DIY Schroeder Tonearm???

[bgruhn]

Originally Posted by Jeffrey Davison

a young person demonstrating how aluminum interacts with a regular magnet and a neo magnet.

from the mouth of babes

http://www.youtube.com/watch?v=a5BtjZWkua0

Hi Jeffrey,

Yes, a neat demonstration of eddy current braking (damping).

Given the aluminum structures the arm magnets are embedded in and the magnetic fields that are present the motion of the arm will be damped. I grant you that I was not looking at damping the motion of the arm. It is similar in results to the damping of the tonearm motion by the silicone bath surrounding WTA's golf ball, without the messy juice. From another perspective though I wonder how much motion can occur per unit of time to generate these braking currents where you have two very powerful magnets pulling on each other and resisting every force to move them out of axial alignment. I would think that this magnetic attraction would swamp any eddy current damping especially where the currents will be so very small, unless I am mistaken about being small.

I've placed small disks of aluminum and copper coins on a smooth table top and also suspended by a thread. Then brought a neo magnet smartly up to or past the conductive non ferrous disks and you can easily see them repelling each other. Key to this one is bringing the magnet up smartly. Move the magnet slowly enough and you won't see anything move. Yes, the galvanometer isn't sensitive enough.

Thanks for your input and the neat video link,

Bill

[jeffdavison]

While looking for some more reference pics for my arm, I came across some new pics uploaded at OswaldMills (Schroeder distributor) of the Reference arm.

Looking at the new large pics, it looks like there is now a brass ring around the upper magnet mounted in the arm tube.

Frank... if you're lurking...and others chime in...

What would be the purpose / function? easier mountin of the magnet? string?Taming eddy currents??? etc......

http:\\www.oswaldsmillaudio.com

Jeff Davison

[binspaul]

Jeff,

What tube are you using: metal or wood ? How did you design the tonearm head shell ?

Best regards,
Bins.

[jeffdavison]

I'll most likely be going with a 3/8" o.d Titanium tube, tho I have a ceramic tube on the way to play with during the design phase.

On the Titanium tube for damping.....

It's 3/8" O.D. and 5/16" I.D.... Inside this tube will be an inner tube, a 3/16" O.D. 1/4" I.D carbon fiber tube which will have a series of "O" rings to keep the central tube concentric and to act as dampeners. The "O" rings will be evenly (or maybe proportionatelly) spaced along the length of the inner tube..

On the outside of the outer tube I may go with external dampening as well. A length of Polyolefin heat shrink tubing worked well for a SME 3009 I rebuilt and may use it here as well.

I'm working up some mechanical drawings to have a headshell fabricated, consisting of two parts... an end of tube part that will allow a cartridge carrier part to be adjusted lengthwise and alloe for rotation of the cartridge carrier to be rotated for the correct offset angle. The cartridge carrier will also be slotted for back and forth positioning once offset angle is set.
The cartridge carrier will have a small drilled and tapped hole for a finger lift rod around 1/16" dia and threded to match the hole... itwill be gently "ergonomically" curved. I'll be making the end of tube part out of 7075 Aluminum for it's strength seeing that it's a small part with small sections for a hold down screw to bear upon.


JD.

Originally Posted by binspaul

Jeff,

What tube are you using: metal or wood ? How did you design the tonearm head shell ?

Best regards,
Bins.

[berlinta]

Hello Jeff,
The picture you were refering to just shows a gold plated magnet, no brass ring...

Have a great sunday,

Frank

[Jeffrey Davison]

Found a few off the shelf parts that may help with builds...:

1. The arm lifter I'll be using is a Rega part, available as a spare from Rega for btween $40 and $50.

2. The anti-skating knob I'll be using is a DIN 466 brass thumbwheel using an M5 threaded hole.. (about $.30 cents each in qty. 1) much cheaper than having one machined. There is a small radius between the head and the body, but that can easily be cut square and/or the top support piece can have the hole that it will fit into beveled or radiused to suit.

3. The top string holder (that fits into the anti-skate thumbwheel)is a vented set screw (grub) 10-32 x 1/4" long (10-32 is the same as M5)... the vented hole in the set screw (grub) is .042" diameter.

4. The string I'll be using is a fiber called "Vectran"... it is stronger and has less stretch than Kevlar. It's use is in ropes and actually astronauts space suits... about $5.00 for 50 yards (on eBay)

Frank... if this is too much info... I'll edit or delete the post (if not too much time has elapsed.. edit function goes away after a short time), just thought this may help some builders without giving "too" much away.

