The Primary Source Expander from Waves is an expander/ducker plug-in designed to provide a simple and efficient interface for live or live-in-the-studio use. By default, it works as an expander, to reduce background bleed and noise when a source is not playing. Alternatively, it can be used as a ducker, to reduce the level of a source when a different signal is playing.

The Primary Source Expander features a sidechain that can be triggered internally or externally. The sidechain offers filters with frequency controls, plus a tweakable delay for the sidechain input. Threshold and range values are adjustable with large, centrally located controls. Meters for gain reduction and sidechain input levels are also in the center of the plug-in window. Three presets for release times are available: Slow (approximately 500 ms), Medium (approximately 250 ms), and Fast (approximately 100 ms).