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Old 3rd August 2011, 07:22 PM   #121
stevieg is offline stevieg  United Kingdom
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Quote:
I'm tempted to build something up with just a crank, rather than a servo, to listen and see if the effort is worth it.
plenty of us reading this thread who could volunteer... I'm still agog - look at all them moving parts. I love to see the human mind at full stretch.
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Old 5th August 2011, 01:16 AM   #122
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Conrad Hoffman: "I've been working on my own version of a conventional pivoted arm design, but with the pivot sliding in a track to the rear. .... Commercially it's probably out of the question."
Instead of a complete arm system, I think it's commercially feasible if someone can come up with a motorized gliding armboard or armbase with sensors. Think of it as an accessory product. Imagine the Rabco arm upside down minus the pivot arm section. Just the base so the user can mount their favorite pivot arm, eg., Rega, Jelco, Graham, Triplanar, FR, etc... I even thought of reusing some part from a couple beater Rabco arms--salvaging the motor, chain pulley, sensor, etc... and add modern linear bearings to the carriage such as THK linear bearings, etc. This way, the user can use their favorite pivot arm and mount that onto this armbase and achieve tangency. Obviously they have to readjust or straighten the offset angle at the headshell on their arms.

Anyway, just a thought.

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Old 10th August 2011, 10:23 PM   #123
RCruz is offline RCruz  Switzerland
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Very good thread
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Old 11th August 2011, 11:53 AM   #124
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Originally Posted by Conrad Hoffman View Post
IMHO, all mechanical nightmares that probably introduce as many problems as they cure. Here's what I've been thinking about, but have yet to build. Pivoted tone arms are highly developed and work well, but I think that offset introduces forces that cause more trouble than can be fixed with anti-skate mechanisms, not to mention the obvious problem of getting the angle right at only two points. Rotating the head of an offset arm is not the answer! You must eliminate the offset. So, keep the standard tonearm, remove the offset and make it a bit longer. Now, just drive the pivot assembly along the back of the turntable in a track. Servo it, probably photoelectrically, but don't try for perfection. Let the tonearm do what it was designed to do. Try for silence and smoothness on the drive. I'm not sure what the best drive method would be, maybe a lead screw and DC motor, maybe a thin thread and drum drive or even a linear motor track. This is basically an electronic forum, so moving the pivot under motor control shouldn't be too difficult. The angular error will be very small even if the pivot location is off a couple mm- no human can align a cartridge that close regardless of what they claim. Maybe line the track with Teflon tape and/or make it a V-way for better stability. IMHO again, any system that tries for a rigid coupling between the tone arm and track (like air bearing arms) creates a horizontal mass problem that's completely unnecessary. The arm in this design can be a unipivot or whatever you like.

Conrad
Conrad, I would agree with near everything you said here. As for a matter of driving the pivot assembly of the arm along the track, I believe it is not necessary applying any kind of motor of another artificial drive, because the spinning record groove itself , been driven by the TT platter, is already a perfect kind of smooth driving system, both powerful and accurate. All we need to do is to figure out and apply some kind of mechanical device (let's say, a bar with a carbon brush), which will drag the arm pivot assembly.
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Old 12th August 2011, 12:35 PM   #125
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Default tangential pivot tonearms

As I see, there were not so few record groove driven linear arm designs, before introduction of electronically guided motor driven ones. However, it seems that purely mechanical systems were still far from optimum.
Back to semi-tangential pivot arms, they seem to be resolving the horizontal angular mistake problem. However, still having skating-antiskating forces problems, not very dissimilar from those of conventional arms. If perfect antiskating device will be applied, would it solve them? What is a main problem with creating such a device?

Last edited by walterwalter; 12th August 2011 at 12:38 PM.
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Old 12th August 2011, 01:13 PM   #126
oshifis is offline oshifis  Hungary
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? Tangential (be it pivoted or not) arms have no skating problem by definition.
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Old 12th August 2011, 04:11 PM   #127
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Hi,
The problem with creating the "perfect" skating force compensation device is that skating force depends on a multitude of parameters(i.e. vinyl formula, modulation level, stylus profile and quality of polishing,etc...), resulting in a compensation varying from 4-18% of the VTF. ALL antiskating mechanisms are a compromise with about 8% of VTF being a reasonable compromise.
@oshifis: It is NOT correct to assume that tangential tracking omits the skating force issue. I.e. , both the Thales and the Simplicity arm do compensate for the skating force that(in both cases) is decreasing with radius, despite the tangential tracking(tracing would be the more appropriate term). Both feature an offset angled headshell, so only if the design features NO offset angle, hidden or not, will there be no skating force. Essentially all servo driven linear trackers require a deviation from tangency to trigger the correction. This deviation results in a force comparable(qualitatively, not quantitatively) to the skating force present on pivoted arms with an offset angle. Only if another arm(preceding the actual tracking arm) reading(optically or mechanically) the (non constant) pitch would "govern" the tracking arm via a servo/feed forward circuit, the issue could be avoided. But a certain amount of tracking distorsion remains unavoidable irrespective of achieving perfect tangency.

Have a fun weekend,

Frank
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Old 12th August 2011, 06:34 PM   #128
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Thank you, Frank. So, simplifying a bit, in both conventional and semi-tangential arms skating force varies:
a) depending of groove-from-record-center distance stylus placement on each record,
b) it varies differently on different records, depending on the different modulations (apart for now from stylus shape and vinyl components). How much it varies on later (percentage)? Assuming we achieved a "perfect" antiskating device for one record, how accurate it would be on others?
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Old 12th August 2011, 06:35 PM   #129
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Default motorized linear tracking

Quote:
Conrad Hoffman: "Rotating the head of an offset arm is not the answer! You must eliminate the offset. So, keep the standard tonearm, remove the offset and make it a bit longer. Now, just drive the pivot assembly along the back of the turntable in a track. Servo it, probably photoelectrically, but don't try for perfection. Let the tonearm do what it was designed to do. Try for silence and smoothness on the drive. I'm not sure what the best drive method would be, maybe a lead screw and DC motor, maybe a thin thread and drum drive or even a linear motor track."

I was trying to see if there's any existing product out there that is thinking along the same line. All I can think of is the Pierre Lurne designed Audiomecca SL5 tonearm. If you gut the tonearm part out and keeping just the track and gliding mechanism and the armbase, what you have is close to what you have in mind. I would like to have something like the SL5 and try it with a high quality conventional pivot arm, say, a la Clearaudio Satisfy, straighten the offset angle at the headshell and mount that on the Audiomecca and simply using it as a linear tracking base with sensor. So the end product is more like an accessory that allows mounting different arms. If we use a 12" arm the whole thing can be assembled outside of the plinth of the turntable as a tonearm island.

Here are some interesting pictures.


Click the image to open in full size. Click the image to open in full size. Click the image to open in full size.

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Click the image to open in full size. Click the image to open in full size. Click the image to open in full size.

Click the image to open in full size. Click the image to open in full size. Click the image to open in full size.
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Old 12th August 2011, 08:25 PM   #130
DIYNick is offline DIYNick  United Kingdom
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^^^^^^I want one of these
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