A three stage power source for an electric arc welding process comprising an input stage having an AC input and a first DC output signal; a second stage in the form of an unregulated DC to DC converter having an input connected to the first DC output signal, a network of switches switched at a high frequency with a given duty cycle to convert the input into a first internal AC signal, an isolation transformer with a primary winding driven by the first internal high frequency AC signal and a secondary winding for creating a second internal high frequency AC signal and a rectifier to convert the second internal AC signal into a second DC output signal of the second stage; with a magnitude related to the duty cycle of the switches and, a third stage to convert the second DC output signal to a welding output for welding wherein the input stage and the second stage are assembled into a first module and the third stage is assembled into a second module connectable to the first module.