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DIY Air Bearing Linear Arm
DIY Air Bearing Linear Arm
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Old 10th February 2015, 05:26 PM   #41
super10018 is offline super10018  United States
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Join Date: May 2010
I donít think my device adds mass. But it is not important. I also donít think it is important how you call it damping or bias.

My tests show there is a fundamental misunderstanding about groove and stylus interaction for a linear tracking arm. This is important.

I did another test last night. Everything else was same as my 1st test except I use my diy ball bearing arm. The cartridge was Denon DL-103R, a low compliance cartridge. There was no damping device. Here is the results. Please see attached images. The distortions do exist in left channel only. It means that the needle was slammed onto right side of groove wall and lost contact with left side of groove wall.

The result is same as what I did on the air bearing arm. I believe that my tests are highly repeatable. Therefore, they are not tonearm specific. These tests raise questions which challenge fundamental thinking laying under linear tracking arm design.

The consensus for linear arm damping( or biasing) is that the stylus will be slammed either on left side or on right side. Therefore, damping should be applied on both sides with equal force. I thought the same until I did the tests.

However, my tests show otherwise. A stylus of linear arm will be slammed on right side of groove wall only. So, if the damping applies equal force on both sides, it wonít correct the problem. The stylus isnít still centered in groove. This is why I said all the damping devices for linear arms on the market are not correctly implemented.

I would like to see someone else do the same test.
Attached Images
File Type: jpg ball bearing1.jpg (359.7 KB, 406 views)
File Type: jpg ball bearing2.jpg (342.1 KB, 387 views)
File Type: jpg ball bearing3.jpg (342.3 KB, 377 views)
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Old 11th February 2015, 02:55 AM   #42
richard38 is offline richard38  Netherlands
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Join Date: Nov 2013
Location: netherlands
Real nice work , I love it when peoples can their own tone arms , I want a fracemacihine(Dutch translatation), only I ain't got the space and my parents ain't waiting for one.
Keep up the good work.
Greets richard
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Old 11th February 2015, 01:35 PM   #43
niffy is offline niffy  Europe
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Join Date: Nov 2013
Location: Dartmoor
Hi Super.

Very interesting results. It puts me in mind of the clearaudio arms. As these are mechanical, like your ball race arm, damping is supplied by the mechanical contact between race and tube. Clearaudio recommend that the rail is set very slightly downhill towards the spindle. This would supply a bias similar to that which you have implemented in you air bearing arm. It would appear that they have come to the same conclusion as you. Could you repeat the test on your ball race arm with the rail sloped and see if this has a positive effect. It would cause the VTA to change slightly as the arm traversed the record but if the benefits outweigh the negatives it would be worth implementing.

Niffy
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Old 11th February 2015, 02:28 PM   #44
Max Headroom is offline Max Headroom  Australia
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Location: West Australia, near to the beach, natural ambient sounds mostly.
Your tests look more like lack of tracking weight, or dynamic mispositioning of sled is causing over deviation of cantilever positioning which is causing dynamic mistracking.
It seems that passive linear tracking is flawed due to too high sled/arm/cartridge masses, especially for eccentric discs.
Lowering moving masses will serve to raise system resonance frequencies up into audio band.
Sled damping needs to be velocity dependent, and not frictional.

Dan.
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Old 11th February 2015, 06:03 PM   #45
niffy is offline niffy  Europe
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Quote:
Originally Posted by Max Headroom View Post
It seems that passive linear tracking is flawed due to too high sled/arm/cartridge masses, especially for eccentric discs.
I would disagree with this statement. I calculated the cantilever displacement purely due to carriage mass eg bearing friction, arm wires resistance etc not included. I based the calculation on my arm and cartridge with a record with a 0.5mm eccentricity which will cause the stylus to dance back and forth by 1mm, quite a severe case.

