This paper presents a new voice conversion algorithm. This algorithm allows voices to be adapted using a small amount of adaptation data. Only a few short adaptation units (phonemes or short words) are needed. The voice conversion is performed using a time-variant digital filter, topological feature maps and a map of filter coefficients. The filter coefficients of the time-variant filter are selected by the feature map dependent on the short-time spectrum. The spectral envelope of the input signal is modified by a time-variant filter using the selected coefficients.