If your photo or scan of a sketch with clear lines is unevenly lighted, just applying Levels will never get you the desired result. It'll always be a compromise between noise, leftover shading and losing lines.
First, pick a color channel with the least noise. Blue usually has the lowest quality. Green is often the best. Copy the color channel into a new document.
Then follow these steps (numbers are provided for your image, adjust as needed):
- Duplicate layer — "paper" layer
- Median (2 pixels) — to remove small noise, depends on image resolution and noise
- Maximum (10 pixels) — to remove lines, adjust until you see almost all lines disappear
- If there're some shaded areas, use Spot Healing Brush to clean up
- Gaussian Blue (15 pixels) — to clean up circles/squares, range bigger than used in Maximum
- Invert colors, then set layer blending mode to Liner Dodge (Add) — to subtract the paper from the source
- Levels (65 to 235) — to remove noise and increase contrast
Notice that there's no paper texture left, unlike in your result, and no lines became too light, unlike in KJP's result.