Softube has developed DSP-card and software for simulation of the sound of old tube amplifiers. The purpose with this thesis is to design a compact amplifier with high power to function together with the DSP-card and build a unit that can replace several other guitar amplifiers. The advantages with that kind of solution could be that it would be cheaper, easier to carry and easier to stor than the real amplifiers. The amplifier that has been designed is of class D type with digital PWM modulator. There is several types of amplifier modules on the market that use analog input signal, but if one of them would be used a D-A converter is needed on the DSP card.