Opus 3 Cantus parallel tracking arm - Page 49 - diyAudio
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Old 4th August 2012, 12:14 AM   #481
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Isn't there an argument that the tube should incline slightly uphill to provide some antiskating force? Though that would increase wear on the outer face of the groove, which is probably already doing most of the heavy lifting....

Maybe level is a good compromise?

Bill, is there any reason not to go with the old-fashioned weight-on-a-screw approach used in regular arms? Could be as simple as a nut on a bolt sticking forward or backwards from your counterweight. Pick your thread rate appropriately and you could have a scale on it!

Or a small weight that slides up one of your pen shafts. Although the mind might require one on both shafts to rest easy....

Terry
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Old 4th August 2012, 01:23 AM   #482
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There's no antiskate on parallel trackers - the antiskate is only present on regular pivot arms exactly because of the geometry....
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Old 4th August 2012, 03:54 AM   #483
bgruhn is offline bgruhn  United States
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Terry, The problem comes in that the common little balance that comes with cartridges just doesn't work for setting VTF on the Cantus. The vertical motion of the arm is constrained by the bearings in the static (non playing) state to the degree that the balance doesn't respond. You place the stylus on the balance beam and adjust the cw to where the balance goes down, but because of the constrained vertical motion and the VERY small zone of relatively free motion provided by the radial play in the bearing you can't reliably detect any restoring force that tells you you're on the spot. You fiddle with it and adjust cw and fiddle some more and try to guess if the amount of downward deflection of the stylus cantilever is enough or too much but you are never sure what you have. If it seems to play OK you settle for that. Any or all of the CW adjusters you suggest will adjust VTF. My arms have nice easy to use counterweights. But that isn;t the problem. If you could put the stylus on the gauge and adjust it with the platter spinning---fat chance! In the static state you can by watching the bearing inner race detect when the bearing is riding vertically and centered in the radial play zone and with the CW set the stylus to ride just in contact with the surface of the record. Then when you put the measured weight on the head shell with the record spinning the stylus drops into the groove with the precisely measured VTF. Doug has experienced this problem and bought a digital scales which he has used successfully. At midnight I couldn't run off to Harbor Freight and buy one. Besides I'm way too much a cheapskate.

Your sliding weight on the arm tube is a good fine adjuster. Perhaps with care and my measured balance weights I could put calibration marks on the tube eliminating the weight bin.

You may want to read this discourse a few times to get the full import of it. Once you have it it is a simple easy to do adjustment. Sorry to have not explained it properly in the first place.

BillG (the long winded one)(and a flute player too)

Quote:
Originally Posted by Terry McGee View Post
Isn't there an argument that the tube should incline slightly uphill to provide some antiskating force? Though that would increase wear on the outer face of the groove, which is probably already doing most of the heavy lifting....

Maybe level is a good compromise?

Bill, is there any reason not to go with the old-fashioned weight-on-a-screw approach used in regular arms? Could be as simple as a nut on a bolt sticking forward or backwards from your counterweight. Pick your thread rate appropriately and you could have a scale on it!

Or a small weight that slides up one of your pen shafts. Although the mind might require one on both shafts to rest easy....

Terry

Last edited by bgruhn; 4th August 2012 at 04:00 AM. Reason: left a verb out
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Old 4th August 2012, 05:19 AM   #484
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Ah, yes, (I think) I see what you mean. In your approach, there is four-point contact between bearings and runway - the front and back edges of the left and right bearings. So the arm cannot freely hinge vertically.

Do I remember correctly that the original design shows bearings ground to a point at the front edge, reminiscent of rail-car wheels? Is this intended to allow the carriage to "teeter" on the two point contact, providing vertical hinging, as well as presumably reducing rolling friction?

A vertical hinge that didn't permit horizontal deflection could be as simple as the pin in your flute key....

Terry
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Old 4th August 2012, 06:33 AM   #485
bgruhn is offline bgruhn  United States
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You are getting it! Remember though that there is the small amount of radial play in the bearings that lets the arm follow warps freely. It is in this zone that the arm is free to respond to VTF adjustments. For major warps the bearings ride up the track. The original design has been discarded as being inferior ln performance. This has been documented in earlier posts.
BillG



Quote:
Originally Posted by Terry McGee View Post
Ah, yes, (I think) I see what you mean. In your approach, there is four-point contact between bearings and runway - the front and back edges of the left and right bearings. So the arm cannot freely hinge vertically.

