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This paper proposes an adaptive robust output-feedback controller for the position-tracking problem of a magnetic levitation system with a current-feedback power amplifier. The system is governed by a single-input single-output second-order nonlinear differential equation which is different from the standard output-feedback form, since there is a position-dependent nonlinear uncertainty multiplied by the control input. Only the position measurement is available for control. The controller is designed by a backstepping procedure with a robustifying modification of the conventional K-filter approach. The boundedness and the guaranteed transient performance of the error signals are achieved by the nonlinear damping terms, and the ultimate position-tracking error is reduced by the adaptive laws. Experimental results are included to show the excellent control performance of the designed controller.