The White 2A Leveling Amplifier is based on a legendary vintage unit that is entirely tube-based. It's a totally different device in terms of construction where all the compression magic happens inside an optically coupled element formed by a fluorescent panel and some photocells: the famed T4A element. There is no electronic circuitry involved with the compression itself. It's just a tube amp with photo-resistors, lighted by a fluorescent panel driven by the output signal. At the time of this invention, there were not many ways of making an audio compressor: only variable-mu and optical. Optical was the simplest one, and if proper elements for both the light emitting panel and the photocells were matched, magic happened.
Originally intended for use in broadcasting, it immediately became a truly signature unit for music recording and mixing. It's a must-have for both tracking and mixing vocals, but it's also tremendously effective on electric bass, upright bass, piano, winds and many other tracks where you need a gentle compression with great warmth and consistency. This is one of the few units in the audio industry that nobody will allow to be substituted with anything else.