There have been a few recent murmurings regarding the advantages of zero offset tonearms, together with related talk about the advantages this brings regarding the issue of anti-skate compensation.
So, what does zero offset mean? Put simply, the zero offset concept refers to a tonearm with the cartridge mounted such that the stylus and cantilever, (and usually the headshell and armtube) form a straight line with the arm pivot. This contrasts with a typical arm in which the cartridge is offset at an angle to a line from pivot to spindle.
For example, the classic DJ turntable has this characteristic, as does a linear tracking tonearm with its arm tube at 90 degrees to the groove radius.
The difference between these two cases is that while both have zero offset cartridges, only the linear tracker has zero overhang at all points of its travel because it tracks a radius. The DJ arm, and any other similar pivoted design, like the ViV Lab, may have zero offset, but, because they are pivoted, they must move through an arc. If that arc intersects the spindle, the arm has zero overhang. Otherwise it has either positive overhang like most pivoted arms, or negative overhang, i.e. underhang.
Most zero offset arms are set up so that there is a point on the arc where the armtube is at 90 degrees to the spindle, which means the stylus is sitting on a radius. This a null point. The stylus and cantilever are tangent to the groove at this point and only this point, and it is dependent on where the arm is mounted.
In other words, as the cartridge moves away from this single null, the angle between the pivot, the stylus, and the groove tangent increases (let's call it angle PSG), and instead of zero overhang like a linear tracker, this leads to underhang.
As the arm swings across the record, it describes an arc of around 30 degrees. If the null is around 70mm from the spindle, then there will be an increase in tracking error as the stylus moves away from this point and the error will be more at the outer edge than at the runout. The stylus will underhang as opposed to overhang the spindle.
Because antiskate forces are generated by the PSG angle not the cartridge offset, then there will be forces acting on the stylus which will try to rotate the arm. On a straight arm with underhang this will mean that the arm has a force acting on it inwards or outwards at points on the arc depending on where the null is positioned.
This means firstly, i t will need less compensation as tonearm length increases. Secondly , it will need less compensation than a typical arm with overhang. Thirdly, it will have more distortion at the beginning and end of side than a typical arm. Fourthly, a spherical stylus may work better than a fine line stylus.
