Unipivot tone arms

@panzrwagn 

You’d be correct regarding the VPI’s stability in that they act similarly to a lab scale—they have a “stable” balance configuration. As such, they are constantly counterbalancing any change in vertical height. Take a digital stylus force gauge and any record with a slight warp and measure the difference in VTF of the VPI’s unipivot between the high and low portions of the disc. You’ll measure a difference for sure!! In fact, you can even do this experiment with a flat record and measure a difference between the inner and outer disc grooves. The tiniest fluctuation in elevation will change the VTF with a balanced-stability tonearm. So yeah, they are stable in the static academic sense, but we are discussing a dynamic system. They are unstable in terms of tracking force. 

Think about the fact that VPI’s upgrade/flagship arms are now gimbaled. There’s good reason for that other than the “wobble” that so many found disconcerting. 

“Tonearms come in three three balance flavors: "stable" "neutral" and "negative". A "stable balance" arm is one where the center of gravity of the moving system is located below the pivot point. That's true with most unipivot arms, where the lower center of gravity aids stability. A "negative balance" tonearm is one where the center of gravity is above the pivot point, and a "neutral balance" arm is one in which the center of gravity is in line with the pivot point.

If your tonearm is "stable balanced," the further from the record surface you measure tracking force, the less accurate will be your result. That's because a "stable balanced" arm wants to return to its resting position on the record surface. The further up from the record surface you measure the tracking force the greater will be the measured force because the arm wants to return to its resting point on the record surface.

In practical terms that means if you measure 2 grams well above the record surface, the tracking force will be lower and perhaps too low at the record surface. That is one reason the simple Shure "teeter-totter" device is not accurate with "stable balanced" arms.

A "negative balanced" arm means the higher up you measure VTF the lower will be the tracking force at the record surface. A "neutral balanced" arm doesn't care where you measure VTF. It will remain the same high or low.

If you have a unipivot arm like a VPI, Kuzma Stogi S, or pre "Magneglide™" Graham arms, it will be "stable balanced". Kuzma's 4 Point is negative balanced. Most gimbaled arms close to neutral balance. Graham's Phantom Supreme is neutral balanced. Each of these balance conditions produces different results under dynamic conditions such as when encountering a warp, but that's best discussed in a tonearm review.”

—Source

You can achieve neutral balance with a unipivot, as you achieve it with other designs.  Graham arms, for example are neutral balance arms.  The Graham does not rely on the center of balance being below the pivot point to minimize wobble; that is done with magnetic stabilization.  There is no one inherent quality of unipivot arms that someone has identified here that cannot be addressed by correct design.  A good unipivot arm is a good arm, as is the case with all other designs.  

I was lucky enough to get one of the last production Schiit Sol tables after they fixed all their qc issues. The arm is mildly "stable balanced" and I know to adjust for measuring correctly for increased height. Same is true for measuring vta. I will say setup of this table was tedious just due to being able to adjust so many parameters in a very mechanical way, but the sound is fabulous. Lively, clear, and very consistent. No concerns with it being a unipivot. Old school, simple design much like a well functioning carburetor on your car. 

dynacohum:

One of those Keith Monks mercury-dampened arms came with a vintage Thorens TD124 I bought. Scared the heck out of me, though I appreciated the inventiveness of the design.  I called the city and a guy came out in a hazmat suit with a specialized vacuum and sucked up all that Hg. They treated it as a spill, which, thank heavens, didn’t happen. Needless to say, I rebuilt the Thorens and added an Ortofon (Jelco) arm and a Cadenza mono cart. To play all those used mono’s I keep buying 🙄.  Happy listening!

@helomech First, you need to quantify the amount by which VTF varies over any vertical displacement (warp), say 5mm. That would be a seriously warped record.

Next you need to assess any impact that has on the cartridges tracking ability. And the map those, if any, to changes in SQ that can be attributed to those variations in tracking force, and not the warp itself. 

Finally, you need to assess any variation in VTF as a function of arm length, as well as changes in Stylus Rake Angle (SRA). 

Let's throw in one last variable, F=MA. To function at all, the stylus and associated motor, MC, MM, or MI, must move relative to the cartridge body and arm. Ideally the arm mass would be infinite, and the resistance to movement vertically or laterally should be zero. Obviously mutually exclusive requirements. Instead, we are left with a mass (the arm) suspended by a compliant spring (the cantilever, stylus, and it's suspension) that results in a resonant system that has been objectively determined to be optimal around 9-10Hz. This enables the relative motion and generation of the electrical signal, while not being excited by the lower warp frequencies. Get that too low and even a small warp can throw the stylus out of the groove. Too high and 'the tail wags the dog', the system is too stiff and the sylus/cantilever pushes the tonearm, wiping out the bass. A dancing bear, to be sure - the miracle being mechanical not that it dances well, but that it dances at all.

With that data, you can begin a conversation on this dynamic system. Until then, the model is incomplete, and the discussion theoretical at best.