(or, a tale of gain, faders and plugins output controls)
Once upon a time, mixing was something done on large and uber-expensive consoles, by people who had spent years learning the ropes.
These days are long gone.
Today everyone with a cheap computer and a $100 interface has more sonic power on their hands than most hardware consoles ever had. Which is overall a good thing.
But: all that power without the knowledge to use it, is at best useless and sometimes damaging. It also leaves lots of people with basic questions which are embarrassing to ask. The most basic of all is: what actually is mixing? In a little more detail, why do I have both faders, gain controls, plus plugin gain controls etc? When am I supposed to use one or the others? All too often I come across people who, in good faith, have no idea (or doesn't even make the question) on why you have faders and you have gain and whether or not it's ok that a plugin suddenly increase the track level by 3dB when turned on (it's not).
So let's fix that.
Mixing is about the faders
Let's get the cat out of the bag first: a mix is simply the relative position of the faders at any given time.
No more, no less.
"At any given time" because static mixes are the ones where you set the faders at the start and leave them there, while dynamic mixes are the ones where you use automation to move the faders a little during the mix, usually in order to bring out certain parts or move others to the background.
So you start with all faders at "0", and then you move them a little up and down to get the blend of tracks you want, then you print the result to tape/hard disk/whatever and that's your mix.
If you need or want to change some levels during the song, you move ("ride") the faders while the track is printing, and again the result is your mix (this time, it will be a dynamic mix).
Since people have usually ten fingers, if you want to move more than ten faders you may need the help of someone else - and since that is boring, automation is there to.. well, automate the moves so you don't need to use your fingers anymore.
In short, mixing = setting faders position.
All tracks begin with the same level, or must be brought there...
This idea is based on the fact that when you begin to mix, all well-recorded tracks should have pretty much the same overall level.
Yes, you read well.
In your mix, with all faders at 0, all tracks, or more precisely, all recorded microphones and/or virtual instruments should present pretty much the same level on the meters when played. With 24 bit digital recordings, that level should be in the ballpark of -18dBFS average and max -12dBFS peak (why these numbers? Read How DAWs destroy recordings to find out). So you start with the same signal level and then you proceed to mix by moving the faders, so that the loud bits gets louder and the quiet bits get quieter.
So how do we get all tracks at the same average level?
Make a proper recording...
The main way is to simply to make a proper recording: when you record, you set the preamplifier gain so that, when the instrument is played, you get the desired average and peak levels in your meters.
This works also for virtual instruments - it's just easier since with these when you can control average and peak level even more precisely and consistently.
This is by far the best way, and that's why a good mix always begins with good recordings.
...or fix a bad one...
The other way to do it is to correct a bad recording at the beginning of a mix.
It's a fact of life that sometime people record too hot or too quiet.
The biggest mistake of beginner (and not so beginner) recordists is that they track way too hot, under the completely mistaken impression that signals not peaking at 0 dbFS aren't good (again, see How DAWs destroy recordings if you want to know why).
...by using the channel gain knob...
Which is why every self-respecting DAW has a gain knob in every channel.
That knob can be used to increase or reduce the track's gain with all the faders still at zero - basically changing the original gain setting of the preamplifier.
In this case, before touching any faders you simply go thru every track and move the gain so that the meters hit the average/peak range you want.
...or placing a gain plugin in every track
If it so happens that your DAW does not have a virtual channel gain knob, no problem.
You can simply place a gain plugin at the first place in your inserts box, in every track, and use that to decrease (or increase) the gain of each track as needed.
In digital audio world, a gain operation is simply a multiplication, so as signal processing go, it's pretty cheap.
Note that there are complex plugins (for example analogue channel simulations of expensive consoles of the past, or some other supposedly magic old piece of hardware) which apparently can do the same thing - i.e. changing the output fader in the plugin changes the output level.
However, often it's not exactly the same.
Analogue console and effects (and as a consequence, many plugin effects which emulate them) were made in blessed times, when people understood recording level, and there weren't any hopeful souls trying to express their artistic aspiration with Ableton and some headphones.
