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.