Arm geometry and null points

Halcro, I normally look at the tonearm alignment situation from the other direction. Lofgren A, Lofgren B, Stevenson and other alignments can be said to be a set of null points at specific radii that are selected (usually by the tonearm designer, in some cases by you) for certain priorities, such as end-of-side distortion, lowest peak distortion or whatever. If the priorities are different, the radii of the null points will likewise be different.

Once you (or the tonearm designer) know what priorities you want to optimize for, and the null radii have been picked (Lofgren A, Lofgren B, Stevenson etc.), you can calculate the overhang and angular offset required to hit those radii within the constraints of the tonearm's effective length (the sum of the overhang and spindle-to-pivot distance).

Usually, as the tonearm length increases, hitting the same null radii will require the overhang to be shorter and the headshell offset angle to be shallower.

Practically, unless you are using an SME/Graham type tonearm base which can be moved fore and aft, or a baseplate with an offset hole of the kind found on many vintage Japanese turntables, you will have little freedom to alter the spindle-to-pivot distance (for example, if the dealer drilled the tonearm hole in a slightly different location from what the tonearm manufacturer intended). You may therefore need to recalculate and adjust the overhang and offset angle to compensate for the discrepancy in spindle-to-pivot distance and accurately hit your target null radii.

IME, many vintage Japanese tonearms (Audiocraft, Yamaha, Micro-Seiki, FR, Ikeda etc.) do not use Lofgren A or Lofgren B, and so if you want to use these alignments, you may need to recalculate the tonearm geometry.

regards and hth, jonathan carr

The respective null points will be at the very same locations - no matter whether 9", 10" or 12" (or anything in between). The derivation from that zero error arc however will be the less the longer the effective length of the given tonearm. Each of the respective different length tonearms however have to be adjusted for overhang and azimuth independently - according to the geometry desired.
As Jcarr pointed out in his post, several japanese tonearm designers went for very "individual" calculations in their tonearms geometry. The FR-60 series tonearms in particular should be recalculated before put to use. This will better their geometry by a magnitude. The same is indeed true - as mentioned by Jcarr - for many tonearms from the 1970ies and 1980ies (especially so, but not only of japanese origin).

Cheers,
D.

Null points (Baerwald and otherwise) are defined by measuring a specified distance along a radius centered on the TT spindle. The tonearm has nothing to do with their location.

Null points do not change whether you adjust spindle-to-pivot or overhang correctly or incorrectly. They do not change regardless of which tonearm you use, or even if you use no tonearm at all! The null point is the null point.

This literal answer is so simple that I suspect you intended to ask some other question than the one you actually wrote. :-)

Here's another way of putting it: with a single pivot tonearm, the stylus scribes an arc. This arc will have at most two points where it intersects the straight line which corresponds to zero tracking error

The various schemes set these two points at different disc radii according to formulations which attempt to minimise either simple errors or the consequences of these errors.

Different tonearm lengths change the curvature of the arc but the position can always be adjusted so the arc has the same two intersection points..

This last statement assumes the radius of the arc is not made impracticably small.

Mark Kelly