An experimental investigation on active control of naturally occurring Tollmien-Schlichting (TS) instabilities on a NACA 0012 airfoil is presented. A real-time closed-loop control system has been implemented using the filtered-x Least Mean Squares (FXLMS) adaptive algorithm based on Finite Impulse Response (FIR) filters. Two surface mounted microphones provide the unsteady surface pressure measurements, which are used as reference and error signal, respectively. The controller directly drives a Dielectric Barrier Discharge plasma actuator positioned on the suction side of the airfoil. In contrast to the conventionally modulated sinusoidal signal, the actuator is powered using a signal continuously adapted in real time by the controller. In this manner, the inherently unsteady plasma force production is capitalised in order to dampen the incoming wave-train of Tollmien-Schlichting instabilities. Time-resolved two-component Particle Image Velocimetry is used to characterize the flow in the vicinity of the actuator. Several cases are tested using both open- and closed-loop actuation. In the case of open-loop control, the actuator is operated with a non-adapted single-frequency sinusoidal signal. Freestream velocity ranges from 17 to 25 m/s, equivalent to a chord based Reynolds numbers between 0.2 to 0.35 × 10⁶ respectively. The experimental results indicate a suppression of the discrete component of the unstable TS waves with closed-loop actuation. A maximum amplitude reduction of approximately 50% is achieved at low velocities. In contrast, open-loop control appears to accentuate the growth of discrete instability modes when the frequency of actuation is in the instability region for the given conditions. An additional stretching effect is observed on the TS waves for both open- and closed-loop control as a result of the inherently directional plasma body force.