When a system contains two or more pieces of equipment that are grounded, whether via power cords or other ground connections, a “ground loop” will likely be formed.

This post from prosoundweb.com explains what they are and how to get rid of them.

http://www.prosoundweb.com/article/pesky_ground_loop_problems …

The proper use of EQ is a fundamentally simple yet often a misunderstood and critical aspect of the mixing process.

This article from prosoundweb.com runs us through the different types of equalizerrs and how they can be used.

http://www.prosoundweb.com/article/improving_your_sound_with_eq