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A simple control strategy was studied for harmonic disturbance rejection in magnetic field compensation systems for low-field magnetic resonance techniques. The strategy is based on the simultaneous action of a conventional PID and a selective harmonic-compensation controllers. The system consists of a set of compensating coils fed by independent current sources driven by a digital controller. A series of hall magnetic sensors close the control loop. Despite its simplicity, it is shown that the performance of the dual controller improves within the frequency range where the waterbed effect becomes dominant, by selectively enhancing the rejection of the harmonic component. The proposed solution is particularly useful for selective harmonic rejection of slowly varying frequency and amplitude dependent harmonic perturbations. An extension to multiple-harmonic components perturbations is possible.