A car's graphic equalizer is used for the stereo. The equalizer helps you achieve a more pleasant-sounding experience from your speakers.

Clarion makes a car graphic equalizer for under $100. You can find them at www.nextag.com/graphic-equalizer/products-html and volumestereos.com/store/product_info.php?products_id=61

You need to be qualified in Electronics, then you must find an schematic for that specific equalizer.

The Pioneer DEQ-7600 is a Digital signal processor with EQ and spectrum analyzer

A computer graphic equalizer calculates the graphic equalizer settings on your computer. After, it gives the most realistic playback of sound, compensating for both your sound system and the acoustics of the room