An electrical discharge machine for applying a pulsed voltage to a machining gap between an electrode and a workpiece disposed opposite thereto. An insulating dielectric fluid, comprising an oil or water with compound additive, is disposed in the gap. A positive polarity voltage and an opposite polarity voltage are applied via switches and respective impedances to the machining gap in order to generate a discharge for machining the workpiece. A high frequency alternating current voltage is applied between the electrode and the workpiece when the first polarity and second polarity voltages are not being applied.