DIY Schroeder Tonearm???

[Paul Ebert]

Originally posted by berlinta
The drawing is very good indeed, leaves out some important details though. And the shape of the upper polepiece is not quite correct. Build it and tell me if you managed to achieve equal VTF and azimuth from beginning till the end of the record...

Thinking about these statements led me to one of those 'stupid' questions that one can only ask, usually because said one is completely missing something obvious. So, here goes...

Consider what happens when an arm such as we are considering experiences a warp. The front goes up causing the upper magnet to tilt towards the back. Since the magnets are configured as opposite polarity, the back experiences an increase in attraction due to it's gap being reduced and the front experiences a reduction in attraction. If the differential in attraction is enough this will cause the arm to continue to pivot until the magnets touch in the back. Even if it is not enough to lift the front of the arm, it represents a force that the arm must overcome when it goes down the other side of the warp.

This strikes me as a problem. What am I missing in this picture?

As to achieving equal VTF and azimuth across the record; I'm stumped. Unless it has something to do with the horizontal angle of the cartridge with respect to the arm tube axis (what's this angle called?).

Thanks.

Paul Ebert

[berlinta]

Dear Paul,
Not a stupid question at all!.. But, as with any Unipivot arm(this design is NOT a true unipivot, though!), the center of gravity of the whole moving assembly is below the pivot point. As a result, the tracking force increases as the cart/arm "travels" up hill. The effect you described would ameliorate this "unipivot "- flaw (to a degree at least) and should therefore be welcome. In practice it is inconsequential, since the variation of the tracking angle caused by warped records is very small , consequentially the upper magnet doesn't get tilted enough to show such behaviour. On top of that, the other end is raised by a similar distance so that the overall flux density remains the same(close enough ...).

As for your second question, - the armwand can rotate(slightly) around it's axis and horizontally too, off course. If the magnets used aren't perfectly centered/drilled(same for the pole pieces), or if they're not cut perfectly the effect you described first will then indeed have a negative effect. Imagine two discs(the magnets) with magnetic "hotspots" on their surface(same as imperfections which are weak spots). As they get closer to each other while the arm travels across the record, the attracting forces vary and, depending upon their location on the surface, the VTF and/or Azimuth will be affected.
In other words, adjusting the magnets is no simple task.
Build it and then measure VTF at the beginning and then the end of the record. No change? Well done!

Cheers,

Frank

[RogueAngel]

Hi all

I'm a new member hereand I ran across this thread. I thought you all might be interested in seeing my version of the "schroeder" tonearm.

It replaced the standard tonearm on a Denon DP-62L. I just finished construction this past weekend and am still in the process of fine tuning.

I did the test mentioned by Frank and found my tracking force varies by 0.2 g with no apparent change in azimuth.

Has anyone else tested theirs? or would like to exchange experiences?

Bobbi

[Paul Ebert]

Looks great, Bobbi! Very well done! Care to fill us in on materials used and design decisions / features?

Also, how did you measure the VTF?

Thanks.

I've been thinking about the problem of magnetic strength variation within a ring and I wonder if a smaller diameter magnet would be better. Taking this to an extreme, perhaps a cylindrical pair of magnets of, say, 1/8" diameter would work well. Can anyone think of a downside to this approach?

Paul Ebert

[RogueAngel]

Looks great, Bobbi! Very well done!

Thanks Paul!

Care to fill us in on materials used and design decisions / features?

Big question! As I think you can tell, the support structure is acrylic....chosen mostly for ease of tooling. I thought for appearance, the mixture of clear and black would look good and I am quite pleased with the look. The arm is a brass tube (originally intended to be aluminum - but the tube was too thin) with the headshell assembly fashioned from solid brass stock. The counterweight is stainless steel. Wiring is 33ga Cardas with Cardas cartridge clips and Eichmann bullet plugs at the phono input end. The magnets I chose are non-sintered - so, I gave up a bit in field strength, but they seem to be working well. All set-screws etc. are stainless steel. The lifter was obtained from the manufacturer in Japan.

Adjustability is almost too much .... as I have spent hours, literally, adjusting each parameter. Offset angle, VTA, VTF, overhang, and antiskating are all adjustable as in the Schroeder except that in mine the VTA adjustment is loaded with a stainless steel spring in lieu of the concealed adjustment in the original.

Also, how did you measure the VTF?

VTF was measured with a digital guage with +/- 0.1 gm accuracy. I think the thickness of the scale is influencing the readings and there may not be quite as much variation as the scale indicates. I have found the the angle of the vertical support can produce large variation in VTF. I played with the angle and got as much as a 1.0 gm variation from inner to outer groove.

Having only attempted speaker construction previously, I was a bit reluctent to commit a lot of funds to this project, not knowing if it would even work. The difference over the original arm is quite startling, especially in much tighter bass, much better detail, improved dynamics and a much lower noise floor. The improvement over the original Denon arm is NOT subtle! In fact, due to my beginner's luck, I'm now thinking about making a second arm, with much better materials and a stronger magnet.

I've been thinking about the problem of magnetic strength variation within a ring and I wonder if a smaller diameter magnet would be better. Taking this to an extreme, perhaps a cylindrical pair of magnets of, say, 1/8" diameter would work well. Can anyone think of a downside to this approach?

If you can find a 1/8" diameter magnet with adequate strength, I think the smaller the better. I believe it would lessen any areas of field variations and would also lessen any problems maintaining the consistency of the gap between the magnets during arm rotation. Has anyone tried various magnets to see what effect they have?

Bobbi

[Paul Ebert]

Bobbi,

How did you implement anti-skating?

Thanks.

Paul

[RogueAngel]

Paul,

The knob above the suspension string is threaded into the top suspension arm. There is a smaller allen-head type bolt that is threaded axially through this bolt. The "hook" that captures the string is passed through another axially drilled hole that passes through the smaller bolt. Basically, one is used to adjust the gap between the magnets and the other is used to place a small counter-clockwise pre-twist on the string. This pre-twist acts to balance out the arms natural tendency to travel inward. If you read the user's manual posted on Frank's website, he gives a great description of how to make this adjustment.

Bobbi

[krishu]

Hello to everybody and especially to Frank Schröder

I am lathing a schroeder inspired tonearm at the moment and have not decided which magnet to use yet.
There are two sizes ready to go in: D15×H5mm and a smaller one measuring just 10mm in diameter. Which one should i use? Both are Neodym N40 or N50 (i do not remember exactly).
What would be advantages/disadvantages of a bigger/smaller magnet? Will increased magnet force lead to any problems?

Your help is appreciated.

Cheers
Christian.

[berlinta]

Hello Krishu,
Making the magnet larger means you have to increase the diameter or rather the strength of the thread(=higher friction). Eventually you will end up with a thread that is too thick to allow for a suitable adjustment range/evenness for the antiskating.
You could make that thread longer but then it will pick up more airborne vibrations.

Increasing magnet size/strength also increases the damping(sometimes there is too much of a good thing..)

What do you do about shielding the magnetic field?

Properly implemented 10mm x 5mm(N50) should suffice...

Cheers,

Frank

[krishu]

Hi Frank;

thank you for your quick response.

I will make all metal parts out of bronce, "behind" the magnets small steel cylinders in the same size as the magnets. I will use standard steel (like C45, ST35 ...) for that. This is what I understood from your patent paper.

Using the smaller magnets will make it easier to glue in the arm pipe (more contacting surface). the arm pipe will be carbon fibre.

the magnets i have are N48 15x5 and 10x4mm. So you say the smaller one would be sufficient?

Or should i go for 12x5 N48? (do not know if this is available yet)

Thanks again
regards
Christian.