Guitars: This is a very full-frequency instrument, whether it's acoustic or electric. Guitars should be clear, full, warm, tight, big... you name it. Be creative. listen, adjust, experiment and compare to commercial cds. Think wide mids from 150 to 400hz, but don't let it get boxy. If it sounds boxy, cut from 250 to 500hz. Add mids from 900hz to 3.5k for clarity and bite. Keep it smooth (and good mic pre's really help). Easy on the top end but add if it makes a subtle difference. Don't over compress the guitars in tracking - lean more toward doing that in the mix. Again, the clunker strings have got to go, unless you're going for that 50's Guild sound.
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