A power take-off device for use in a wave energy converter comprises a buoy (1) and a means (2, 3) for converting linear motion from movements of the buoy (1) into rotary motion comprising an elongated means (2) and rotary devices (3) interacting with the elongated means, wherein whereby the rotary devices (3) in the power take-off device preferably are flexibly suspended by wire (33) in support structure (30a) fixed with the buoy (1). By providing means (7a) for applying an essentially constant pre-tension force on the buoy (1), connected to a rotary device (3), and means (7b) for applying a variable damping force on the buoy (1), connected to a rotary device (3), wherein the variable damping force is superimposed on the pre-tension force, whereby, during operation, a unidirectional torque is applied on the rotary devices (3) by the combination of the means (7a) and the means (7b), to combine control of the resonance frequency of the buoy as well as control of the damping to extract power, with the purpose to increase the power capture and reduce design complexity. Optionally, means (6) are connected to a rotary device (3) for changing the effective mass of buoy (1) to control the resonance frequency.