An absolutely small headphone amp, based on a single Opamp - 4580. This one I tried to fit
into the smallest box I could find, and yeat fit in a mains transformer and all that jazz.
The darn bastard didn't want to work at first but I noticed that the print on the PCB was a bit blurry why the markings for the 78L12 and 79L12 got mixed. Swapping the regulators made the trick.