Do I remember correctly that the original design shows bearings ground to a point at the front edge, reminiscent of rail-car wheels? Is this intended to allow the carriage to "teeter" on the two point contact, providing vertical hinging, as well as presumably reducing rolling friction?

A vertical hinge that didn't permit horizontal deflection could be as simple as the pin in your flute key....

Terry
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Old 4th August 2012, 07:38 AM   #486
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Hmmm, I am wondering about giving it a go myself. Too bad my regular off-cut materials, African Blackwood and sterling silver, would be a bit on the heavy side. Could look striking though!

I guess the critical components are the bearings. What's the current thinking on a spec? I see things like 10mm OD, 3mm ID, 4mm thick, 5 ceramic balls and a non-metallic cage that doesn't require lubrication. Are we getting close?

Any recommended source? (I can try here in Australia, but the mark-up on specialist items here usually dwarfs the price of postage from the original source.) Postage on such a massive weight shouldn't be too frightening.

Terry
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Old 4th August 2012, 01:35 PM   #487
bgruhn is offline bgruhn  United States
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Terry,
Great! I'm sure you will be a valued contributor. You will get a great arm in the process. Yes you are on the right track. Go to Ball Bearings | Ceramic Ball Bearings | RC Bearings | Tungsten Carbide | Powerpole Connectors | Silicon Nitride by Acer Racing and find their ceramic ball bearings page. Look up the 4x10mm (MR104) part for USD 7.99. There are similar parts all at the same price with slightly different sizes. With the 10mm bearing you will have to relieve the mounting shaft to provide clearance for the top of the track. See my pics. You will have to flush them clean in acetone or other solvent and blow them dry. All trace of lube must go. Make sure the corners of the outer races are flawless and clean. I assume you are able to cut some glass or can get some cut for you. I love the african blackwood. Drill some holes to reduce weight if needed. These are hybrid ceramic ball/steel races bearings. Full ceramic are far more dear. Keep us informed plz.

Good Luck
BillG





QUOTE=Terry McGee;3114420]Hmmm, I am wondering about giving it a go myself. Too bad my regular off-cut materials, African Blackwood and sterling silver, would be a bit on the heavy side. Could look striking though!

I guess the critical components are the bearings. What's the current thinking on a spec? I see things like 10mm OD, 3mm ID, 4mm thick, 5 ceramic balls and a non-metallic cage that doesn't require lubrication. Are we getting close?

Any recommended source? (I can try here in Australia, but the mark-up on specialist items here usually dwarfs the price of postage from the original source.) Postage on such a massive weight shouldn't be too frightening.

Terry[/QUOTE]

Last edited by bgruhn; 4th August 2012 at 01:41 PM.
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Old 4th August 2012, 11:51 PM   #488
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I'd be tempted to go with a metal tube at first, as I can easily mill the slot. Any recommendations on internal diameter?

2 x $7.99 for the bearings shouldn't break the bank - the postage will probably be as much again! Thanks for that.

Any suggested source for the little connectors that go over the pins on the back of the cartridge. It would be wise to have an ample supply of these. Anything else I need to order up front?

Now, I guess once you make an arm, you need a test disc to evaluate it? Any recommendations for a suitable test disc? (Even if I could find my old one, it hard a hard life back in the days of AM radio. Then again, there might be uncharted territory at the bottom of the groove!)

On the playing weight front, how about mounting the cartridge on an ordinary arm, setting the nominal weight and measuring the clearance under the front edge of the cartridge, then loading up the linear arm (with regard to free-play zone as you had talked about) to give the same clearance?

A wedge style thickness gauge might work well here. You cut a right-angle triangle 10mm high and 100mm long on the hypotenuse, and mark a scale on the hypotenuse. (In this crazy new world, you can probably buy one readymade for less than the materials - anyone aware of such a thing?)

Actually, all we really need is a thin wedge of wood. Mark it with a pencil where it just touches the cartridge bottom on the pivoted arm and aim for that mark on the linear tracker.

Hmmm, my excuses seem to be running out....

Terry
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Old 5th August 2012, 12:02 AM   #489
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Location: Ireland
A Dsub connector from an old computer or similar yields a god supply of cartridge tags. Usually gold plated too!

Hifi news and record review disc seems the standard still...expensive though, but very useful.


Fran
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Old 5th August 2012, 01:36 AM   #490
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Thanks Fran - I'm sure I have one of those around here somewhere!

The HiFi disc seems more suited to the non-technical user. I also see one called The Ultimate Analogue Test LP which seems more suited to the technically equipped user. I'm more in the second class.

Ultimate

Comments anyone?

Terry
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