Compliance in robot actuation provides a solution to perform safe physical human-robot interaction. Conventional compliant actuators (variable stiffness actuators, series elastic actuators) used more than two motors or closed-loop controller to modulate both stiffness and equilibrium position independently. These actuators are complex, lack of energy efficiency, and have limited stiffness range. In conjunction with an active, positive stiffness modulation, implementing a passive negative stiffness element enabled a compact design of the compliant actuator. This paper suggests a power transmission design of fast and energy-efficient stiffness modulation based on this new compliant actuator concept. First, the double slider-crank mechanism made fast stiffness modulation and high energy-efficiency. Second, positioning the leaf spring’s bending location to the center also enabled the fast stiffness modulation speed and wide range stiffness modulation. Third, optimized elliptical cam with compression spring generated negative stiffness in output. We provide theoretical modeling of each mechanical drivetrains and characterization of positive stiffness modulation (range and speed) and negative stiffness with corresponding power consumption experimentally.