For the manufacture of our transformer core coils t84 to t150 we use nothing but FERON which is extremely low in losses and distortions. This material is a grain-oriented silicon iron with a sheet thickness of 0.35 mm. This high quality initial material is subsequently heat treated in a special process. In this way, distortions in the material's crystal lattice structure, caused by rolling and stamping, are eliminated. In this way, FERON is provided with a crystal lattice structure which displays optimum magnetic properties. Such a core material is used for the manufacture of high quality audio transformers. It is very seldom used in "normal" transformers, except where very low power loss is important. In this construction type, coils can be manufactured with very low internal resistances and extreme load-bearing capacity. These are the preconditions for a precise deep bass. The ratios of inductivity to internal resistance achieved here can often not even be realised with pure air cored coils, or only at considerably greater financial cost. Vacuum impregnation, described in detail at the end of the chapter "Zero Ohm Coils", can be recommended without reservation as a supplementary treatment for transformer core coils to enhance their high quality.