How to use: Measure the distance from your tonearm pivot to the spindle centre. Print the PDF at exactly 100% scale — verify with the 100mm scale bar before use. The two alignment grids show where your cartridge stylus should sit with zero tracking error.
What is an arc protractor?
An arc protractor is a printed template for aligning a phono cartridge. It shows the arc the stylus travels along as the tonearm swings across the record, plus two alignment grids at the null points: the two radii where the cartridge sits exactly tangent to the groove and tracking error is zero. Because it is calculated for your tonearm's pivot-to-spindle distance, it sets both overhang and offset angle for your specific arm, unlike a generic one-size protractor.
How to use it
- Find your pivot-to-spindle distance. Choose your tonearm from the presets, check the manufacturer's specification, or measure from the centre of the tonearm pivot to the centre of the spindle.
- Choose an alignment geometry (see below), then generate and print the PDF at exactly 100% scale. Check that the scale bar measures 100mm.
- Cut out the spindle hole and place the protractor on the platter over the spindle.
- Set overhang: slide the cartridge in the headshell until the stylus tip follows the printed arc as you swing the arm across it.
- Set offset: lower the stylus onto each null-point grid in turn, and twist the cartridge until its body and cantilever line up with the grid lines at both points.
- Tighten the screws and check both null points again, since tightening can shift the cartridge slightly.
Which alignment geometry?
All three geometries place two null points where distortion is zero. They differ in where those points sit, which trades distortion in one part of the record against another:
- Baerwald (Löfgren A): keeps the peak tracking distortion equally low at the start, middle and end of the side. The most widely used choice and a good default.
- Löfgren B: gives the lowest average distortion across the whole side, at the cost of slightly higher distortion at the outer and inner edges.
- Stevenson: places the inner null point at the very last groove, giving the lowest distortion at the end of a side, where inner-groove distortion is most audible, at the cost of more distortion elsewhere.
The IEC and DIN groove standards set the innermost and outermost groove radii the geometry is calculated for. IEC is the usual choice; DIN allows for records that run closer to the label.