Adjusting treble and bass

[Lemon Herb]

I realize this has nothing to do with synths, but this is the only list I
could find who might have people on it that know what I'm trying to do.
I want to build a simple mixer (4 track.)  I'm mostly doing this to just
learn some of the basics of electronics.  Anyway, I want to be able to
adjust the treble and bass on each track.  Can anyone tell me how to do
this?

thanks,
corey