TC – It could stand for taut control. But Joe Skubinksi of Abyss Headphones has it as total conciousness. With this suffix to their AB-1266 Phi, we get a "newly designed 66mm planarmagnetic driver of the absolute highest resolution of any headphone ever made". The aluminium frame and lambskin ear pads have been revised as well and the finishing is a two-tone black polymer ceramic.