Record player - Null Points
Making the Adjustment

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

D Vautier
07/2017
email


General

Most turntables are designed with a bend in the tonearm between 26 and 30 degrees which can be in the arm itself or built into 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 quite adjustable and changes 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 apparant arm length which is approximately 1 and 1/2 longer than the actual arm length. Itís just geometry, not style, convenience, tradition or magic.


 

There are two points on a properly adjusted turntable where the tangent becomes zero.  These are called null points.  Null points are determined by a number of factors but mainly cartridge angle and 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.  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 in or out from center but both these adjustments work together, and to get null points where you want them requires manipulating both.  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, the null points can be moved by a combination of lengthening or shortening arm length and adjusting the offset angle of the cartridge which is already mounted in a headshell and already 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 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.  There are no record manufacturers today that 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 at 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.

Published Null Points

The three common alignments and their null points are:

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

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 in the inner groves which may be more compact and more in need of proper adjustment.

None of these alignments make a lot of sense to me because they calculate a final groove at 60.325 which is an incorrect assumption.  My calculations and tests come up with a preference for an 85 and 120 mm null setting.  This takes into account an actual playing area of 79 to 145 mm rather than the 60 to 145 that the methods above are based on.

Testing

There are various techniques described everywhere for null point adjustment and I think it may depend a lot 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.  In general changing the pivot length will increase or decrease the tracking arc and also move the null points.  Rotating the cartridge will move the null points in and out and also somewhat effect the distance between points.

 

 

I first extended the cartridge out to the very end of the headshell giving the arm a maximum length. I then 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.  The only way to get a pointer adjusted is by sighting down the cartridge, fixing the pointer over the stylus and adjusting the far end of the pointer until it aligns with the stylus angle, not the headshell angle.

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

 

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.