Jeff Davison

[berlinta]

Hi Jeff,

Nothing that I would delete, but the advice on the standard knurled brass antiskating wheel is prone to cause DIYers a little headache. The threaded hole in those is most often rather generously sized(slack!) and the suggested thread itself is quite a bit larger than what I use(M3). Same for the hole through the grub screw, 1mm is way too large! Besides that, the holes in standard parts like the brass wheel are often not perfectly centered...
Vectran is usable but only comes in "loose" fibres, making it difficult to lead through two holes that allow for NO slack.

Cheers,

Frank

[K.A.B]

Hello Frank,

First of all thanks for sharing your knowledge about your tonearm too us diyer I have almost completed my clone. I have one question about the arm lifter can or will you recommend an supplyer. I can try to built one but i have no experience of this kind of part.

Anders

[Jeffrey Davison]

Any Rega distributor can help you as their arm-lifter is readily available.
I came across a dealer or two in the U.K. I got mine from a Dealer (Needle Doctor) in the US who has them drop shipped from the USA Disributor (Sound Orginasation). It would probably be less expensive to source them from a European source..


yep, Frank, I agree Vectran is a bit tricky to work, but my fingers are rather dexterious. ( play Flemenco guitar ). A hard twist of the fiber let me "thread the needle", so to speak, an a 4ply knot worked well for the .042" hole in the grub screw and just a drop of cyanacrylic adhesive binds the fiber together in side the well of the grub and to the grub itself. Testing has shown it is extremely well anchored and won't pull through. Tho not technically accurate.. looking under magification, the string looks centered.

Now when the thumbwheels come, we'll know for sure how concentric the holes are and see how tight the tolerences are... after all they are certainly cheap enough to play with.

Also it looks like the pull on the string from the magnets should be more or less a straight line on the string .. might not be "perfectly" centered, but probably somewhat workable and "close enough". within a RCH here or there (email me privately and I'll tell you what the unit of measurement RCH is ;^P ), .. the tricky bit will be when I try to anchor the sting in the arm... trying to work out a solution other than drilling thru the magnet.. I have some ideas I'm working out on paper i.e. AutoCAD


Jeff Davison

Originally Posted by K.A.B

Hello Frank,

First of all thanks for sharing your knowledge about your tonearm too us diyer I have almost completed my clone. I have one question about the arm lifter can or will you recommend an supplyer. I can try to built one but i have no experience of this kind of part.

Anders

[Jeffrey Davison]

Frank, or anyone else who've built the tonearm following Franks work...

I was going through the patent and have some questions..

The Patent refers to "high permeability pole pieces" that are place below the bottom magnet and over the top magnet.

Has anyone used such? Can the arm be succsesfully realized without them? are they vital? Frank, is there a special metal you use? I have some thin sheets of Mu-Metal that I can hole-punch to make discs that I can stack to get an appropriate thickness. Trying to find "off the shelf" shims or discs is almost impossible and going to a specialty house requires large minimum orders.

On the last point... I'm looking into out-sourceing the parts for my build to a mahine house for the head shell, arm-wand central piece, top and bottom support, central block, bottom block central support rod and bottom magnet holder. Doing one-offs are very costly as most of the cost is in set-up time.
If I have 3 of each part made the piece price comes way way down. example.. say one of the parts will cost $200 to make one, but 3 will cost $250, and 5 pieces of that part will cost $260. I have 6 different parts to have fabricated, so this arm will cost a "chunk-o-change"... The arm should perform better than any other commercially available arm for that money, but it sure would be nice to share the burden with others... I will not do this if it is not OK with Frank.
I will never share my CAD work or drawings I developed as that is one of the prerequisits I agree on with Frank and I wish not to violate any trust, nor will I ever sell parts once this project is finished.I don't want this to look like I'm selling parts, only splitting costs with a couple of others, not unlike a group buy. but the tonearm is propretary and patented and I wish not to in anyway be infringing or profiting. But what I would like to know from Frank, if it's ok to offer the other couple of each part I've produced to help offset some of the costs.

The parts I talked about above were quoted using www.Emachineshop.com.
In the meantime I'm trying to find a local "hungry machinist" to do cost effective one-offs for me so I won't have to go the more costly rout and avoid Emachineshop if possible. I've used them before for some auto related projects and they do fine work, albeit pricey.

a note to Frank.... if you'd like I can forward the Autocad .DWG drawings to you that I'm working from if you'd like.


Jeff Davison