An active damping system assembly is configured to dampen a vibration of at least part of a structure. The assembly includes a plurality of active dampers each including a sensor configured to measure a position quantity of an interface mass mounted on the structure; and an actuator configured to exert a force on the interface mass in dependency of a signal provided by the sensor, wherein each of the plurality of active damping systems is connected to the interface mass. The structure may be a projection system of a lithographic apparatus.