Acoustic Matrix Professional This now classic system features an onboard equalizing preamp, which installs in a small window cut in the side of the guitar. In addition to four bands of EQ, it also offers a phase switch for feedback control and a low battery indicator. A preset sub-bass control lets you choose between a Natural I voicing (deeper bass) and a Natural II voicing (less bass). (Dimensions - 2" x 3" - 52 mm x 80 mm)