Blog; Unipivots, V8s and straight sixes…… July 31 2016
A customer asked for an explanation of how The Wand Tonearm® compares and is different from other unipivots and ‘conventional’ arms. (There will be separate blogs about why tonearms matter and the other design aspects of The Wand Tonearm)
In my mind, that an arm is or isn’t a unipivot is a bit like thinking a car is good or bad just because it uses a straight six rather than a V8 (or whatever) engine. Yes, there are apparent advantages in each approach but is really how you implement that technology that has more influence than the particular path.
The most common fear about Unipivots is that they are ‘less rigid’ than arms with rolling element bearings. On a slightly mischievous note, I would argue they are more rigid! Yes, many Unipivot feel that they wobble and those where the cueing is out the side unfortunately accentuate this feel. But for a rolling element bearing to be able to roll, it must have clearance or it will jam. Even the best grade bearings have clearance that is in the same order of magnitude as modulations of a small signal on the record groove. A unipivot has zero movement (which usually means a thin film of grease or oil acting incompressibly).
If the concern is that the rotation of a Unipivot arm is only countered by the arm’s weight/ inertia. It is worth considering that this is also the case in all arms in the vertical plane, where it is not usually considered a problem.
I suspect some of the negative aspects people associate with Unipivots is more to do with the design choices of the era when most were designed ie; 1960-70s. At this time there was a ‘space race’ to make technically perfect cartridges with vanishingly low tracking weights and perfect tracking on test discs as the aim rather than music*. This was the mechanical equivalent to the 1970s high feedback, ‘zero distortion’ amplifiers or ‘flat response at all costs’ speakers. Generally they didn’t do much for music either.
At this time, cartridge designers were pursuing higher compliance and thus arms became more and more spindly and lightweight (Higher compliance means ‘more easily moved’ stylus, so the arms needs to be lighter to keep the resonant frequency away from the warp frequency of the record but ho much this really matters is another blog to come…). This theoretical approach also encouraged two design practices used in conventional bearing arms that are worth critiquing; High centre of gravity arms and zero friction bearings.
High centre of gravity arms sounds great as a concept. The argument goes that to aid tracking over warped records the centre of gravity of arms should be at the same height as the bearing pivot point. This would mean that as a cartridge lifted over the warp, the centre of gravity would go down, reducing the balancing effect of the counterweight and so increasing the effective tracking force to help the stylus track down the other side. Much marketing was made of this. Unfortunately, for Unipivots where this ideal was approached become less stable on normal (non-warped) records as they don’t have the lowered weight necessary for keeping them stable. Digging into this pours an ice cold bucket of water of reality over the theory. If we have a really badly warped record, with a 6mm (1/4in) high warp, for a 9in arm, the angle the arm moves through is less than 0.1 degree and changing a 2g tracking force to around 2.0002g. Even if you doubled the figures (a crazy warped record) the effect is negligible.
I can’t argue with the apparent desirability of zero friction bearings, but it is only one factor of tonearm design. Many Unipivots aim for low friction by having a very sharp spike of steel sitting in a jewelled cup, sometimes referred to as ‘stick in bucket’. Unfortunately these spikes taper down to a fraction of a millimetre which means they can flex, affecting what you pick up.
The Wand Tonearm (Series 2) uses a considered low friction bearing, something more like a hip joint. A hard ball sits in a hardened tungsten carbide cup. The rolling action within the cup (like your hip rolls when your walk) gives a bearing that is higher friction than most unipivots, but lower than most ‘pivoted’ arms. Importantly it copes well with the dynamic forces exerted during its operation, a far more important role than how it functions at rest. The contact point in The Wand unipivot forms a vector (Straight line) oriented towards the stylus , so won’t deflect in operation.
So how you implement a Unipivot (or any arm design) matters more than whether it is a Unipivot or rolling element bearing arm.
*Exempting the Telarc 1812 overture which didn’t bear much relation to music, the playing of the orchestra giving the impression they know people only care about the cannon to come…..
©Simon Brown 2016 (may be reproduced with permission and acknowledgment)