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Old 1st September 2012, 05:26 AM   #1
mortron is offline mortron  Canada
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Default So when designing a tonearm...

I have read some stuff on tonearms, various builds etc. I plan to make a unipivot, but don't see too many measurements i can borrow from so had a question or six....

Does the pivot to stylus distance have to be predetermined and set in stone? I intend to work angle into my headshell, and am shooting for about 12". How far back past the pivot should the counterweight end go? Shorter the better or? Or are we shooting for a certain weight to the weight and that determines stub length? Thanks
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Old 1st September 2012, 10:25 AM   #2
SY is offline SY  United States
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There are several "standard" lengths, but nothing magic about them. Longer arms have less tracking error but higher inertia. It's all tradeoffs...
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Old 1st September 2012, 10:38 AM   #3
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I'm quite possibly talking nonsense here, but surely the aim should be to copy the geometry of the cutting lathe?
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Old 1st September 2012, 10:45 AM   #4
SY is offline SY  United States
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That was the idea behind linear tracking arms. As a practical matter, they carry their own set of engineering challenges, especially regarding friction and horizontal inertia. Still, I'd love to have a high quality one...
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Old 1st September 2012, 10:52 AM   #5
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I've got an old SL5 you're welcome to!
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Old 1st September 2012, 11:00 AM   #6
SY is offline SY  United States
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Don't laugh, put an EPC205CIII or IV into one of those old Technics tables and you've got a pretty damn nice analog setup. I have an SL10 on the East Coast waiting for me to pick it up.
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Old 1st September 2012, 02:25 PM   #7
mortron is offline mortron  Canada
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I just got an SL QL5 up and running for a friend the other day. So nice.

I guess the things I am worried about are of little consequence, because once the arm is built, I can then design a protractor to align the cartridge? I am looking at the Rats Paw DIY tonearm on AudioKarma, and I think I am going to contact him and get some questions. Thanks
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Old 1st September 2012, 04:13 PM   #8
marekst is offline marekst  Poland
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Hi morton,
You can use any practical length and calculate other dimensions. Below is a link for Excel spreadsheet calculator
Free Cartridge Protractor And Speed Discs!

I believe that shorter counterweight distance and heavy weight (lead) is better.
Marek
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Old 1st September 2012, 11:23 PM   #9
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The effect of counterweight mass on inertia is usually overestimated; doubling the CW mass and halving the stub length will generally only reduce inertia by around 10%.

A more practical reason for going the short stub / heavy CW route is to reduce the real estate required for the rear end of the arm.

It has its drawbacks, for instance it increases bearing reaction force and therefore friction.
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Old 2nd September 2012, 02:49 PM   #10
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Hi everybody,

Why are we looking for low inertia?
Are we talking about whole tonearm inertia or cartridge inertia?

I am building a high mass unipivot (see "man on wire" thread) because I wanted a good tracking of the grooves, which causes a very slow lateral motion, but I seek for high inertia of the cartridge in order that the stylus senses and reacts better to the micro-accidents of the groove.
Am I completelly wrong here???

Thank you,
M
Analog débutant.
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