The zapco ST-4B 4-channel bridgeable 2 to 6 channels, amp brings the sound quality and reliability that built the zapco legend; and an amplifier that everyone can afford. The extruded aluminum chassis is compact to fit today's smaller cars. The feature set provides versatility including built in crossovers, bass boost and preamp outputs. The ST-4B amplifier is misfit powered, uses class nab designed hi current bi-polar outputs, pass through preamp outs, and high voltage speaker level inputs. Th.D. noise: <0.03%, signal to noise ratio:>82Db, stereo separation:>45Db.