Gain Explained

[flemingmras]

You know, too often I see lots of guys that have the misconception that gain=volume, when this is simply not the case. You will spot these guys as they tend to set the volume of each channel with the gain pots instead of the channel faders.

Gain in a live PA is signal strength. A microphone converts sound pressure into electrical signals that correspond with the frequencies/sounds picked up by the microphone element. These signals are sent through a mic cable into the mixer board. However, these signals are so weak in amplitude(again signal strength) that they need some sort of gain to increase the amplitude of the signal so that it can drive the gear down the signal chain. If the signal flow down the signal chain is not strong enough, the gear it runs through will be less effective and it will not be able to process the signal to its full capacity. This is because the weaker the gain in the signal, the less musical content it will contain for the gear to process.

With gain also comes higher signal to noise ratio. Signal to noise ratio(commonly referred to as S/N Ratio) is the ratio between signal content and noise content. All gear produces a small amount of "noise", which is the common "hiss" noise you hear in systems with no signal going through. The better the gear, the lower the amount of produced noise in the system. A signal to noise ratio of 1:1(1 part signal to 1 part noise) would mean that the level of signal and the level of noise are at an equal, which is definitely not desirable as it will "cloud up" the signal. However, the more you turn up the gain, the stronger the signal becomes and the higher the S/N ratio. A 90dB-100dB or higher S/N ratio(90dB-100dB of signal to 1dB of noise) is a much more desirable S/N ratio for optimum performance and superb clarity.

Also, with more gain comes more voltage "swing", or dynamic range. This is where the gain structure comes into play. On a mixer board, you'll see that all the faders are labeled in negative dBs, until you get about 3/4 of the way up on the fader travel, which will be a 0dB or a "U" mark. Mackie's tend to use the "U" mark, which stands for Unity Gain. In the law of Unity Gain, this law states that the only place in the signal path that the signal should receive ANY kind of gain is from the "Gain" or "Trim" control on the channel. If you push the fader past the 0 or U mark, the signal will then be receiving gain from the fader itself, which can overdrive/clip the main mix buss inside the board. On all mixers, with the channel faders set at the 0 or U mark, and with the Main Mix fader at the 0 or U mark, and with no boost from the channel EQ, whatever signal level is coming from the Gain or Trim control is what will be seen at the mixer's output and at all various points in the mixer signal flow path. With maximum possible gain coming from the trim control itself, this will feed each channel EQ with maximum signal, making the channel EQs MUCH more effective to where you won't need to boost at all from the channel EQ.

As a matter of fact, when I run sound, I take whichever bands I want to stand out more and leave them completely flat, then cut everything else around it. With max gain coming from the Gain or Trim control, your EQ cuts become MUCH more effective. For instance, on drum channels I typically leave the lows and highs flat. I then put in about 3-6dB of mid cut(depending on the drum), and as I'm listening to the drum I'm EQing, I sweep the frequency control until that drum sounds nice and punchy with plenty of attack definition and clarity(usually around 200-400Hz...each drum is different). With only EQ cut put into the EQ, I can now punch some more gain from the Gain or Trim control through the channel, allowing for an even more powerful signal.

Most gear down the line from the mixer will feature input and output controls. Most of these controls have a 0 or U setting. Set these at 0 or U and that same signal coming from the mixer will flow through each piece of gear from input to output while maintaining the same signal level it had coming from the Gain or Trim stage in the mixer.

The crossover, however, is a little different. The low band, since it typically only passes through frequencies from 40Hz to 100Hz, will have less signal energy than the high band as the high band is passing through a lot more frequencies than the low band, so you may need to punch up the low gain on the crossover so that the low band will clip at exactly the same time as the high band. On my crossover, the input and high bands are at 0dB and they both clip at the same time, but the low band required about 5dB of gain in order to clip at the same time with the high band and the input.

Next comes the setting of the amp's input attenuators. Another common misconception of these controls is that they somehow control the OUTPUT of the amp and if you don't max them out then you won't get the full power output from the amps. This is not the case at all. These controls simply control the input sensitivity of the amp. The input sensitivity rating specifies how much voltage is required at the input of the amp to get the output to swing to full clean power before clip. Most amps are rated anywhere from 1VRMS to 2VRMS with the attenuators maxed out. Well, most +4dBu mixers with the gains maxed out and all faders at unity usually put out around 9-10VRMS of signal. With the amp's input attenuators maxed out, you're gonna hit clip long before the mixer puts out its full rated output. Once the amp hits clip, you can't go anymore...PERIOD! So in effect you actually end up sacrificing a good 8 7-8 volts of signal strength from your mixer, which results in about an 18dB-20dB loss in dynamic range/headroom, which amounts to much less voltage swing from the amps. This is where the amp's input attenuators come in. By turning them down to the point where the amp will clip at exactly the same time as the mixer/house graphic/crossover, you can now get the full 9.8 volts of signal from your mixer down the signal chain and into the amps, resulting in max dynamic range/headroom and a much larger voltage swing from the amps, which results in a much more powerful, cleaner sounding system with lots of punch and clarity. By dialing down the input attenuators, the amps will STILL put out full power...it will just take a much larger signal to get it to put out full rated power, which is what you want. A big high gain signal results in a signal with much more musical content for the gear to work with, making each piece of gear MUCH more effective, allowing you to get the most performance out of your gear.

Another one I see a lot of is guys that max their trims out, but then set the main mix fader at unity and control the overall mix volume with the channel faders. Again, you're limiting your system this way plus making it harder on yourself to control an overall mix volume. Typically what I do is I subgroup vocals to groupd 1/2 (1-Left 2-Right) while subgrouping drums to groups 3/4(3-Left 4-Right). I then set all vocal faders plus the vocal subgroup to the 0 or U mark while maxing out the gain on all vocal channels, which will give me max level from the vocals. This way if the vocals are getting drowned out, I know to pull the instrument channel faders down to better balance with the vocals. All drum channels I set to the 0 or U mark with max Gain/Trim, except the snare and overhead, which will almost always be too loud with the fader up that high, so I start with the snare and overhead faders halfway between full down and the 0 or U mark. I then set the overall volume of the drum mix with groups 3/4, which almost always end up about -5 to -6dB below the vocal subgroups. I then set the guitar and bass faders about halfway up. With max level coming from all channels, I can now control the overall mix volume with just one fader, the Main Mix fader, while maintaining the same balance as the volume changes throughout the night. This fader setup technique always starts me out pretty close to a good mix. Then as the band kicks off the night I fine tune the balance/overall volume, slight EQing of the vocals(almost always Low/Low Mid cut is needed for the vocals to cut through, then dial in the effects last. Usually I have the band dialed in by the end of the first song one shot go.

To Summarize:

Gain does not equal volume

The amp's input attenuators DO NOT control the amp's output level

Always max out all gain controls for the most clean and clear signal from the source

Set the loudest instruments in the mix's faders to 0 or U on the channel faders, then use the remaining faders to balance the rest of the instruments to those instruments

Control your overall mix volume with the Main Mix fader, not the channel faders

Jon

[5150loveeddie]

Your my hero Jon

TABARNAC

[Hassan Chop]

Another very common error is to mistake gain for distortion.

(I want the T-shirt version of your "Son, one day you will make a girl very happy..." image.)