They are commonly referred to as the 'holy grail' of outboard gear. The rarity of Fairchild compressors has made them highly desirable and very valuable, with a used 660 now selling for $20,000 or more and a 670 selling for $30-$40,000 or more. The Fairchild 670, introduced shortly after the 660, is a dual-channel version. The 660 was the first intelligent automatic volume control limiter.
The Fairchild 660 is a tube-based single-channel audio compressor invented by Rein Narma and manufactured by the Fairchild Recording Equipment Corporation beginning in 1959. Fairchild Recording Equipment Corporation