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VPI 3D Dual Pivot Conversion Kit

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  • garylkoh
    replied
    The origins behind this idea is not just about off-center holes. Stiction (static friction) or I also call it "striction" as in constricted movement is also a result of the compliance of the cartridge cantilever and the inertia/momentum of the tonearm effective moving mass.

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  • Brf
    replied
    The vpi conical tip is shaped to approximate a very small sphere. The ruby tip just has a larger circumference when compared to the SS VPI tip. Too me, the material difference, size etc is a moot point because I can't hear any difference between the various types.

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  • Waxxy
    replied
    Originally posted by Brf View Post



    With respect to your previous question as to why the sphere diameter matters when spheres have a theoretical infinite contact. You answered your own question, material deflection under load.
    The sphere diameter doesn't matter. Any sphere will have a smaller contact point than a sharpened tip. Two theoretically perfect spheres touching each other will only be in contact on the atomic level. And obviously material deflection of a ruby ball will be much less than that of a stainless steel tip. The stainless plate will have deflection, but the ruby ball will still have a smaller contact point that the pointed steel tip. None of it matters because the device is flawed from the get-go....the eventual arc carved into the plate makes this obvious.

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  • Brf
    replied
    Originally posted by Waxxy View Post

    I have to disagree. The vast majority of records have off-centre holes which causes the arm / cartridge to swing back and forth with every rotation of the record. With each rotation, the natural inward swing of the arm must come to a stop, and then swing back towards the outer edge of the record, come to a stop again, and then swing back inwards....over and over 33 1/3 (or 45) times per minute. On some records this is obviously visible and on some it's happening on a much smaller scale, and may be invisible to the eye, but it's still happening. Static friction is certainly at play here.
    That’s a fair point as long as the non-concentric record hole causes a deceleration of the dual pivot that results in a change of direction. On some records, this can be obvious, on others, not so much… but none the less, a valid point.

    With respect to your previous question as to why the sphere diameter matters when spheres have a theoretical infinite contact. You answered your own question, material deflection under load.

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  • Waxxy
    replied
    Originally posted by Brf View Post

    Stiction: I do not believe that the coefficient of static friction is an issue as the rotational torque generated by the lead in groove puts the dual pivot into motion prior to the stylus hitting a modulated music groove. I addition, the dual pivot is in constant motion throughout playback.


    .
    I have to disagree. The vast majority of records have off-centre holes which causes the arm / cartridge to swing back and forth with every rotation of the record. With each rotation, the natural inward swing of the arm must come to a stop, and then swing back towards the outer edge of the record, come to a stop again, and then swing back inwards....over and over 33 1/3 (or 45) times per minute. On some records this is obviously visible and on some it's happening on a much smaller scale, and may be invisible to the eye, but it's still happening. Static friction is certainly at play here.

    Leave a comment:


  • Brf
    replied
    Nice work Joe, love the diagrams. I have played around with jewelled pivots and sapphire window skate plates since the dual pivot’s introduction. A few comments.

    Stiction: I do not believe that the coefficient of static friction is an issue as the rotational torque generated by the lead in groove puts the dual pivot into motion prior to the stylus hitting a modulated music groove. I addition, the dual pivot is in constant motion throughout playback.

    “the contact between the spike and surface is less than 1/2" long playing a record” – I am not 100% sure what you are implying but the relatively short path travelled by the dual pivot compared to the path of the stylus provides a mechanical advantage which will significantly negate the kinetic friction of the dual pivot. With respect to the kinetic friction caused by the dual pivot’s interface, I do not believe it to be that great (if properly loaded) as it does not cause any noticeable stylus deflection when employed.

    I guess it all boils down to the friction of the dual pivot interface once in motion, and does its reduction have a positive result. I first tried a wet lubrication to reduce the friction caused by stainless steel on stainless steel interface. The lubrication significantly reduced the interface’s coefficient of friction, but long-term affects will be compromised, as a wet lube will attract air borne contaminants. I then swapped out the regular SS dual pivot with both a jewelled and a grade 3 Si3N ball, and added a sapphire window skate plate. To be 100% honest, I could not discern a different in sound between the different materials. I am not discounting others who report a sound improvement, but as much as I wanted to hear an improvement, I could not.

