The Yamaha SPX90 is a 16-bit digital multi-effect processor introduced in 1985. It operates at a 31.25kHz sampling rate and its frequency response is 20 Hz to 12 kHz. The SPX90II (1986) has double the memory, allowing for longer delay times.
The SPX90 is most famous for the "Symphonic" chorus algorithm (also found in the Yamaha REV series effect processors). While its reverbs are known to have a metallic sound, the "Ambience" and "Pitch Change" algorithms are great for adding air and width to mono signals. The SPX90 has been used by Brian Eno, Depeche Mode, Howard Jones, Steve Vai, and Thomas Dolby among others.
The SPX90 can be upgraded by Jim Williams to reduce its noise floor:
Syntaur carries replacement parts for the Yamaha SPX90.