I picked up one of these up a few days ago. It was on consignment at a local shop. Since it's missing from Bongomania's list and a search finds no practical other info about it, here's a short review. It is a 2-channel compressor. Each channel has one of: compressor/limiter, gate, expander, and duck, which is used for an external side chain. The channels can be chained by simply adding a patch cord from the output of channel 1 to the input of channel 2 which will allow you to use 2 effects. Each channel has threshold, ratio, attack, and release controls. There are no input or makeup gain controls. It's an old unit with a toggle switch and, without opening it up, I assume it has a Valley People VCA. I ran it through a MarkBass rig (LMII, 102P, 151HR). The best sound was in the parallel effects loop, blending it with the clean sound. Overall, it sound very good. There is no loss of low end, and it seems to color the low end in a good way. There is an slight audible artifact when the VCA kicks in. I almost returned it when I noticed it but at medium or loud volume, it's not very noticeable and actually began to like what it adds to the sound when I dig in hard on the strings. I haven't gigged with it yet and don't know if I will yet because a Rane DC24 is on its way here that I'm anxious to try. Attempting to minimize the artifacts, I tried running it from an Avalon U5 and a Sansamp RBI as preamps directly into the LMII, thinking the input impedance or volume was the problem, but that had no effect on the artifacts. Here is a partial summary of the difference between it and the 501 (from http://www.audioproz.com/AP.php?Article=31&Tab=Educational). There is more interesting info/opinion there, btw. "These compressors are almost the same but the 501 will have more adjustment controls and range over the 522. Although sharing the same VCA, the 522 does not have as sophisticated RMS circuit (which looks at the incoming audio). The better audio detection circuit in the 501 (often call somewhat erroneously RMS detector) can allow a wider range of dynamics to be recognized and more carefully tracked or "voiced" to control the VCA, (voltage control amplifier, which affects compression of audio dynamics). The 522 main claim to use is its versatility to duck, noise gate and other VCA acrobatics. Both units are the older style compression techniques like Drawmer, Ashley, Orange County, etc., but personally I think they did it better. Other companies priced their products and marketed More! More! More!"