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Thread: Different length tonearm why ?

  1. #41
    Join Date: Jan 2013

    Location: Carlisle - UK

    Posts: 1,956
    I'm Ken.

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    Rothwell - I see what your getting at, in describing the twist.

    The amount of force required to move the cartridge sideways, to keep up with the spiralling groove is very small, albeit multiplied by the arm length.
    The air bearing however is designed to keep the parts (Tube and rod) separated so they don't lock up, and can support the whole weight of the arm, many times the force applied by the cartridge, so it shouldn't be an issue.

    The design used on my arm doesn't actually have a tube, its a piece of equal angle extrusion that runs on the top edge of a square profile track, that has holes in it, bit like a saddle, but it would offer similar resistance.

    Coming under attack - feels a bit like that, not me personally, but the concept of a parallel tracker.
    Some folks obviously feel its a Marmite situation, I don't mind that, the banter has been constructive, so it's not a problem for me.
    I think people know where I stand on the subject and all seem to agree there are issues with any design.

    Still, its supposed to be a thread about the merits of different arm lengths.

  2. #42
    RothwellAudio Guest

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    Quote Originally Posted by Qwin View Post
    Still, its supposed to be a thread about the merits of different arm lengths.
    Absolutely correct, so here are some very interesting papers:
    http://www.audiomods.co.uk/regafacts.html#theory
    they're near the bottom of the page under "arm geometry".
    Pickup arm design techniques by TS Randhawa (Wireless World, March 1978) is a very good one.

  3. #43
    Join Date: Jan 2009

    Location: Norwich

    Posts: 2,814
    I'm Hugo.

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    It's interesting to read the Stevenson pieces, but his assertion that 'a pickup arm should be constructed so that its inertia or effective mass at the stylus tip is as small as possible' would not hold water today! Impressive mathematics skills though.

  4. #44
    Join Date: Jan 2013

    Location: Carlisle - UK

    Posts: 1,956
    I'm Ken.

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    Enjoyed the Wireless World article, essential reading for anyone swapping out arms/cartridges.

  5. #45
    Join Date: Jan 2009

    Location: Essex

    Posts: 31,861
    I'm openingabottleofwine.

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    Quote Originally Posted by Ammonite Acoustics View Post
    It's interesting to read the Stevenson pieces, but his assertion that 'a pickup arm should be constructed so that its inertia or effective mass at the stylus tip is as small as possible' would not hold water today! Impressive mathematics skills though.
    I think it's a sensible assertion, provided the design doesn't sacrifice rigidity for low inertia. One can always increase the inertia (and hence effective mass), but is difficult to reduce it.

    Stevenson does make some small angle approximations which are not necessarily valid; but in practice they are largely self-cancelling.

    The first and original paper on tonearm geometry was written by Loefgren. Unfortunately, being written in German, Loefgren's work was not widly promulgated. Baerwaldt later independently derived the same equations and explicitly derived formulae for the arm overhang and offset angle, and for the two null points used to design an alignment protractor.
    Barry

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