A couple things I wanted to share: Whilst perusing around the treasure trove of info that is SOS, I came across a great article discussing compression circuits, and how to apply this knowledge with Logic's default to compressor to aid in emulating the behavior of certain hardware synths. And here's an article that discusses compression in general. I know there's a million of these "compression explained" articles in existence on the interwebz, but I found this one to be particularly interesting (apologies up front if this has been posted previously). Finally, if you're looking for a quick and dirty set of definitions for Logic's compressor circuit types, look below. Note: The descriptions below were pulled from the previously mentioned SOS article and have been placed here for educational purposes only. Logic Pro Circuit Types These are based on the electronics that different compressor types use to perform gain reduction. VCA. Uses a Voltage Controlled Amplifier. Known for their fast gain‑reduction abilities, examples include SSL’s famous bus compressor and the Dbx 160. FET. Uses Field Effect Transistors. Compressors based on these designs have a ‘valvey’ sound, but are also capable of pretty fast response times. Examples include the Universal Audio/UREI 1176. Opto. Uses a lamp and photoresistor. By their nature, optical compressors react quite slowly to transients, which can be a good thing in some cases! Examples include Teletronix’s LA2A and the Joe Meek/Ted Fletcher designs. Platinum. This is Logic Pro’s original compressor ‘model’ and it can still be useful in some situations, as it has a fairly transparent quality. ClassA_R & ClassA_U. Quite what these emulations are based on is anyone’s guess, but the names suggest variable ‘mu’ devices combined with Class-A amplification, similar to devices from Manley Labs.