The increasing use of power electronics devices tends to aggravate high-frequency harmonics and trigger resonances across a wide frequency range into power systems. This paper presents an active damper to suppress multiple resonances with unknown frequencies. The active damper is realized by a high-bandwidth power converter that can selectively dampen out the wideband resonances. A cascaded adaptive notch filter structure is proposed to detect the frequencies of resonances, which makes the active damper different from the resistive-active power filter for harmonic resonance suppression. The performance of the active damper is validated by implementing it to suppress the resonances in a grid-connected inverter with a long power cable. The results show that the active damper can become a promising approach to stabilizing the future power electronics based power systems.