DIY Schroeder Tonearm???

[bgruhn]

Originally posted by berlinta
Hi Bill,
(Eddy Currents) This could be the single most important reason to use aluminium. "

Right on the money...

< Thanks


Counterweight impedance matching is another issue....

<Frank, I am not with you for understanding counterweight impedance matching. How can we assign an impedance value (Z1) to the counterweight where it is a fixed mass of an inert physical object? True we move the position of the mass on the arm tube to balance the distributed mass of the cartridge and the arm tube (Z2) forward of the fulcrum. Yes it is possible to use a lower weight CW and position it further away from the fulcrum to achieve balance. Actually to select the desired unbalance needed for correct tracking force. Are you suggesting there is an optimum relation between CW weight and position for best performance of a tonearm of some given cartridge weight and tonearm forward of the fulcrum?

If you treat the tonearm as an active transmission line with signals (mechanical vibrations) running along the arm wand from the cartridge (Z2) to the CW (Z1) and being reflected and re-reflected until being absorbed and disipated as heat, then I can understand the importance of impedance matching to eliminate the reflections as much as possible. In a microwave transmission line we can put in an isolator, a one way street, which absorbs the reflected wave.
We can also alter the physical structure to eliminate the reflections from either the source or the load, or we can insert a significant lossy element to absorb this standing wave. But, we are not dealing with microwaves. So if this transmission line analogy is what you are talking about in counterweight (CW) impedance matching then I would think that introducing a lossy resistive element to absorb the vibrations (signals) preventing them from reaching the reflective element (the load) of the counterweight is the only aproach to solving this problem. In other words decouple the mass of the CW from the armtube.

Is this what you are referring to when you talk about impedance matching of the CW? If not, please explain.

Regards and thanks,

Bill


Have fun,

Frank

[berlinta]

Hello Bill,

"Are you suggesting there is an optimum relation between CW weight and position for best performance of a tonearm of some given cartridge weight and tonearm forward of the fulcrum? "

Yes, there is. To put it differently(more precisely), the mass distribution is of vital importance. Choosing a low slung counterweight for the sake of lowering the center of gravity of the entire suspended/pivoting system, to achieve"greater stability"(meaning: high restoring force = massive VTF variations due to record warp) is rubbish.
It only makes sense if your aim is to design an arm for defined inertia ellipsoids. Due to the fact that there are "taller" and not so "tall" cartridges, the need to adjust arm "height" and the need to shift the counterweight forward or backward to adjust VTF, any design can only come close to the theoretical ideal.

".....In other words decouple the mass of the CW from the armtube."

All my counterweights allow for a variation of coupling between them and the endstub. But, the counterweights are not as softly suspended as in old SME arms or via soft rubber O-rings, like Rega.

You can clearly hear a difference between the counterweight tightly coupled or with the set screw just barely tightened.

And there are lossy elements in my armtubes...the variable end stub - counterweight interface is only one of them(no, there is no sand anywhere :-)

Have a great weekend,

Frank

[Badge]

My arm is composed of a 3/8" diameter carbon fiber tube. It is very light. I got the idea about the sand from the Well Tempered thread. That arm is supposed to be filled with sand and sealed with wax.
My first build will be to fit my Linn with a Ittok. I will fashion the size to utilize it's mounting board. You can find arm data at the Vinylengine. Mine has a effective length of 229 mm. Mounting distance is 211.

[binspaul]

Hello Frank,

I have got the N38 magnets with the following values:

* 10mm diameter.
* 5 mm thickness.
* < 2 mm hole diameter.

Which one is the best model to go for, the single string version or the double string version ?

Best regards,
Bins.

[berlinta]

Hi Paul,

You need to build both and decide for yourself. I prefer the single string type, but I do have a couple of friends who like the divided string version just as much, or better.

If you can't find a thread material that has near zero stretch, go for the single string type...

The devided string bearing can easily be built to exhibit no restoring force, not easy at all with the single string type.

Have a great weekend,

Frank

[binspaul]

Hi Frank,

Are the N38 magnets (10 mm diameter; 5mm thickness) strong enough for the single string type arm ?

Regards,
Bins.

[berlinta]

Hi Bins,
Yes they are. And why should there be a difference between single thread and divided thread bearing? You just have to keep the gap smaller. Stronger magnets = higher damping for the same distance between the magnets.

Resistance against displacement is a slightly different story. Here, the stronger magnets(N50, 52+) have the edge. But with most carts you don't need max. possible damping anyway.

Cheers,

Frank

[binspaul]

Hi Frank,

Thanks for your reply. I have a few more doubts regarding the arm wand:

1. Is there any specific reason for choosing aluminium for the later portion of the arm wand ?

2. Do we need to use an aluminium rod ? Can we use an aluminium tube instead ?

Best regards,
Bins.

[bgruhn]

Help on arm mounting an overhang needed. My Schroeder clone is working pretty well now. I'm very pleased with the performance but I feel that there are a few things that could be cleaned up a bit. The one area I'm still not convinced is right is where I have the arm positioned. The arm is 10.5 inches long and I have no information on what the overhang should be. For 9 inch arms it seems that something near .5 to .6 inches is correct.

To position it I set it up on a moveable pad and put the "enjoy the music" protractor on the table. Then I moved the arm base around on the plinth until I found a mounting position where the headshell and stylus matched quite precisely both grids on the protractor. I then transfered this position to the plinth and completed the mounting. There were very few other positions that permitted such a good agreement with the grid markings. There appeared to be only one right place. The problem is that in this position the overhang exceeds 1" by a small amount. From what I see in manufacturers literature that is about double the overhang there should be. The arm length for these shorter overhangs is not spelled out. Is there anything wrong with the method I used to find the mounting position?

I am not completely satisfied with the distortion at points on the record but the problem could be I don't have the antiskate set right yet. Reducing the overhang to about .5" to .7" seems to give more uniform sound across the record even though the stylus tip falls a bit short of the protractor grid mark and overshoots the other grid mark by the same amount. (You could position the stylus right on one or the other grid marks in which case it would way over shoot or undershoot the other grid mark).

to Frank Schroeder: What overhang do you feel is correct for a 10.5 inch arm?

Please folks give me the benefits of your thinking on this topic.

Thanks,

Bill

[K.A.B]

Maybe I can help you. For an 10.5 tonarm the overhang is 0.52in the offset will be 19.24in zero tracking error points in inches from the records center first 2.33 and second 4.59 gives you 2 degrees maximum trackin error.

Anders