Record player - Null Points
Making the Adjustment

"If you come at the king, you best not miss"
           Omar - The Wire

D Vautier
07/2017
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General

Most turntables are designed with a bend in the tonearm between 26 and 30 degrees which can be built into the arm itself or in the headshell. Some arms have no headshells. The arm overhangs the spindle by around 12 to 14 percent of the pivot length (pivot to spindle, or PtoS). The PtoS is a constant which is built into the turntable and is defined as the distance from arm pivot to spindle center.  Arm length (effective length) here means the actual distance from pivot to stylus and this is usually adjustable and can change the overhang.

Turntables are designed with angular arms and overhang as a way to increase the apparent arm length and thus reduce tangential error since the apparent arm length is projected out to be longer than the actual arm length.  In the diagram at right we observe an apparent arm length which is approximately 1 and 1/2 longer than the actual arm length. Itís just geometry, not style, convenience, tradition or magic, but they have been doing it this way since stereo came along and tangency became important. 


 

There are two points on a properly adjusted turntable where the tangent becomes zero, that is, the stylus rides in the groove at exactly the right way.  These are called null points.  Null points are determined by cartridge angle and arm length (actual arm length).  Arm length can be adjusted quite a bit on tonearms with standard headshells but arms without headshells have little ability to change arm length unless they have slotted mounting holes.  Changing arm length also changes overhang. This significantly effects null point adjustment.  The ratio between PtoS and overhang is just as important as cartridge angle.

Here is a standard headshell.  the cartridge mounting slots are about 12 mm long.

headshell displacement

Changing the arm length changes the distance between null points.  Changing the attack angle of the cartridge moves the null points together in or out from center but both these adjustments work somewhat together, and to get null points where you want them requires manipulating both arm length and attack angle a number of times.  The objective is that a record is best played with a minimum amount of tangential error across the playing surface, or better said, the stylus should remain as perpendicular as possible to the groove during play.

Here is a picture of the issue (not to scale)

diagram of null point relationship to spindle and pivot point

 

Description

As mentioned above, null points can be moved by a combination of lengthening or shortening arm length and adjusting the offset angle of the cartridge which is mounted in a headshell and already be at an angle.  Since a standard headshell allows about 11 to 13 mm arm length change and up to 15 degrees angle change, it is quite possible to arrive at just about any set of null points.

The effective playing area begins at 145 mm from center and ends at 65 to 70 mm from center.  There are few record manufacturers today that dare cut closer than 65 mm from center.  Even though the inner groove standard is supposed to be 60.325 mm from center, most turntables will go into return cycle before that point.  It is for this good reason that no record is cut closer than 65 mm, even though high end turntables usually have no return mechanism so they don't have to worry the amount of deadwax.

Published Null Points

The three advertized alignments and their null points are:

Baerwald 66 and 120.9
Loefgren 70.3 and 116.6
Stevenson 60.325 and 117.42

From what it seems Baerwald tries to even out all distortion by attempting to spread it out in the middle.  Loefgren minimizes distortion by trying to average it out over the entire record and Stevenson reduces distortion by favoring the inner groves which may be more compact and more in need of good alignment.  Manufacturers may recommend one of the other type but I have a hard time seeing why it would matter.

None of these alignments make a lot of sense to me because they calculate a final groove at 60.325 which is not born out by fact.  Records are just not cut that way.  Deadwax begins way before 60 mm, usually around 68 or 69 mm which is even pushing it.  My calculations come up with a preference for an 85 and 120 mm null setting given that I do favor the inner grooves a bit.  This takes into account an actual playing area of 147 to 70 mm rather than the 145 to 60 that the methods above are probably using.

The Template

Take a sheet of lined paper and punch a hole on the upper right side on the top line or use one of the holes.  This becomes the spindle center.  Then mark your null points on the line.  Here I use my preferred points (85-120) but you may want to use your own nulls like 90-125, which would emphasize the second and forth piece on a five selection side, or perhaps an 80-115 which gets a fatter part of the surface.  Project a right angle out from your two points. You now have a template. 

 

The Pointer

Now you need a way to project the true perpendicular from the cartridge cantilever, NOT the headshell.  You can do this with a small strip of cardboard or a paper clip just so the tip lines up with the cantilever.

    

Testing

There are various techniques described everywhere for null point adjustment and I think it may depend on the turntable, tonearm and cartridge but it does involve a template, a number of repetitive length and angle adjustments and then some performance listening.  As described above, changing the arm length will increase or decrease the tracking arc thus moving the null points closer or farther apart.  Rotating the cartridge will move the null points in and out and somewhat effect the distance between points. When the distance between your null points is good your overhang is good.  Rotating the angle of the cartridge changes the position of the two points. 

 

Here's another way. I first extended the cartridge out to the very end of the headshell giving the arm a maximum length just to see what would happen. It was really bad. I then adjusted to the middle of my headshell and began to change the angle until I got my inner null point and then repeated the process against the outer null point.  When I got satisfactory adjustments of both points I aligned the pointer to the position of the cartridge and marked off each angle.  If your pointer is taped to the headshell you have to readjust it every time you change the angle of the cartridge by sighting down the cartridge to the cantelever, fixing the pointer over the stylus and adjusting the far end of the pointer until it aligns with the new stylus angle.

When taking measurements it is important to carefully sight down the spindle axis first holding the turntable firmly on the line because this makes a huge difference in the tangent angle.  Then place the stylus exactly on its point lock the TT and mark off the displacement.  

Next I moved the cartridge all the way up on the headshell to get the shortest arm length and repeated the tests. It was hard to get null points at both extremes but it gave me a good idea where the "soft spot" was.  I decided that about 2/3rds down the headshell was about the best spot and I did a third set of tests at that location.  The results were quite satisfactory because all readings were within one degree of tangent (and that is pretty good).

Another perhaps simpler technique can be described like this.  Position the cartridge midway on the headshell at zero degrees.  Place the pointer and tool.  Now by moving the arm and tool discover and mark the location of current null points.  From here you can make what additional adjustments are needed.  Remember that arm length change moves the nulls closer or farther apart.  Angle change moves both nulls in or out.  

calculation tangent error on null point adjustment
Here is my final cartridge adjustment.  it is about 5 degrees clockwise and in about 1/3.cartridge correctly alligned for null points  

checking null point pointerI'm checking the alignment of the pointer with a ruler.

Here is my template/worksheet

null point template/worksheet

Here is a scatter chart of tangent error taken at minimum arm length.

Here is a scatter chart of the optimum tangent error.  All samples are within one degree.