Gain staging – the digital realm versus the analogue world
We have all encountered a house of worship where you hear a distinctly audible audio hiss when you walk into the room. Or perhaps the microphones get distorted when the lead vocalist hits a powerful note during worship. These problems are almost always due to poor gain structure.
Gain (or trim) is the amount of amplification applied to an original source signal usually by a preamp (for microphones) or – in the case of a line level signal – an op amp. What many sound system operators fail to realise is that the most important amplifiers in the system are not the big amps in the rack connected to the speakers. It is the little preamps in the mixing console. These circuits are capable of boosting a microphone level signal as much as 1,000,000 times – or more. Yes, you read that right. One million times more. Remember that decibels are logarithmic. When dealing with voltage, a 3dB boost is a doubling of electrical voltage. A 6dB boost is four times more. Jump up 20dB and you have increased the power by 100 times. Another 20dB (for a total of 40dB) is a 10,000 times jump and a 60dB increase is a million times more voltage.
Setting the preamps is the single most critical step in your entire sound system gain structure. Get this right and everything downstream will work beautifully. Get it wrong and nothing will behave like it should.
Nearly all microphone preamps are – by nature – analogue because the vast majority of microphones are analogue. In a digital mixing console, the signal is converted to the digital realm after the preamp. So, the same rules apply when setting the input trim or gain of a microphone into your system. In a music-based environment, you want to set your input trim so that when you are getting a normal level signal at the microphone, your signal will be about 20dB below maximum at the incoming level meter. The 20dB is known as “headroom” – the additional amplification available in case the source suddenly jumps up in level. 20dB of headroom helps prevent clipping should there be a sudden increase.
The important word here is “normal”. What is normal for one source is weak for another. So, you must set up each channel based on the particular source – whether it is a singer, musician, drums or multimedia source. As an example, my worship leader is a trained professional baritone operatic singer. When he hits a strong note, there is hardly any need for amplification at all. His microphone is set at very low amplification – about 16dB. Our lead female singer, on the other hand, is not a professional and while she has great frequency range, she doesn’t have the volume that our worship leader does. So, on an identical microphone, I need about 35dB of gain on her channel. The witness/announcement microphone, which is used for spoken word only by members of the congregation, requires nearly 60dB of gain because those who use it usually speak in very low levels holding the microphone pretty far away. The computer audio comes in at a pretty hot level and we only apply about 5dB to it.
Different brands of consoles present their input trim and fader levels differently. On our console, the input trim shows a gain of –10dB to +60dB. The faders show +20dB at the top with –60dB at the bottom and a prominent mark on the 0dBu position. When I set the gain at 16, it means I have amplified the source signal by 16dB.