The sidechain offers a a low cut filter to reduce pumping of the signal by the low frequency content, a hard / soft knee switch and a slave function for stereo linking. Two Expressors can be linked using a jack-to-jack cable. The sidechain send & return are on jack sockets as well. The HFX system gives high-frequency expansion to reduce dullness when using large amounts of compression and the amount & frequency of the expansion are adjustable

In use this is highly flexible, responsive compressor that sounds great. Control calibration is excellent and it’s easy to get the kind of compression you want, from soft, slow gain-riding to pumping drums. It remind me a little of the UREI 7110 – both are underdogs that are unjustly ignored. I don’t believe Aphex made a stereo Expressor but that would have been quite a piece of kit. If you find a pair of these 651s they can be stereo linked with a jack cable