Sourness can also come from water that's too hard or too soft. If your tap water is heavily mineralized or tastes flat, try filtered water. Temperature matters too—if you're at high altitude, water boils at a lower temperature, so you may need to use just-off-boil water rather than letting it cool.

Another possibility: your beans. Light roasts are naturally more acidic, and if they're too fresh (less than a week off roast), they can be hard to extract. Try a slightly darker roast or let your beans rest a few more days.