C core is made of Hi-B oriented silicon steel strip. C core is assemabled by 2 U shape cores, and there is gap between two U shape cores. C core is also called cut core, because it looks like a rounded rectangle cutted at the center. Air gap is not always bad for transformers. Sometimes it does good. C core is good at making choke and audio output transformer, because of it's adjustable air gap. Certain air gap can reduce the flux saturation. Easily assembled Prepare 2 coils and insert the core. That is simple. There is no need to insert EI laminations one by one like EI transformer. Air gap adjustable The air gap in C core can be adjusted as you wish to make different applications.It is suitable to make choke and audio output transformer.