Harmonics generated by converters have become a serious problem for the AC mains as they can only be removed by using bulky passive filters. Besides, the harmonic compensation by shunt passive filters may cause undesirable interferences such as resonance with the AC mains.. To solve this problem some authors have proposed a combined system of a shunt passive filter and a series active filter. This system is self-adaptable and only small power active filter is needed. The choice of the control algorithm, together with the AC mains and non-linear load parameters determine its performances. In this paper the compensation characteristics of this system are studied. The filter control system with some peculiar features is detailed, simulation and experimental results obtained by a single-phase prototype are presented.