Unipivot tone arms

@helomech  Re: "Some unipivot tonearms (VPI for example) are poorly designed because they are inherently unstable, thus the tracking force is in a constant state of fluctuation as the tonearm traverses the record." 

Uhhh, no. They are no more unstable vertically, the axis of tracking force, than any other tonearm. There is no mechanism that accounts for what you describe, varying the vertical force simply from the arm moving laterally across the disc

As for the 'wobble', no one has ever been able to quantitatively measure any impact of the unipivot bearing during playback. It only makes inexperienced users uneasy during cuing. 

Finally, a unipivot bearing, by design, is incapable of rattling, chattering, or moving forward and back during playback. 

That's it X, Y, and Z axes. There is no 'inherent instability' in a unipivot arm during playback. 

What you describe sounds more like a cartridge compliance/arm mass issue which I'll affect any improperly paired arm and cartridge. The classic example being a low compliance MC mounted in a low-mass tonearm. We knew this 50 years ago when people tried mounting an Ortofon or Supex MC in a Grace 707 or Infinity Black Widow tonearm. Those bad matchups yielded a very high arm resonance frequency that wouldn't track well and lacked bass. The reverse was an ADC XLM or Sonus Blue (both very high compliance carts) in a Technics or Denon medium-high mass arm. The too-low resonant frequency was extremely susceptible to footsteps, and often the cantilever suspensions just failed from trying to push around a high mass arm. 

If you want an arm whose tracking force is unimpacted by warps and such, then get an arm with spring loaded rather than gravitational VTF, like a Rega 330. 

@dynacohum : The Keith Monks unipivot arm with its four mercury baths is certainly unique! I wish I had kept mine! I sold it because I had problems with the liquid  silicone contaminating the mercury contacts, causing channels to drop out. I bought the Australian JH Formula 4 unipivot instead. 

@dynacohum : I have a NIB Mayware unipivot awaiting installation, also a NIB Shure V15 mkV that would match well with the Mayware compliance-wise.

@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.