Carriage mass. 50g
compliance. 20um/mN
cantilever length 5mm
speed. 33.3rpm

This case would result in the cantilever being displaced by only 0.02 degrees. Well within the zone where the cartridge is still linear, in fact a lot less than the deflection observed due to groove modulation. With this arm /cartridge combination the cantilever resonance is just under 5hz safely above the 0.55hz driving force caused by disc eccentricity.

Passive linear tracking is not flawed and any short comings it may have are massively outweighed by the advantage of being able to utilize very short armtubes.

Niffy
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Old 12th February 2015, 07:41 PM   #46
super10018 is offline super10018  United States
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Hi Niff

I did the test as you requested last night. I tilted the ball bearing arm less than 1 degree and adjusted azimuth to make sure the stylus is perpendicular to the record. Here is the result. Please see the image.

For air bearing arm, the distortion from left channel starts at about 7 db signal because frictionless bearing. Air bearing doesnít have any damping. But for ball bearing arm, the bearing and glass rail are not frictionless. The distortion starts at 10 db. This friction acts as damping for the arm. If I tilt the arm more, the distortion will switch to right channel. It means overdamping(or basing). The stylus hits the left side of groove wall.

So, for an air bearing arm, damping is a must, but for ball bearing arm, you may get away without damping. However, you must realize that the ball bearing may have high friction since damping is not necessary.
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File Type: jpg Image 00111.jpg (278.2 KB, 321 views)
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Old 12th February 2015, 08:31 PM   #47
niffy is offline niffy  Europe
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Hi Super.

Thanks for the test. The sub1 degree tilt has certainly improved the plots.
The result would suggest that the force required to track from outer to inner groove is significant even with a virtually frictionless bearing. This seems a bit counterintuitive especially as the distortion always appears in the left channel suggesting that this force is more significant than that due to eccentricity.

I Have the begining of a theory as to why this may be but will have to think on it a bit more before committing it to paper.

Niffy
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Old 12th February 2015, 08:48 PM   #48
super10018 is offline super10018  United States
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Quote:
Originally Posted by Max Headroom View Post
Your tests look more like lack of tracking weight, or dynamic mispositioning of sled is causing over deviation of cantilever positioning which is causing dynamic mistracking.
It seems that passive linear tracking is flawed due to too high sled/arm/cartridge masses, especially for eccentric discs.
Lowering moving masses will serve to raise system resonance frequencies up into audio band.
Sled damping needs to be velocity dependent, and not frictional.

Dan.
I donít think the stylus lacks tracking weight because the right channel is almost perfectly tracked. VTF was correct. It isnít caused by moving mass either. The cause is the interaction between groove wall and stylus. I can do another test to approve my point. But I need to change the cartridge to a cheap one. I will run the cartridge on a grooveless disk. I expect that the cartridge will be stationary and will not move sideways.

A cheap ink jet printer can do 600 dpi or 1200 dpi. The resolution is well enough for tracking records, but the tracking system may be very difficult to implement for diyers. I am sure active tracking arm will have some kind of problems too.

I donít think the linear tracking armís mechanism is flawed, but little complicated.
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Old 12th February 2015, 08:52 PM   #49
super10018 is offline super10018  United States
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Quote:
Originally Posted by niffy View Post
Hi Super.

Thanks for the test. The sub1 degree tilt has certainly improved the plots.
The result would suggest that the force required to track from outer to inner groove is significant even with a virtually frictionless bearing. This seems a bit counterintuitive especially as the distortion always appears in the left channel suggesting that this force is more significant than that due to eccentricity.

I Have the begining of a theory as to why this may be but will have to think on it a bit more before committing it to paper.

Niffy
Tilting the arm means adding damping force(biasing) on left side of groove wall. Therefore, the distortion on left channel is gone. For right channel, the stylus is being slammed on the right side of groove wall. Extra damping is not necessary.

Last edited by super10018; 12th February 2015 at 08:55 PM.
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Old 12th February 2015, 08:56 PM   #50
niffy is offline niffy  Europe
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I think the bias applied buy the tilt is more significant than any additional damping effect.
Little grey cells engaged and working the problem.

Niffy
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