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Why Do our Arms Work?
#1
This seems appropriate for "General Audio".  Recently a question came up on a Facebook post about "Why Does every cartridge seem to work on VPI tonearms".  There is an actual answer and it was too long for a quick facebook response so here is some more "behind the currant at VPI.   I know I did share this on the previous forum and elsewhere but always good to talk about it some more.

Back when my dad/Harry was designing his first arm he wanted to make a design that was top shelf but easily adjustable.  Also at the time I believe he preferred the sound of a Uni.  He focused on what would later be called the "JMW" (Jonathon Mathew Weisfeld) Memorial Tonearm.  A standard Unipivot type tonearm with the lowest about of contact/friction.  When he was designing it he had multiple cartridges to experiment with.  Every experiment was met with measurements and listening tests to confirm what he was trying to accomplish.   One of those cartridges was the Sumiko Black Bird.  He had three of them and had a hell of a time getting all three of them to sound good.   Angry

He made adjustments to the arm and eventually made it work.  It wasn't until he actually checked the stylus of the cartridges that he realized all three of those cartridges were drastically different!  Keep in mind this was MANY years ago and is no longer the case for Sumiko but at the time it was a nightmare and dad made the adjustability.  SO we can thank the (at the time) inconsistency of Sumiko for helping us design a better/cartridge universal tonearm.  Cool 

Harry feel free to chime in with any additional info on your tonearm design experience.  Smile
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#2
I'd love to hear Harry's thoughts, and recommendations as to set-up, regarding heavier (12g and more) cartridges and the 10" unipivot arm.
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#3
Very cool insight!
Whoa listen to the music...

Wachuko
VPI Scoutmaster Refurbish, Prime Signature Motor (300 rpm), Prime Aluminum (20 lbs) Platter and Bearing, Delrin Record Clamp, JMW 9 3D Arm with Sumiko Blue Point No.2, Upgraded Classic Feet, Periphery Ring Clamp
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#4
(05-12-2022, 05:30 PM)jupiterboy Wrote: I'm struggling to get up to speed with the various VPI arm offerings. I've had a Rega arm for decades, and being someone who spins lots of records, I've had lots of cartridges. I started off getting the best I could afford, but then realized how quickly they would show wear and need to be replaced. I would always look at the standard graphs for resonant frequency and didn't think that much about it as long as it was in range. Over the years I've developed more specific ideas and found I'm not alone in targeting a resonant frequency right around 9.5-10Hz. There's something about that compliance match AND a good gain match that seem to sound more right. I'm always spinning jazz records and so often a horn player will come in a little hot, and this is where it pays off, on these very hot passages.

My question is this—is this a perception related to the Rega arm or all arms? I've never lived with a unipivot and don't know if my hard earned opinions will carry over to another arm/table combo.

I know I'm certainly tired of sales people bulldozing me and telling me I can stick any cart on any arm, but based on this post and opinions of some other arm designer/makers I may be too stringent in my thinking.

This would be a good topic for a new thread.
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#5
(05-12-2022, 06:10 PM)seamonster Wrote:
(05-12-2022, 05:30 PM)jupiterboy Wrote: I know I'm certainly tired of sales people bulldozing me and telling me I can stick any cart on any arm, but based on this post and opinions of some other arm designer/makers I may be too stringent in my thinking.


When I started on the original forum I mentioned I wanted to try my old Stanton 681 cartridge on my Prime.  Brf explained why that would never work (arm/cartridge compliance mismatch).  As I was swapping my worn out Delos out last summer, I mounted the Stanton for fun...and not only did it sound horrific, it was impossible to dial in.  This is hearsay, of course, but I am believer in pairing a well-matched cartridge to an arm.
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#6
I believe it is important to distinguish the difference between what “works” and what “works optimally” for the purpose of this discussion.

When determining cartridge and tonearm compatibly, many users look at cartridge compliance and tonearm effective mass to see if the combination produces an acceptable resonance frequency.  With a cartridge with a stiff suspension, you want a tonearm with a higher effective mass so that the groove modulations moves the stylus and not the tonearm. 

A simple analogy would be a car with a suspension that’s too stiff for its mass.  When the car hits a bump, the energy produced by the bump cannot be fully absorbed by the suspension and is passed through to the car chassis causing a rough ride and poor handling.  If the car’s suspension is too light for its mass, the suspension bottoms out and again the energy is pass onto the chassis.  Matching a car’s suspension to the car’s weight is the same principle as matching cartridge compliance with an arm’s effective mass.

So why do Uni-pivots “work” better with a wider range of cartridge compliances than a gimbal tonearm of the same effective mass?  It all has to do with the position of the arm’s center of gravity.  Uni-pivots have a lower C of G (center of gravity) than captive bearing tonearms.  Having a C of G below the pivot will create an opposing force when the arms is displaced from its rest position.  This is why Uni-pivot tonearms have different measured VTF at the record surface than when raised slightly.  Now combine a Uni’s effective mass and add an opposing force when the cartridge/arm is displaced, creates a greater resistance to change than a captive bearing arm with the same effective mass.  On the surface, the this might appear to be desirable, but the opposing force caused by cartridge/arm displacement is not constant and ever changing, therefore, causes inconsistent VTF. 

