

Good to have another viewpoint.

But IMO a non starter for the following reasons :

a) AC signals through the field coil would be very difficult to suppress.

b) As far as I understand an "airfilm" 5um gap requires
very high precision and finish in the slide and guide.

c) once you introduce a sevo system is begs the very obvious
question of servoing the horizontal positioning, and this has
been comphrehensively done before.
Including servo absolute centering of the record on the platter.

I'm really interested in if a passive magnetic approach is possible.
As far as I can tell so far the basic idea with an adjustable
field coil could be a practical simple workable solution.

sreten.
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Old 19th April 2004, 05:25 PM   #30
hifiZen is offline hifiZen  Canada
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Hm. I think you're right.

After sleeping on it, I've decided that the possibility of having vibrations introduced directly to the tonarm carrier by the servo coils may well outweigh any benefits. As I recall, the easy way to tell when a CD/DVD head had gained focus lock is the 'hissing' noise it makes, by virtue of the fact that the objective lens and it's coil act just like a tiny tweeter... once the loop is closed and the servo feedback takes over, any noise in the system manifests itself as that hissing sound. As you can imagine, SNR in these little feedback loops isn't wonderful. Considering the system complexity, and add to that the reasons you give above... it doesn't look too good. But, it was fun to think about!
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