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DIY Air Bearing Linear Arm
DIY Air Bearing Linear Arm
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Old 29th January 2015, 08:22 AM   #11
Max Headroom is offline Max Headroom  Australia
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Location: West Australia, near to the beach, natural ambient sounds mostly.
Quote:
Originally Posted by super10018 View Post
He wrote:" The one glaring omission in the Air Line's design is a damping trough. A linear design—even one using a stationary rail—must laterally move a large mass. The grooves of most LPs are not concentric because of pressing inconsistencies, which means the groove is constantly shifting the tonearm's mass back and forth. Because the stylus is at the end of a spring mechanism (the cantilever's suspension), if you use a cartridge with too high a compliance—too floppy a spring—you can have the tail wagging the dog, in that the motion of the groove spiral will move the cantilever from its central position. The risk is then that with a frictionless bearing the stylus might be slammed from one groove wall to the other.
Exactly as above are my concerns also with this frictionless air 'lubricated' linear bearing concept with relatively heavy cartridge/arm/sled system.

In the condition of off centre record, this high mass is encouraging sled lateral overshoot/resonance and stylus cantilever deviation from it's central position within the cartridge magnetic circuit, all due to cantilever lateral compliance.
Distortion and mistracking are the results of this behaviour.
Higher tracking force is a remedy, but not the proper solution.

There are numerous production examples of motorised arm mounting sled driven by tracking error signal generated by angular offset sensors (usually optical) of more conventional twin pivot arm.
Lateral run out of the disc will cause a tracking error signal AC component, which is heavily filtered and the resultant averaged/smoothed DC component drives a sled motor servo system....typical CD players also use this method.

My solution would to to use the tracking error signal to drive a centre reading MC meter (or other suitable display)
With the stylus placed in the final groove of the record, and the record rotated manually, it would be a simple matter to precisely centre the record whilst viewing the display.
The metering system would require DC offset and gain controls.
The platter would require an undersized centre spindle.
Once the record grooves are exactly centred, horizontal stylus deviation from neutral position is of much less concern, if not eliminated.

My other concern of these very short arm systems is VTA modulation caused by record warp.
To cure this problem I would look to a vacuum clamping system....not too difficult to implement in my opinion.

One day when I get a round tuit, it I will give it a go.

Dan.
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Old 29th January 2015, 08:25 AM   #12
Max Headroom is offline Max Headroom  Australia
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Originally Posted by Arch Stanton View Post
Or maybe a simple vertical wing, like a dorsal fin, to do pneumatic damping.
This damping, or other damping systems will not cure the problem.
Sure overshoot is reduced and magnitude at resonance is reduced, but undershoot is increased.

Dan.
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Old 29th January 2015, 08:51 AM   #13
vynuhl.addict is offline vynuhl.addict  Canada
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Super,


That is a very nice arm. I must say it is more heavily engineered than even the lathes that a laquer is cut on . From what I have been reading thus far is that some lateral friction/damping is needed to be added to the air bearing?. This was my observation even with using bearing that there is a happy medium between over and under damped lateral motion. I also learned the same applied to a lesser degree to vertical motion. With the 2 bearing 2 tube I did this with displacement to center of gravity. As Fremer states correctly, in a nutshell , the larger the mass the harder to stop the inertia. In a bearing you can also carefully tailor the two to respond ideally.


Colin
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Old 29th January 2015, 10:44 AM   #14
Hiten is offline Hiten  India
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Since we want arm to be moved easily on air bearing but also want to damp it Wouldn't it be little difficult to achieve ? A little offbeat newbie approach. Can the wire coming out of tonearm be used to dampen the movement with varying stiffness of the wire ?
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Old 29th January 2015, 07:32 PM   #15
super10018 is offline super10018  United States
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I have done some experiments in regarding damping and disk eccentricity. I played a eccentric LP with the air bearing arm. The air bearing kept moving side way from left to right and from right to left. The distance of moving is between 2-3 mm. I used a Lyra high compliance mc cartridge at recommended VTF 1.7 g. The sound was normal and had no noise. But I couldn’t tell if there were distortion or not. Then, I changed VTF from 1.7 to 1.2 g, which is much lower than manufacture recommended VTF. I could hear some kind noise at a particulate frequency at same area when the needle was being thrown on right side of groove wall. This experiment indicates that as long as you keep the correct VTF, the noise caused by lack of damping isn’t audible usually. In most of cases, it will be fine even without damping. I couldn’t tell if there are mis-tracking and distortion by just listening. In theory, I believe mis-tracking and distortion exist. In other words, it is ok not to damp as long as VTF is correct in most of cases.

