Vinyl Asylum

Posted by Helge Gundersen (i) on February 08, 2001 at 05:30:50:

In Reply to: Are you serious??? posted by Mr. Walker on February 07, 2001 at 16:41:48:

Hi,

First off, the 93.4 mm was tied to the example I used with null points of 66 and 120.9 mm; the Polaris Plus uses different null points, so the sight line should not be placed at 93.4.

Rulers differ slightly, but by comparing all of my rulers and measuring as carefully as I'm able to, I've come to 91.9 mm for the sight line, 63.9 mm for the inner point, and 120.5 mm for the outer. These figures must still be taken as approximations, as the measurements are not exactly done with laser or something. Also, the spindle holes may have been cut very slightly differently from sample to sample.

The instructions say that you can verify your alignment by using the outer point (i.e. spindle hole C) "if you wish". Every time I do this, the stylus will not hit the centre of the point, but is slightly too far forward. Maybe that's okay in practice if you know about it beforehand!

As far as I can see (and Brian Kearns believes the same thing), the sight line should be placed smack dab in the middle between the two spindle holes, which you can easily see that it isn't; it's closer to A than to C. A line like this is located at the so-called linear offset, which is the product of the effective length of the arm (no matter what this length is) and the sine of the offset angle (whatever that trigonometric stuff means). It also equals the average of the two null points.

With an inner point of 63.9 and a line on 91.9, the outer null point should be expected to be located at 119.9 mm. With an outer point of 120.5 and a line on 91.9, the inner point should be expected to be located at 61.9. With null points of 63.9 and 120.5, the line should be expected to be placed at 92.2. This assumes that the line is to be placed at the average of the two points (you just isolate X in the equations by multiplying and subtracting).

There are in principle three options:

Align as per the instructions, and accept that the real outer null point is at 119.9 (I have done this so far).
Align at the outer point instead of the inner, and accept that the true inner point is at 61.9.
Draw a new line (completely parallel) and use both points as they are.

I'm posting from home, and need to look at some Excel stuff on my workplace machine..., but the first option seems to be the best of these. As Martin Bastin says, the inner point is the most critical (because of the notorious inner-groove distortion), so I wouldn't use alternative two, although this is what Geo-Disc does. Also, an inner point of 61.9 seems to be farther inwards that I would prefer. And in particular, the null points would be way too widely spaced. The latter point is possibly valid for the third option too. Bastin does write that the outer point is less accurate, but I don't know if he is referring to the same thing as we do. If so, why did he place the outer null point where he did?

An inner point of about 63.9 is not "incorrect" anyway; it gives you a good performance on the inner grooves, but, relative to the 66/120.9-alignment, at the cost of more distortion elsewhere. It's a kind of compromise between the Rega-style alignment and the 66/120.9-alignment. I will see what the outer point is doing to this, what it adds to the distortion pattern.

I have a (new, cleaned) record with a more sibilant vocalist than usual, and the sibilance grows during the middle portion of the record, and may seem to reach a high somewhere around the middle peak distortion point, while I think it's better at the very end (about 65 mm). I'm thinking that the balance could be too much in favour of the innermost grooves, but that's speculation for now.

I haven't called Martin Bastin.

The Polaris Plus is really excellent in many ways: The stylus mark is a small circle, not a black dot or a cross. There are plenty of gridlines, including crosslines for aligning the front or back of the cartridge. There is a line for aligning the cantilever, and a nice touch is that this line is positioned slightly to the side of the circle; in this way, you can view the line behind *and along* the cantilever. The infamous sight line is fairly thick, just right for the pillar of my Nottingham Interspace arm. It's also long.