AC Adaptors (or "wall warts" as they are often called) are usually unregulated. You usually need to run the power from these through a voltage regulator to get the precise voltage you want. You will also need a few filter capacitors as well to ensure a clean, consistent supply. Depending on what your circuit is, you may need to attach a heat sink to the voltage regulator. If you provide a schematic or more details of your circuit, we can recommend a voltage regulator that would suit your needs.