This form generates precision arc protractors for aligning phono cartridges on turntables with pivoting tonearms. Simply measure your turntable's pivot-to-spindle distance, enter the value and any other desired settings in the form, click "Generate Protractor", and print your custom protractor PDF at 100% scale.
How It Works
Based on your tonearm geometry and chosen alignment method, the generator creates a PDF with an arc matching your stylus path and alignment grids precisely positioned at the null points (where tracking error = 0). The protractor allows you to set both the overhang (the distance the stylus extends beyond the spindle) and offset angle (cartridge rotation) simultaneously by aligning the stylus with the arc and grid lines at both null points.
How to Use the Protractor
- Place the Protractor: Centre the protractor on your turntable spindle, ensuring it's the correct one for your tonearm's pivot-to-spindle distance.
- Loosen Cartridge: Slightly loosen the cartridge mounting screws so the cartridge can slide forward and backward in the headshell.
- Align at Outer Null Point:
- Lower the stylus onto the outer alignment grid
- Adjust the cartridge's forward/backward position (overhang) so the stylus tip sits precisely on the arc
- Simultaneously adjust the cartridge's angle so its body or cantilever is parallel to the grid lines
- Check Inner Null Point:
- Move the tonearm to the inner alignment grid without moving the protractor
- If the stylus doesn't align perfectly with the arc and grid, adjust the cartridge position again
- Iterate: Return to the outer point and recheck alignment, then back to the inner point, repeating until the stylus tracks the arc and aligns with both grids perfectly.
- Tighten: Once aligned at both points, carefully tighten the mounting screws without shifting the cartridge position.
Printing Instructions
- Print the PDF at 100% scale (NO scaling/fit-to-page)
- Verify the 100mm scale bar measures exactly 100mm with a ruler
- Use the protractor to align your cartridge following the steps above
Alignment Geometries
Baerwald (Löfgren A) - Default
- Minimises maximum tracking error
- Best for general-purpose listening
- Null points: 66.04mm, 120.90mm (for IEC standard grooves)
Löfgren B
- Minimises RMS (average) tracking error
- Best for full-side listening
- Null points: 70.29mm, 116.60mm (for IEC standard grooves)
Stevenson
- Minimises tracking error at inner grooves
- Best for records with long inner groove sections
- Null points: 60.33mm, 117.42mm (for IEC standard grooves)
Alignment for Other Record Formats
The form provides preset groove standards and allows custom values for specialized formats.
Standard Groove Specifications
IEC/RIAA (60.325-146.05mm) - International standard used for most modern 12" LPs manufactured worldwide.
DIN (57.5-146.05mm) - German standard with smaller inner groove radius, historically used by some European manufacturers.
About Custom Groove Radii
The form provides preset groove standards for convenience. For these standard values, the protractor uses precisely optimized null points derived from published alignment standards.
When you enter custom groove dimensions (different from any preset), the generator approximates optimal null points by scaling from the nearest standard. This scaling preserves the distortion balance characteristics of each alignment geometry—the mathematical relationship between null point placement and tracking error distribution.
Common Custom Format Examples
Select "Custom" groove standard to enter these values:
7" 45 RPM Singles
Typical groove radii: 57-58mm (inner) to 88-90mm (outer). These use the same groove specifications as LPs but with smaller dimensions due to the record size.
10" 78 RPM Records
Typical groove radii: 60-62mm (inner) to 117-120mm (outer). For single-track recordings (one song per side), Löfgren B alignment is recommended as it minimizes average tracking error across the entire side.
12" 78 RPM Records
Typical groove radii: 60-65mm (inner) to 135-145mm (outer). These were typically used for classical music and orchestral recordings requiring longer playing time.
Note: 78 RPM records pre-date standardization and groove radii varied by manufacturer and era. The values above are typical approximations.
Example Tonearm Mounting Distances
9" tonearms:
- SME 3009 S2 Improved (early): 215.7 mm
- SME 3009 S2 Improved (late): 213.25 mm
- Technics SL-1200: 215.0 mm
- Rega RB300: 222.0 mm
- Audio-Technica AT-LP120: 215.0 mm
- Pro-Ject 8.6" tonearms: 200.0 mm
12" tonearms:
- SME M2-12R: 295.6 mm
Python Script
This web form uses the open-source arc protractor generator Python script. The script is available for download if you prefer to run it locally from the command line:
