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Any Special Considerations For Aligning An S-Shaped Arm?

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Any Special Considerations For Aligning An S-Shaped Arm?

Postby Dazed » 14 Sep 2011 09:06

I'm trying to figure out how to mount a L.A.D/Jelco SA-370D arm on my Pro-ject Debut.

Are there any special considerations I'll have to take when aligning an s-shaped arm, or can I just use Hoffman's template generator for this too?

I've tried it, and it looks good. The data sheet for the tonearm says the arm has an effective length of 229mm and an overhang of 15mm, so I entered (229-15=) 214mm as the mounting distance in the template generator app. Is that correct?

When using Löfgren A/Baerwald alignment, i get 231.9mm eff. length and 17.9mm OH. Fine, I guess, but what I'm slightly worried about is the 23.7 degree offset angle it uses. The headshell is specified with a 22 degree offset angle. But 1,7 degrees of difference really is minuscule, right? I'll probably be able to mount the cartridge at any angle I need, don't you think?

Thanks in advance for any help. It will be appreciated.
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Postby JaS » 14 Sep 2011 09:52

Are there any special considerations I'll have to take when aligning an s-shaped arm, or can I just use Hoffman's template generator for this too?

Luckily, aligning an S or J shape arm is exactly the same as aligning a straight arm (except for DJ arms with no offset angle) so no problem there.

There are three ways (or more) to approach the mounting

(1) Use the original mounting distance 214mm (229-15mm) and mount the tonearm / print a protractor for this distance. For a 214mm distance Baerwald geometry requires an extra 3mm of overhang and 1.7 degrees offset which may not be possible with all cartridges. In this case a Stevenson arc /universal protractor (+1mm, +0.7 degrees) could be used. Accurately aligned, the difference in error between the two geometries is minimal.

http://www.vinylengine.com/tonearm_alig ... =calculate

(2) Calculate an optimum mounting distance for the alignment geometry you want to use. For an effective length of 229mm, a Linn mounting distance of 211mm should work well for Baerwald. This should leave the cartridge more central in the slots once aligned, although it will still require more offset angle than original.

http://www.vinylengine.com/tonearm_alig ... =calculate

(3) Mount a cartridge centrally in the slots, measure the effective length and use this figure to calculate an optimum mounting distance as in (2). The advantage of this method is that you aren't relying on the accuracy of the manufacturers figures.

note measuring the effective length can also be used to double check the published effective length before using methods (1) or (2).

Fine, I guess, but what I'm slightly worried about is the 23.7 degree offset angle it uses. The headshell is specified with a 22 degree offset angle. But 1,7 degrees of difference really is minuscule, right? I'll probably be able to mount the cartridge at any angle I need, don't you think?

If you want to align accurately to the null points on the protractor you need to set the overhang and the offset angle. Any other angle will result in different, or even no, null points. Even 0.5 degrees can have quite an effect on the resulting error.

For example, if you mounted your arm at 214mm and adjusted the overhang to perfectly follow a Baerwald arc, but ignored the offset angle grid and left the cartridge square in the headshell, you would end up with no null points and more than double the tracking error distortion!

http://www.vinylengine.com/tonearm_alig ... =calculate

Regards,
JaS
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Postby Dazed » 14 Sep 2011 10:07

Excellent advice as usual! :D Thank you very much!

I couldn't think of any reason why alignment of s-shaped arms would be different from straight ones, but was told that ordinary protractors would not work for such arms, and wanted to check. Thanks again.
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Postby JaS » 14 Sep 2011 10:20

Dazed wrote:I couldn't think of any reason why alignment of s-shaped arms would be different from straight ones, but was told that ordinary protractors would not work for such arms, and wanted to check.

I think this misunderstanding is based on users of S-shaped arms trying to align to universal protractors without adjusting the offset angle? For a given mounting distance it's only possible to align to two null points with one specific overhang distance and offset angle. Trying to keep the cartridge square in the headshell while adjusting the overhang back and forth is a futile exercise in any arm with a radically different offset angle to that required by the protractor.

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Postby Dazed » 14 Sep 2011 10:32

Could be. I'm satisfied knowing that the "usual" protractors will work. :)

I like the "measure the actual effective length and calculate the optimal mounting distance" approach. I guess I wouldn't even need to mount a cartridge to do it, since I know the distance between the cartridge bolts and the stylus tip. :)

Thanks again!
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Re:

Postby Dazed » 10 Feb 2012 23:10

JaS wrote:(1) Use the original mounting distance 214mm (229-15mm) and mount the tonearm / print a protractor for this distance. For a 214mm distance Baerwald geometry requires an extra 3mm of overhang and 1.7 degrees offset which may not be possible with all cartridges. In this case a Stevenson arc /universal protractor (+1mm, +0.7 degrees) could be used. Accurately aligned, the difference in error between the two geometries is minimal.

http://www.vinylengine.com/tonearm_alig ... =calculate


I'm digging this up again, as I've bought, and am waiting for a used Jelco SA-250ST for my Debut III.

Do you think it's going to be a problem aligning tha sa-250st to Baerwald null points from a manufacturer recommended 211mm mounting distance? Would you recommend that I calculate my own Baerwald-optimized mounting distance, or is it safer/OK to use 210mm and use the Stevenson Alignment?

Thanks in advance.
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Re: Any Special Considerations For Aligning An S-Shaped Arm?

Postby davidsss » 11 Feb 2012 01:37

No idea about your specific arms but I've only ever owned S shaped arms, just happened that way really. If an S shaped arm is designed for a particular mounting distance and for Baerwald geometry then it should align fine with the cart straight in the headshell. I'm very lucky in that my arm (Stax UA7) has a little dot on the headshell which you align the tip of the diamond to. When I do this the arm lines up perfectly with the null points on protractors and I have also printed out the Conrad Hoffman arc protractor and it all lines up. The S shaped arms are just using a different approach to implementing an offset angle to a straight arm with an offset headshell.

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Jelco SA-250ST Baerwald null points w/210mm mounting distanc

Postby Dazed » 11 Feb 2012 12:20

Thanks. Yeah, I understand that. I actually has strayed a bit off topic. (Sorry about that!)

What I was wondering now is if I will be able to align the cartridge to Baerwald null points with the Jelco SA-250ST (which actually has a straight arm tube) mounted at the recommended 210mm, or if i need to/should calculate another mounting distance based on the actual effective length of my arm and cartridge.

Thanks again.
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Re: Any Special Considerations For Aligning An S-Shaped Arm?

Postby JaS » 11 Feb 2012 13:52

Hi,
If Jelco's figures are accurate (!) the standard alignment is very close to the overhang and offset angle needed to align to an IEC Baerwald protractor. In fact the inner null point is between Lofgren A and B and should give very low tracing distortion.

I'd mount it and see if it aligns OK with a Baerwald protractor, and if not use Stevenson if you need less overhang, Lofgren B if you need more :)

http://www.vinylengine.com/tonearm_alig ... =calculate

Regards,
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Re: Any Special Considerations For Aligning An S-Shaped Arm?

Postby Dazed » 11 Feb 2012 15:31

Thanks! You're right. According to that, 210mm looks spot on. I'll just take a quick measurement of the arm when it comes, then, and if it looks right, I'll just use the mounting template from Vinylengine.

I'm giong to have to expand the mountig hole (at 200mm) to an oval, ro reach 210mm, so I want to get it right the first time. :)

Thanks again.
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