The early, primary technology used to distinguish Boulder from other audio manufacturers was the use of the 990 discrete gain stage, based on the original work of Deane Jensen.
In subsequent years, Boulder Amplifiers have shifted their focus from studio electronics to high-end audio electronics utilizing an updated third- and fourth-generation variant of the 990 gain stage and are primarily known for creating products with no limits on retail price, extremely complex engineering, and chassis design which doubles as industrial art.
- Fremer, Michael. Boulder 2008 phono preamplifier & 2010 preamplifier in Stereophile, July, 2002
- Fritz, Jeff. Boulder Amplifiers 1060 Stereo Amplifier and 1010 Preamplifier in Soundstage!, January, 2005