Input Gain Fundamentals

Input gain matches the source level to the input circuits of the audio console.

The input section of any mixer is the cleanest amplification system in the audio console.

The input section allows for the largest amount of amplification within the audio console.

Correct input gain yields good audio.

Incorrect input gain yields bad audio. If the gain is too low in volume the audio signal will have a great amount of noise. If the gain / input volume is set to too high it will have incredible amounts of distortion.

"Gain Structure" is the technical term used to label the setting of the audio input gain. Gain Structure is the alignment of electronic circuits to maximize the signal-to-noise ratio and to also minimize / alleviate  distortion due to overdriven circuits.

Signal to noise ratio is the difference between the loudness of a desired audio signal and the inherent noise within an electronic circuit. Every amplifier or analog electronic circuit has an inherent noise that will translate to the signal flowing through it. This inherent noise is called the noise floor. The design of the input circuit of an audio mixer is to provide the cleanest amplification possible with the least amount of noise added to the analog signal.

If the desired audio signal is of very little volume when it passes through the input amplifier and is mixed with the inherent noise of the amplifier circuit the output will yield an audio signal with noise mixed in at the same level reducing the quality of the original audio signal and in some cases rendering it unusable. This noisy audio would have a very low signal-to-noise ratio and could be in the range of less than 20 db.

If the desired audio signal is of a good volume when it passes through the input amplifier and is mixed with the inherent noise of the amplifier circuit the output will yield an audio signal with so much good sound that it will completely mask out the inherent noise of the amplifier. This clean or good audio signal would have a very high signal to noise ratio which would be in the range of 40-50 db or higher.

At this point you would think that it would be best to run the level of the input amplifier as loud as possible but as each circuit has a minimum operational limit it also has a maximum operational limit. Meaning that if the audio is too loud it will lead to a distortion known as clipping or overdriving. This condition is cause by exceeding the operational capability of the input amplifier.

Grasping all that has been previously stated helps to build your foundational knowledge to have an understanding of what it takes to make good decisions about your audio levels. The main goal is to set the gain structure so that the audio signal is loud enough to mask the noise floor and not so loud as to cause distortion. Sorry to say, but it will not be the same settings for every setup or for every piece of equipment. This level setting will be dependent on the source and type of audio signal being fed into the audio mixer, camera, or any audio recording device. To set gain structure one must use a VU meter and headphones or speakers to monitor the audio. While the headphones or speakers are not required they are the only true way to verify the quality of you audio.

In some cases a simple way to set a good level is to increase the volume until the clip light starts to flash or the VU meter is in the red area. Then turn the volume back slowly until the vu meter is out of the red or the clip light stops flashing. After which listen to the audio to see how it sounds. It may be wise to back the volume down just a little more to keep any sudden loud peaks from distorting your audio. In this manner we have set our gain structure for the optimum operation of the input amplifier circuitry. Giving clean sounding audio without distortion and minimizing noise.

In other cases, such as with some musical instruments or sports announcers that inherently may have wide variances in volume, it may be necessary to turn down the input volume / gain even more to create a more cushion for those loud peaks. But you must be careful not to adjust the input gain to low or the audio will be at the same level as the noise floor and you audio will then be filled with noise.

For the more technically inclined a good general audio level on a dbFS (decibel full scale) VU meter is a variance of the audio between -12 and -30. This will yield a headroom of 12 db for peaks. If you continue to have distortion due to overdriving target and audio variance of -18 to -30 db. But try to keep the low audio levels above -50 because most of today's analog circuitry has a noise floor around -50 db.


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