Some say it’s the mic pre-amp. Others might say the microphone or the speakers. When you get right down to it the most important piece of studio gear is your EARS. Hearing the tones, the balance of the mix and the arrangement beats all the other pieces of gear in your studio hands down. If you can’t hear the tones or interpret what you are hearing all else is moot. So, start to train your hearing early. Musicians learn to hear differences in pitch. If they can’t, how can they tune their instrument? Once that is learned then they begin to hear the balance between themselves and the other musicians. Audio engineer engineers start at this point. The engineer needs to listen to the individual instruments and make them sound real, or not, through the speakers. Balance or mix those instruments according to an ideal. Whether it is the producer, the band, the musician, the genre/style, the music public or themselves.
How do you go about training your brain to hear all this? Start by listening to everything! How loud is the traffic compared to the bird singing, what about that buzz or oscillation you hear when walking into a warehouse or office building? When you go to a music event listen to the mix coming out of the speakers. Can you also hear the sound coming from the onstage monitoring system? What about the reflections of the sound system bouncing off the back wall? Like I said everything.
Soon you will be able to know what frequencies affect what instruments and be better able to make adjustments to the audio that you are hearing inside your mind.
There are several training systems out there to help you along the way. My favorite is Dave Moulton’s Golden Ears system. Do a search on the web, you’ll find it.
One more thing. Take care of your hearing.
It is the most important asset you have.