    I encourage anyone curious about the dual pivot modification to try out Joe’s jewelled assembly, as the cost of entry is a bargain and who knows, it just may prove to be beneficial in your system.

    Thanks Joe for pushing the envelope. BTW, I had a peak at you website http://www.kosmic.us/ .......very nice!
    .

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  • Joe Pittman
    replied
    To understand the scale of the problem, the contact between the spike and surface is less than 1/2" long playing a record. The "stiction" at the point of contact is significant. Zero Friction Perfection at the molecular level is the goal.

    Click image for larger version

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  • Joe Pittman
    replied
    I finally got around to replacing the polished metal Pivot Surface of the Dual Pivot Assembly with Sapphire. When used with my Ruby Ball Tipped Screw, I now have a sapphire-to-sapphire contact. The performance of the stock kit is taken to a higher level. The Sapphire Pivot Surface is laboratory quality with ultra smooth and flat surface matching my Grade 5 Ruby Ball smoothness. Overall friction/stiction is extremely low. Very exciting.

    Click image for larger version

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    Click image for larger version

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  • Waxxy
    replied
    Originally posted by Brf View Post

    The challenge I found with a jeweled pivot was finding a ruby ball small enough to approximate the polished stainless steel tip. Commercially available ruby balls that I found offered a smoother and harder surface than the stainless steel tip, but had a contact area much larger than SS tip which results in a larger frictional surface area.
    How can any sphere, no matter how big, have a larger contact area than the stainless steel tip? Unless there is material deflection, the ball should have an infinitely small contact area.

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  • Letsmakeadeal
    commented on 's reply
    I have the same only with brass setscrews (very hard to find) and tiny Grade 3 Ceramic balls...Free if you will cover shipping ($3-$4). I sent some to Brent. Not sure if he ever tried them. I have enough parts to probably make 20 or more of them. Since I am going to Gimballed Fatboys, I don't need them anymore. PM if interented.
    Last edited by Letsmakeadeal; 11-26-2018, 10:35 AM.

  • Stringreen
    replied
    Joe - if you decide to make them available for sale, let me know. Thanks

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  • Joe Pittman
    commented on 's reply
    I made my own ruby tipped screws. And they are easy to make. I'll make some more in a few months...

  • YoungDave
    replied
    Originally posted by Joe Pittman View Post
    The Dual Pivot is now my standard preferred accessory and provides a little better performance IMHO. The ability to precisely adjust azimuth is an added bonus. I finally got around to changing the stock screw points with precision 1.5 mm Ruby ball tipped screw (actually sapphire, Grade 25) on my three VPI arms.

    The arm is constantly moving and causes the point to grind into the thrust plate. This creates a kind of mechanical jitter in my opinion.

    I like the sound of the smooth Ruby ball much better, it also has very low friction. This is a low cost mod.

    Click image for larger version Name:	VPI Dual Pivot Screw with Ruby Ball.jpg Views:	1 Size:	219.3 KB ID:	83852
    Please tell us where one may source the ruby tipped grub screw!

    Thanks

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  • Stringreen
    replied
    Regarding counterweight .......If you need a heavier counterweight for the 3D (or any of its arms) VPI can accommodate. My counterweight has a mini-weight attached to properly balance my Ortofon. This little weight can be unscrewed if one needs to install a lighter cartridge.

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  • JennMartin
    replied
    Originally posted by MylesBAstor View Post

    Yes it is a worthy sonic upgrade. Plus some just like the feel better. I'm hard pressed to think of a better $150 upgrade.

    My only issue has been VPI's going to a round CW like on the new Fatboy and loading the dual pivot. Ugh. But there's no issue with the elliptical shaped CW.
    Thanks Myles!

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