A filter element in a fuel filter has an annularly arranged filter medium that is sealed at the end faces by end plates wherein a venting unit for venting gases out of the filter element area is provided. The systems recover gas andor heat from metal melting andor smelting processes used in the manufacturing andor refining of metals andor their by-products. The improved differential inverter further comprises a controlled bias generator that generates the second vector of input signals in response to a bias control signal.