Tonearm design is all about design compromise.  The near frictionless single point Uni-pivot must be stabilized and supported by mass below the pivot, therefore, necessitating a low C of G, whereas a higher friction captive bearing can have its C of G on the same plane as the stylus.  Pick your poison.
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#7
The other problem I see with unipivot arms, they can move in all 3 axis whereas a gimbal arm is constrained in 1 axis, which is desirable. Unipivot arms can pitch, roll and yaw where a gimbal arm removes the possibility of roll (azimuth) which reduces distortion.
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#8
(05-13-2022, 03:21 PM)Bagehot Wrote: The other problem I see with unipivot arms, they can move in all 3 axis whereas a gimbal arm is constrained in 1 axis, which is desirable.  Unipivot arms can pitch, roll and yaw where a gimbal arm removes the possibility of roll (azimuth) which reduces distortion.
I wonder, is that a problem, or a solution to the fact that LPs are not actually flat at all?  Not even close, and they are inconsistent all over every side.  So therefore the flexibility of the unipivot keeps it in the groove better?  

Just wondering.

(05-13-2022, 03:21 PM)Bagehot Wrote: The other problem I see with unipivot arms, they can move in all 3 axis whereas a gimbal arm is constrained in 1 axis, which is desirable.  Unipivot arms can pitch, roll and yaw where a gimbal arm removes the possibility of roll (azimuth) which reduces distortion.
I wonder, is that a problem, or a solution to the fact that LPs are not actually flat at all?  Not even close, and they are inconsistent all over every side.  So therefore the flexibility of the unipivot keeps it in the groove better?  

Just wondering.
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#9
(05-13-2022, 09:29 PM)suntea Wrote:
(05-13-2022, 03:21 PM)Bagehot Wrote: The other problem I see with unipivot arms, they can move in all 3 axis whereas a gimbal arm is constrained in 1 axis, which is desirable.  Unipivot arms can pitch, roll and yaw where a gimbal arm removes the possibility of roll (azimuth) which reduces distortion.
I wonder, is that a problem, or a solution to the fact that LPs are not actually flat at all?  Not even close, and they are inconsistent all over every side.  So therefore the flexibility of the unipivot keeps it in the groove better?  

Just wondering.

Actually, that is exactly what you don’t want in either Uni or captive bearing tonearm. You want the tonearm to hold the cartridge in a fixed plane and allow the stylus/cantilever do all the work and trace the record groove.  In a Uni you have mass below the pivot point which is sufficient enough to stabilize the cartridge to prevent torsional torque from upsetting the cartridge balance.  In a captive bearing tonearm, the arm is fixed in two planes of motion, therefore, torsional torque will not change the plane of motion. 

An argument can be made that torsional torque does upsets the balance of a Uni at a micro level which can translate into diminished performance (lack of bass), and this is why VPI introduce the dual pivot which prevents free rotation around the pivot axis in the longitudinal plane.
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#10
(05-14-2022, 03:10 PM)Brf Wrote:
(05-13-2022, 09:29 PM)suntea Wrote:
(05-13-2022, 03:21 PM)Bagehot Wrote: The other problem I see with unipivot arms, they can move in all 3 axis whereas a gimbal arm is constrained in 1 axis, which is desirable.  Unipivot arms can pitch, roll and yaw where a gimbal arm removes the possibility of roll (azimuth) which reduces distortion.
I wonder, is that a problem, or a solution to the fact that LPs are not actually flat at all?  Not even close, and they are inconsistent all over every side.  So therefore the flexibility of the unipivot keeps it in the groove better?  

Just wondering.

Actually, that is exactly what you don’t want in either Uni or captive bearing tonearm. You want the tonearm to hold the cartridge in a fixed plane and allow the stylus/cantilever do all the work and trace the record groove.  In a Uni you have mass below the pivot point which is sufficient enough to stabilize the cartridge to prevent torsional torque from upsetting the cartridge balance.  In a captive bearing tonearm, the arm is fixed in two planes of motion, therefore, torsional torque will not change the plane of motion. 

An argument can be made that torsional torque does upsets the balance of a Uni at a micro level which can translate into diminished performance (lack of bass), and this is why VPI introduce the dual pivot which prevents free rotation around the pivot axis in the longitudinal plane.

Thanks BRF, that does make sense.  I actually stopped using the dual pivot because it was a pain to setup.  Maybe my 2 Rel subs make up for the bass loss so I don't notice too much.  I should try the dual pivot again though.

On another subject, do you still use and recommend the Project Leather It platter mat?
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