So these consoles and effects were designed in the expectation that people knew how to record and, in particular, weren't smashing their preamps. And even if there was some naughty signal-smashing going on, analogue circuits tended to distort in a pleasant manner so it wasn't a big deal.
Nowadays, your nice SSL Channel Strip plugin does not have the preamp section, because by definition in a DAW when the signal reaches the first plugin, it is already recorded by an hardware preamp. That means that the emulation will expect the signal level to be in a reasonable range (the usual 18 average/max 12 dBFS peak at 24bit). In other words, if your signal was recorded too hot, feeding it to the plugin before jacking down its gain won't give you the expected results and can even made it worse (depending on how the plugin is designed). In contrast, the gain control in the DAW channel or a simple gain plugin as first thing in an insert are designed to attenuate too hot signals, so will work to bring it down with no issues whatsoever.
...and then you mix...
At this point, your faders are at 0 and all your tracks are leveling at -18dBFS average/12dBFS max peak, either by virtue of good recording skills or because you have judiciously adjusted the DAW gain knob in every track or placed a gain plugin in the insert slot of each one.
Now you can start moving the faders and determine the relative level of the various parts, or sum then up in buses (drum microphones, for example) and go on from there. In other words, now you can mix.
...but beware of plugins
However, we're not done with gain.
You may have experienced that if you insert a plugin, the level in that track immediately rises up.
Or often you won't notice the level change, only that the track sound much nicer when you put the plugin in the insert chain.
You go "wow!" when you place the plugin, swear it's the best thing happened to your music and proceed to praise it to all and sunder. Whereas the main thing that's happened is that the plugin gets loaded with an initial preset or settings that increase the signal gain, making the track a few dB louder.
The reason is simply that we like louder sounds better.
We hear them better, so we like them more.
The less experienced among us will tend to mix the fact that the track is louder with the idea that the plugin has made it better. And of course plugin manufacturers are very happy about that!
But remember: in a mix, level changes should be made by using the faders.
The gain at the all various stages of the chain (including between plug-in effects) should stay pretty much the same. So if a signal enters a plugin averaging -18dBFS, it should come out averaging -18dBFS (assuming of course it's not a gain plugin).
In other words, make sure that the levels before and after every effect stay pretty much (in average) the same ballpark.
Again, you can do that simply by using the output level control on the plugin (if it has one) or by placing a gain plugin right after (if it hasn't). For example a classic Pultec EQ does not have an output control - depending on the settings, your average level could jump several dBs. So you follow your Pultec with a gain plugin that brings back the signal to the same average.
Another example is when you insert an effect on a vocal track, and it gets louder, and you like it better. In that case, simply reduce the output gain to the plugin and then increase the fader a little to bring back the loudness.
In this way your mix (i.e. the relative position of your faders) will keep telling you exactly what your decisions are, and taking out that plugin won't change them.
And maybe you'll also find out that, once the level increase is removed, that plugin doesn't really do all that much...
But what's the big deal? Isn't it technically possible to make a mix using gain changes? Sure it is! And for simple mixes with limited tracks and buses it may work fine.
But in general, it's dumb.
Or, if you want the polite version, it makes things needlessly complicated once the mix is a little more than trivial.
That's because if you do use track gain or effect gains to determine the signal level before the fader, the position of your faders will no longer tell you what are the relationships between the various tracks or buses. Removing that plugin will change that relationship unexpectedly and adding another one may change it once again.
This is bad because, once you progress with your mix, it will get increasingly difficult to find out what's "making" the level and change it without affecting all the downstream effects, or sends or other stuff, A/B different timbre and texture ideas and so on. If you add/remove effects to try different timbres, textures and the processing chains, suddenly your levels will go totally astray because it's not just the fader that determines the level, but the effects.
In short, as it grows, the mix becomes and uncontrollable mess, and changing it gets increasingly difficult and unpredictable.
Most plugs have an output level control. It takes a few seconds to turn it so that the output level is int the same ballpark as the input.
If you want to keep your sanity when mixing, I strongly advise you do it!