AP Series Power Amplifiers High efficiency, updated circuit design, front-to-back cooling, Speakon™ and binding post outputs, ¼” & XLR inputs, user defeatable limiter, high pass filter, separate power switch and breaker & soft turn-on circuit (AP4020/4040). Yorkville's continuing research in new high efficiency designs sets new standards for quality, reliability and performance in sound reinforcement. AP-Series amplifiers deliver more power and weigh less, thus representing a significant advance in analog power amplifier design.