Here is one: **broken link removed**
Costs ~200 USD. Cheaper to buy than build your own.
If you are looking for a simple solution to a simple problem, you have to give a lot more information. Do you need to control positon/speed/torque? What is the resolution of your encoder? What kind of DC motor you have? What kind of gear you have? How accurate and how fast the control needs to be? Is it a dynamic (servo) control or a static regulation (like vinyl record player) problem? What is your budget?
It is possible to make a good control with relatively simple electronics, but we need to know all the details to be able to help you.