The primary function of a compressor is to control dynamic range, and by 'control', I mean 'reduce'.
The sounds of real life and music in general have excessive dynamic range. In other words, the louds are too loud and the quiets are too quiet.
An example of this would be listening to music late at night. You can't turn up the volume too far or it annoys the neighbors. But the quiet parts of the music, and the subtleties, are now inaudible.
Listening in a car is another example. If you turn up the volume so that you can hear the quiet parts of the music – particularly in classical music which has a very wide dynamic range – then the loud parts will blow your head off.
Back in the studio, the main problem will be the singer. Singers in general can't control the level of sound they produce very well. Low notes are quiet, high notes are loud.
But a compressor can help. A compressor can automatically raise up the quiet parts so that they are more consistent with the louds.
But the trouble is that the compressor doesn't always get it right. It may be slow to respond, or if you set the attack and release times to faster values, then you might be able to hear it working.
And sometimes it will apply too much dynamic control, or too little.
So what do you need? A better compressor?
No – you need the best compressor in the world, at least as far as dynamic range control is concerned. And that is…
Your finger, on the fader.
By riding the fader continuously, you can control the level of the vocal exactly the way you want. And if you use automation, the process of fine-tuning the dynamics of the vocal is straightforward. Painstaking perhaps, but straightforward.
Using automation on a digital audio workstation is even better. For the want of twenty or thirty minutes of your time, you can get a vocal that is 100% controlled in level, in exactly the way that sounds good to you.
The best compressor in the world, and it didn't cost you one single penny!
P.S. There is a story that top producer Mutt Lange is so pernickety about this that the automated fader is in constant motion and sometimes almost seems to vibrate, such is the detail of the dynamic range control he applies.