So, in my opinion, Michael Fremer’s comment is over simplified. He didn’t consider VTF. Kuzma is right. If VTF is correct, it adds damping. Even for pivot arm, if VTF is not correct, needle will be thrown on the right side of groove wall. But in the meantime, Kuzma’s explanation about damping is not adequate for such high priced arm. Air supply tubing may add sampling but it is not good enough.

In my case, my arm is shorter than Kuzma’s and mass is lower than Kuzma’s. The disturbing force to throw needle off center of groove may be amplified through long arm wand. Once the disturbing force happens, it is more difficult to get the needle back to original position due to heavier mass. This is why I was reluctant to use 3/4” air bushing at beginning and went all the way to make a short arm wand.

In reality, it is probably ok without special damping device. There are a lot of factors contributing to LP playback distortion besides damping. But I am a perfectionist. I want to do it right.

Last edited by super10018; 29th January 2015 at 07:38 PM.
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Old 29th January 2015, 11:02 PM   #16
AVWERK is offline AVWERK  United States
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I see you don,t have a vacuum platter? And with the relatively short pivot distance, you might have some sideways movement due to warps?
I,m in the process of doing a ET2 with a short 1.4" pivot arc distance with silicone damping.
For what it's worth, I will use a big 15" sub that I can see cone movement to adjust the viscosity, so on the worst off center discs I have , a somewhat stable woofer movement will be the goal. This is all low frequency stuff.
Don,t know if your spring system will work
You feel good that the airline to the arm isn,t causing any issue at the start and end of a record?

Regards
David
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Old 29th January 2015, 11:51 PM   #17
super10018 is offline super10018  United States
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Quote:
Originally Posted by AVWERK View Post
I see you don,t have a vacuum platter? And with the relatively short pivot distance, you might have some sideways movement due to warps?
I,m in the process of doing a ET2 with a short 1.4" pivot arc distance with silicone damping.
For what it's worth, I will use a big 15" sub that I can see cone movement to adjust the viscosity, so on the worst off center discs I have , a somewhat stable woofer movement will be the goal. This is all low frequency stuff.
Don,t know if your spring system will work
You feel good that the airline to the arm isn,t causing any issue at the start and end of a record?

Regards
David
David,

I have an outer clamp, which is about 6 lbs. Please see photo 6. Therefore, warp is not a problem for me. Otherwise, I won't try to make short arm. I don't see any problem at the start and the end of a record. In fact, the quality of sound for the air bearing arm is very consistent. I can't hear any difference. I have been using Graham 2.2 for almost 9 years. Over 90% of the times, I could hear the difference when the cartridge was approaching the ends of the records. The only thing I need to be careful is to put the needle down at the start of the records. The air bearing is frictionless. A very light push will move the arm way into the middle of the records.
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Old 30th January 2015, 12:11 AM   #18
AVWERK is offline AVWERK  United States
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Ok , couldn't see the lp edge on this computer
What are you using for tubing to the head shell? It being clamped at both ends it must be very flexible to not affect side way movement.


Regards
David
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Old 30th January 2015, 11:44 AM   #19
super10018 is offline super10018  United States
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I bought it from eBay. It is 1/16" Id 1/32 wall latex tubing. It works pretty good. Go to eBay and search for zwgoods.
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Old 30th January 2015, 01:38 PM   #20
super10018 is offline super10018  United States
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I found out Kuzma added a damping kit to their air line arm now. Please click the link if you are interested.

http://www.kuzma.si/media/uploads/fi...it%2070425.pdf
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