Its very simple.
Ok depending on how you track your stuff can really determine on what control you have for each individual track.
soooo... lets say you track your drums into a completly different mixer. then take that mixer and send it to one track on the main mixer. now apply a Mclass master to the drums mixer. this will apply only to the drumz. now you make your song and can add another Mclass to the total mix and basically do this to any track you want.
now here is how you play with it. Dont forget to pan your shit because compressed tracks sound like shit if the whole song is in the center of the mix soo pan everything that needs to be.
start with one track. for example.. the drumz. take the Mclass and find the appropriate setting for the drumz, lets say Hip Hop.
now open the show devices tab on the Combinator and start playing with the mix, make sure that the mix for the drumz alone sounds nice and tight and if needed add another compressor or EQ or Whatever to make you happy
then just repeat the process to whatever tracks you feel need the Compression or just some brightness.
basically the Mclass is somthing everyone should learn on their own to find their own sound.
Hope this Helps
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