We proposed and analyze the use of adaptive feed-forward controllers based on discrete-time FIR filters to enhance the performance of voltage and current class-D amplifiers to be used in protective relay testing. The feedforward controller compensates for inherent frequency response distortions, allowing the compound system to conduct tests with signal components from DC up to the 50th harmonic. We investigate the use of both, the LMS and the RLS algorithms to estimate the parameters of the feedforward controller for different relay burdens. Results concerning convergence rates of the controller parameters and temporal response for ATP-like transient waveforms for use in protective relay tests are presented.