Why do tonearm specifications list null points?

Several years ago, I experienced the issue discussed by Dover and Larry with a Dynavector DV505 tonearm, probably because the DV uniquely places the vertical pivot so far forward from the horizontal pivot.  The DV was designed with headshell offset for Stevenson or something very close to it.  When I aligned a cartridge according to Baerwald, I of course had to twist the cartridge in the headshell so that its long axis was deviated toward the spindle. I was unhappy with the resulting SQ.  Reasoning that the problem was due to the necessary misalignment of the cantilever with the vertical bearing arc, I then straightened out the cartridge using Stevenson.  Whether this hypothesis for cause is valid or not, correcting the alignment of the cantilever to be in the same plane as the arc of the vertical bearing made for a big improvement in SQ.

@dover , right on. 

@rauliruegas exactly, overall distortion levels are lower with Lofgren B except at the very innermost part of the record. Modern records are not cut that far in. Som older records are but I do not notice significant problems with records cut all the way in and I Use Lofgren B religiously. It is tedious setting up a cartridge correctly especially if you are trying to change alignments. Any tonearm set up for Lofgren A can be made to do Lofgren B but to do it accurately and consistently you need a protractor like the WallyTractor or the SmarTractor. You need a mirrored surface with lines finely etched and magnification.  

This happens all the time: we conflate tracking angle error with "distortion" without actual evidence that the two are perfectly correlated. And what sort of distortion might be correlated with TAE? Suffice to say it seems to be a good idea to minimize TAE, but whether that also minimizes "distortion" in a predictable or audible way is not established. Or if it is, where are the data?

You need a mirrored surface with lines finely etched and magnification.  

and certainty that the diamond tracing faces (assuming advanced profile) are aligned precisely 90° to the cantilever.

dave

Regarding a mirrored surface, I'm currently using the Dr. Feickert generation 2 protractor; while it doesn't have a mirrored surface, it has white etchings on a black matte aluminum surface are very precise and extremely easy to read (for me personally, this is easier to see than white etchings on a mirrored surface on protractors used in the past). I also have a very high intensity work light that I use from different angles (same height as stylus and shooting from the side or front as required) as well as magnification to ensure the diamond tip is in the cross-hairs. I'll admit I think I do have it easier than most because the Clearaudio Jubilee MC cart I'm using has the long cantilever extended out in front of the cartridge body making it very easy to confirm the cantilever's parallelity with the lines on the protractor.

With all this said, I'll be the first to admit that the improvement I think I'm hearing from switching back to to Lofgren A (Baerwald) from Lofgren B is a) either in my head because Clearaudio specified the Baerwald null points in their tonearm specs, or b) as @rauliruegas pointed out, most likely from tiny errors from my hand or sighting with the protractor.

Thanks all for your comments - always fun to discuss these things and learn from each other.  -cheers!