Arc protractor

phono cartridge alignment on an arc protractor






Technics SL-100C
Alignment: CUSTOM
Inner null radius: 58.801 mm
Outer null radius: 113.518 mm
Pivot to spindle distance: 215 mm
Overhang: 15.000 mm
Effective length: 230.000 mm
Offset angle: 22.000°
Linear offset: 86.160 mm

50556065707580859095100105110115120125130135140145150Groove radius (mm) distortion (%) 33⅓ rpm    45 rpm


Unlike 2-point protractors for phono cartridge alignment, which can be used with any turntable, this generator puts out an arc protractor template specifically for your record player.

The great advantage of an arc protractor is that it separates the overhang adjustment from the offset angle. With 2-point protractors, both are set at the same time, which can be confusing and time consuming. The arc protractor makes turntable setup a lot easier.

The turntable input field sets a name for the template, allowing you to identify for which model the protractor was generated.

Pivot to spindle distance is the only variable you actually need to know. It varies from turntable to turntable. Most record player manuals will give you a specification for effective length and overhang. Simply subtract overhang from effective length to get the pivot to spindle distance. Please note that actually measuring the distance may give you a more accurate result.

There are several standards for the radii of the inmost and outmost modulated (i.e. recorded) grooves of an LP record. Simply choose the standard you prefer:

  • DIN (IEC 98) is a German standard and will use the smallest inner groove radius (57.5 mm),
  • IEC (IEC98-1958), an international standard, the largest (60.325 mm). Both use the same outer groove radius (146.05 mm).
  • JIS (S8502-1973) is a Japanese standard and uses a slightly larger inner groove radius than DIN (57.6 mm) and the largest outer groove radius of the three (146.5 mm).

These are the most commonly used dimensions. They were chosen to keep the generator compatible and comparable with other protractors and calculators. DIN and IEC both have been specifying a larger outer groove radius of 146.3 mm since 1964, but this will not make much of a difference.

But which of the three standards should you choose? It depends on your records! Tracking distortion will be minimized in the specified modulated area. This is particularly critical for the inner grooves, as distortion increases significantly beyond the inner null point. So measure a couple of records with a narrow dead wax and choose wisely. For easier measuring, you may print out our record ruler.

If you know what you are doing, you may also enter custom values.

Based on the groove radii, the null points are calculated according to the alignment method. Which alignment you should choose again depends on your personal preferences. They all try to optimize tracking distortion, but in different ways:

  • Stevenson optimizes for the least inner groove distortion at the expense of increased average distortion.
  • Baerwald (also known as Löfgren A) is generally considered to be the best compromise that minimizes distortion across the record and keeps the distortion at the inner and outer grooves low.
  • Löfgren B will give you the lowest RMS distortion, but increased error at the inner and outer grooves.
  • AP™ is our proprietary alignment with reduced tracking distortion at the inner grooves (where tracing distortion is a problem) and increased tracking distortion at the outer grooves (where tracing distortion is low).
  • custom allows you to set your own null points or calculate null points for a known geometry. This is particularly useful if you need to match the stock alignment of your turntable or if you would like to experiment. Note that when choosing this option, the groove specification setting will make no difference.

Make your choices, calculate the specifications of your protractor or generate one right away by clicking on the buttons and print out the arc protractor template at 100 % scale (to be set in the printing dialogue). When printed, the outer square should measure exactly 190 x 190 mm. If it does not, it will be necessary to adjust the printer correction factors accordingly.

Please note that each combination of null points and alignment method will result in a different overhang and offset angle. This is normal. The manufacturer recommended overhang and offset angle can be ignored.

You may also choose "Save as PDF" in the printing dialogue instead of a printer to download and save your custom phono cartridge alignment protractor to your hard drive. To save your alignment template for future use, you may simply save a bookmark. All the parameters of your arc protractor are stored in the URL. To share your protractor with others, we recommend sharing the link to the calculations instead of the protractor link, as this will allow for tweaking the printer scale settings.

To protect your stylus, printing the template onto glossy photo paper is recommended. You may also stick pieces of clear parcel tape to all points on which you lower your stylus, or laminate the alignment protractor.

Now cut or punch out the spindle hole as accurately as you can. Some prefer to perforate the outline of the spindle hole with a needle and then punch out the center. Cut off the excess paper along the dotted line and place the protractor on your turntable.

Decrease the vertical tracking force and set anti-skating to zero.

First, the overhang is set by adjusting the cartridge position so that the stylus tip traces the arc on the template perfectly: Set the stylus on the arc line near the edge of the record. Then lift it and set it down close to the spindle. If it is on the arc, the overhang is set right. If it is short of the arc, overhang needs to be increased. If it is beyond the arc, decrease overhang.

If the slots in your headshell do not allow for setting the correct overhang, you will have to choose a different alignment with an overhang spec that is closer to the stock alignment of your turntable.

If it turns out to be impossible for the stylus to trace the arc, your measurement of the pivot to spindle distance or the print scale may be off.

Once the overhang is set, the protractor must not be moved anymore. Set the offset angle by aligning the cartridge body or cantilever to either grid. The stylus tip needs be lowered exactly to the center of the grid for a good result.

When aligning the cartridge body, greater accuracy can be achieved by taping a pencil lead to a flat front surface of the body.

When aligning the cantilever, it is important to look at the cartridge in a straight line, to avoid parallax error. This is easiest to see from a low angle.

A magnifying glass or phone camera can be of great help here. Set up enough lights to eliminate any shadows under the cartridge. Activate the grid in your camera app and make sure the horizontal lines of the protractor grid are parallel with it. The vertical line, to which the cantilever is aligned, should be parallel with the vertical grid lines.

This is what you want to see:

Alignment of the phono cartridge stylus on a protractor grid

Please note that the cantilever may deflect when a highly compliant cartridge is lowered onto the protractor, especially if the surface is rough and the stylus cannot move on the protractor. Lowering the vertical tracking force should pretty much elimimnate this issue. If it does not, it is best to keep the stylus just above the protractor and to not lower it down or to align the cartridge body instead of the cantilever to the grid.

When you are happy with the alignment, you may double check on the other grid. Make sure that the overhang is still set correctly.

Do not forget to set the vertical tracking force and anti-skate back to the correct weight when you are done.