High quality Kester 63/37 rosin core solder was used throughout. This amp is very well-built and sounds great. I know electronics and can easily tell the difference between a good solder joint and a cold one. and M.S.) in electrical engineering and experimental physics and in the early part of my career I spent some years doing mainly electronic work (electronic design, assembly, testing, and software design) in the research and development department of an instrumentation company in Sweden. It also works very well with many Beyerdynamic and AKG headphones.Īs for my credentials, I have degrees (B.S. Headphone aficionados, the Crack pairs extremely well with Sennheiser HD 600, HD 650, and HD 800 (or HD 800 S) headphones. The Crack is an OTL (output transformer-less) tube headphone amplifier designed for use with higher impedance (>200 ohms) headphones. I am offering for sale a new Bottlehead Crack 1.1 Tube Headphone Amplifier with the Speedball Upgrade.