Gain structure is one of the very important yet highly underrated topics in audio. It’s not nearly as glamorous as EQ, plugins or parallel compression, but if gain structure is out of whack, no amount of EQ, plugins or compression will fix it.
Here I’m going to focus primarily on input channel gain structure (overall system gain structure is another article entirely). The impetus for this article came from a simple question: Is it better to hit the preamps hard then turn down at the main output, or run the mains up around unity and dial back input gain to get the desired sound pressure level (SPL) out of the system?
As a general rule (there’s an exception that I’ll detail in a minute), I would argue the former is the correct (or at least better) method, and here’s why. Most preamps sound best when you hit them pretty hard (at least up to the point of clipping, which is too hard). By running preamps hard – and by hard I mean around -6 dB full-scale on a digital console or within 6 dB of clipping on an analog board – it maximizes signal-to-noise ratio.
And for some reason, they just sound better. Keep in mind, that’s a general rule and your mileage may vary. Now, it’s quite possible that if you dial the input gains up so that all the preamps are running high, the overall system level will be too high.
That’s when you lower the main level to compensate. This method keeps the signal-to-noise (S/N) ratio high throughout the mixing chain and attenuate the signal at the last possible moment.
Before we get to setting up the gain structure, here are my goals for the process. First, I want to maximize S/N ratio, and use up as many of the bits in the analog to digital (A/D) conversion process that I can. Keeping the input level high meets both goals.
Second, I like to mix with faders around unity, which is another key ingredient to good mixing. The fader resolution is highest right around unity, so it’s easy to make small adjustments. If you try to mix with to faders at -20, a slight change in fader position might yield a 3 to 5 dB change rather than the 1 to 2 dB you actually desire.
Finally, I want to be sending a very solid signal out of the mixer to the processors for the same reasons (only in reverse) as the first point. That’s why proper input channel gain structure is important. Now, here’s how I approach the process.
In Digital Console World
For each input channel, I have the musician on the channel play at the loudest level. I then dial up the input gain until I’m within about 8 to 12 dB of full scale (minus 8 to 12 dB on the meters). I like to leave a little room for the musician to play louder when the lights go up (they always do).
Many digital boards also have a trim (or attenuation) control in addition to the input gain. I use the trim to dial the level back to where it should be in the mix with the faders at unity.
Because the entire system is gained properly, the main fader is sitting at unity as well, and all is right with the world. As I’m using DCAs to manage groups of faders (drums, guitars, keys, background vocals, etc.), and those live at unity as well, at least to start. All of this ensures that the signal-to-noise ratio is optimized at the A/D stage (just after the mic preamp), and my starting point for the mix is faders at unity.
Now, if you don’t have a digital trim control on your board, there’s a decision to make. You won’t likely be able to run the mic preamps hard without having too much signal at some point, so it’s necessary to dial the level back somewhere.
Of course, you can always turn the fader down, but then you lose fader resolution. A better alternative would be to use a DCA to keep the fader at unity, though that can get tricky.
Take a drum kit, for example: If you optimize the gain on the kick, snare and hat, chances are, the hi-hat will be way too loud in the mix. But more than likely, you’re using a single DCA for the entire drum kit. So now what?
Break the drums up into zones and use one DCA for each – kick and snare, toms, hi-hat and overheads might work. That way you can pull back the faders at the DCA level (a DCA is really an electronic remote control of the faders) and maintain fader resolution. A similar trick can be done with groups if you have them.
If DCAs are running, break my rule and set the input gain up so that the fader remains around unity for a proper mix. Audio is a lot about compromise, and in this case I’ll give up absolute input S/N to run the faders at unity. I’ve found that to be the wiser trade.
In Analog Console World
Really, the process is much the same, though you’re much less likely to have a trim control after the gain control. In that case, the same rules apply as a digital board without a trim knob. You still want to have good input level coming into the channel (for the most part) and then turn it down as needed later in the mixing stage.
You also want to keep the faders running around unity. Make the trades where you must – in either digital or analog world, what you don’t want to do is underdrive the mic preamps and have to add a lot of gain down the road. Sure, you can push a fader up for a guitar solo, but you don’t want to regularly run input faders at +8, groups at +10 and main at +5 because the input gain is set too low.
Exception To The Rule
Note that all this assumes you’re running on a professional grade mixer that has a mix structure designed with proper headroom. However, if you’re working with an inexpensive mixer, chances are you’ll run out of headroom in the mix bus very quickly.
Setting input gains on these mixers the way you should, when all those hot signals hit the mix bus, is not going to be pretty. The buses quickly saturate and lose all sense of dynamics. In that case, really keep an eye on overall output level and run input gains down accordingly. This isn’t a dig on cheap mixers – they fulfill a need but you can only expect so much for what you pay for them – it’s just reality.
As noted earlier, if you go through this whole process only to find that the overall SPL in the house system is either way too loud or way too soft, you have some work to do at the system processor or amplifier level. But that’s another article entirely…