If your coffee tastes sour, the first thing to check is your grind size. Siphon coffee needs a medium-fine grind, something like table salt. If you're using pre-ground coffee or a coarse grind, the water flows through too fast and doesn't extract enough.

If you have a burr grinder, set it a notch or two finer than you would for a pour-over. If you don't have a grinder, get one—it makes a bigger difference than any other upgrade.

Try that first. It solves most sourness issues.