In a previous work, we presented an analysis and design of voltage and current amplifiers for use in protective relay testing for frequencies up to the 20th harmonic. Beyond this limit, undesired phase and magnitude deviations occur due to the sampling rate, which defines the working frequency range of the amplifiers. To obtain a broader range of protective types of testing is desirable higher signal frequencies. For the current state-of-art of the output IGBT H-bridge, it is not possible to increase the sampling rate. In the present paper, we propose frequency compensation using an LMS-based adaptive FIR filter between the controller and the reference signal, to correct the amplitude and phase distortions. Theoretical and numerical investigations indicate the proposed method can extend the working range, up to the 50th harmonic.