A new principle for an all digital preamplifier and equalizer, to be used together with a compact disc player, is described. The principle makes it possible to obtain an arbitrary gain transfer function together with a linear phase. The gain can be varied 20 dB from point to point, when specified on a logarithmic frequency axis with 30 divisions from 20 Hz to 20 kHz. The deviation in the passbands is max. 0. 2 dB. Taking advantage of the digital signal from the preamplifier, a high-efficiency power amplifier can be developed. A prototype of the preamplifier built with commercially obtainable components has shown promising results.