(Image: Bottlehead)

The Bottlehead Crack amplifier. 

“This is what we call a Skill Level 1 kit, perfect for first time builders. The circuit is so simple you can build it in an evening or two. A simple circuit also means a low parts count, which keeps the price low, too. Crack is an output transformerless amp designed specifically for running higher impedance (100 ohms or higher) headphones like the many Sennheiser, AKG and Beyer designs.”

From Bottlehead