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

Halcro, I share the same opinion as Jfd01. Probably Yip simply doesn't know what alignment was used for the design of each of your tonearms.

A 9 inch arm and 12 inch that are both designed for Lofgren A alignment will have the same null radii.

OTOH, a 9 inch arm designed for Lofgren A alignment will have different null radii from another 9 inch arm that is designed for Lofgren B or Stevenson alignment.

Practically speaking, the null radii have nothing to do with the arm length. The null radii have everything to do with what alignment was used during the design of the tonearm, and that is determined by the priorities (or whims) of the tonearm designer (to keep things simple, I'm leaving tonearm installation errors out of the discussion).

hth, jonathan carr