I remember going to a concert here in Nashville in the early 1980’s to hear The Master’s Five Quartet. They were using a Bose sound system, and the first half of their concert was badly distorted. During the intermission, I eased over to Jake Hess and told him about the problem, and their next set was far better. Apparently JD had turned the microphone trims too high and the master volume too low and it was clipping the preamps severely. There was something reassuring to me that these professional icons were not above having tech problems just like the rest of us.
I have considered doing this article on gain stages and setting levels between various hardware for a very long while, for I have seen and heard both professional and non-professional alike, totally destroy the sound from an otherwise adequate sound system by improper gain setting between stages. Hopefully, this will help.
MICROPHONES
There are many types of microphones, both wired and wireless, and all of them have their own inherent output level. So, an understanding of how mics may differ seems in order. Dynamic microphones are essentially a small speaker in reverse, and are thus totally passive. They require no phantom power or special preamp, however they are infamous for having a very low output level of anywhere from –60 db to –40 db, and thus require a good amount of preamp gain at the mixer. Microphones such as the Shure SM-58 and SM-57 are prime examples of dynamic microphones. Condenser microphones, requiring phantom power, (+48vdc) are mics such as the AKG C-535A, C-451E, Shure SM-81, and countless others. Typically they are far more sensitive and have much higher output levels. In fact some can approach line level with certain high level vocals. An output level between –30 db and –10 db is quite common. Wireless microphones have another set of problems as most often they can be set to have an output level anywhere between low microphone levels of say –50 db to levels that approach or exceed line level of 0dbv to +10dbv. So, it becomes apparent that some method for adjusting the gain between different microphones is required, and this is where the mixer comes in.
MIXERS
Virtually all microphone mixers have input trims (preamp gain) at the top of each channel strip to compensate for a host of input levels from microphones, direct boxes, and line level devices such as CD players. Knowing how to set these trims is paramount to getting your sound system operating properly. In the case of the Mackie boards such as the 1604 VLZ, they offer a method for setting input trims using an expanded metering, and the solo system. This is a great feature, but since not all consoles (mixers) have this feature, I will describe a method that works regardless of the brand.
Turn all power amplifiers to the off position, for we don’t want to destroy your speakers, nor your ears.
Mute all input channels except the one that you wish to set the gain on.
Set both the input fader and the master fader to the unity point. If you don’t know where the unity gain point on the faders are, it will be labeled alongside the fader toward the upper end of the travel as either a “U” or 0db.
Turn off all buss assigns (ie sub 1, sub 2, sub 3, etc) except the Left and Right assign on the particular channel.
Pan the channel all the way to the left or right.
Have the singer, or the musician associated with this particular channel to sing or play their expected loudest passage.
Set the trim so that this loudest passage is about –3 db on the master output meters.
Mute and leave the fader at the unity point and proceed to the next channel you wish to adjust.
Before turning your power amplifiers back on, pull the master fader all the way to the bottom, release your muted channels, and slowly bring the master fader up until a desired level is obtained.
You have now set the gain stage of your mixer correctly. This allows proper headroom, and signal to noise and allows your mixer to operate safely within the design constraints of the mixer. From room to room, your individual channel mix should remain constant, and only the master fader is needed to set the desired room level.
CONCLUSION
If you follow these procedures you will virtually eliminate distortion and noise associated with improper mixer settings. Next month we will continue this theme with how to set levels between crossovers, power amplifiers, foldback systems, etc.
Until next time,
Ben Harris
http://www.southernsoundquartet.com
Author: Ben Harris
| Author's Website: www.southernsoundquartet.com
Written: 02/01/2009
| 
Category: Sound Advice
Comments: 1
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Ben,
Thanks for the great article. I am learning more about running a sound board, and the info here is very helpful. You said the same thing that the sound tech told us when we had our new board installed last December. Now it is in print, so that I can refer to it again.
Blessings,
Mike McIlwain
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