Aligning the cartridge
Especially after having performed all those adjustments, it is a good idea to check the cartridge’s alignment. During reproduction, the needle slides along the spiral groove from the outermost to the innermost position. It is impossible the cartridge will be constantly aligned to the groove all along the way. Theoretical calculations by Baerwald, Stevenson and Lofgren have pinpointed the closest and farthest points from the record’s center where the cartridge must have the best possible alignment, so that the average error between them can be tolerated. The null points are at 66.0 e 120.9 mm from the record’s center (Baerwald). They can be projected on dedicated graphs to be printed and placed on the platter to verify the alignment, such as TNT-Audio’s proctractor (many complicated protractors are available for sale but you could also draw the null points on graph paper). Thorens turntables like mine, which have a spindle to pivot distance of 215.6 mm, could use a specific protractor available on the Vinylengine website. Just center the protractor graph on the record’s bearing shaft and verify that the needle’s tip follows the arc on the protractor; at the same time, the cartridge’s sides must be parallel to the lines drawn on the checking point (better if you are able to check the cantilever’s alignment, rather than the sides’). If it is not so, the cartridge mounting screws have to be slightly loosened so that you can move the cartridge back and forth and twist it sideways to reach the correct alignment.
The cartridge’s lateral tilt (azimuth) is also rather important: an incorrect azimuth may cause a wrong needle-groove contact, tighter on one side, lighter on the other. This could lead to an earlier wear of both needle and record. Azimuth can be evaluated by placing a small mirror or CD on the platter with the needle on it. The reflected image helps in determining if the cartridge’s sides are vertical or not (picture at left, from theanalogdept.com). The Thorens TP60 headshell allows for such an adjustment by slightly rotating the headshell body after loosening the screw at the tonearm insertion point: use a small bubble lever to achieve the horizontality of the headshell’s top – but it does not necessarily mean that the cartridge’s azimuth will be correct. For headshells that cannot be moved, a trick is inserting a narrow spacer that would allow tilting the cart laterally by tightening the mounting screws by different amounts.
If you have a test record you can check the azimuth of your stylus listening to a proper track, that should sound identical on both channels. If you also have a computer and a tape out connection in the rear of your amp, it is easy to “measure” the correct azimuth for your stylus: connect the tape out to the computer’s line in and use a software like Audacity (free) to check the volume levels of the two channels. Azimuth may affect separation. If the test record also has a separate track for each channel with a 1 kHz signal, it is very useful to check the cartridge channel separation, a parameter that is a good indicator on how good a cartridge performs. Most modern cartridges can achieve around 25 dB separation at 1 kHz; 20 dB is the minimum; great if you have 30-35 dB, the maximum achievable! A tape deck with VU meters is also useful to check azimuth and separation provided you have a test record with the relevant tracks.