Older CPUs and logic devices on the motherboard were designed for 5 V operating voltage. Power supplies for those computers regulate the 5 V output precisely, and supply the 12 V rail in a specified voltage window depending on the load ratio of both rails. The +12 V supply was used for computer fan motors, disk drive motors and serial interfaces (which also used the −12 V supply). A further use of the 12 V came with the sound cards, using linear chip audio power amplifiers, sometimes filtered by a 9 V linear regulator on the card to cut the noise of the motors.