West Music Staff Blogs

Greetings! This is the first of a series of articles designed to make your activities as a sound engineer more fun, less stressful, and ultimately more successful.

If you use a console style mixer, as opposed to the all-in-one "top-box" style, you have certainly noticed that the knob at the top of the channel strip behaves like a volume control. Your fader at the bottom of the channel strip also controls volume. Why do we have two controls that appear to do the same thing?

You have all heard systems that issue a fairly audible hiss in an otherwise perfectly quiet room. You have all heard microphones that distort when a performer speaks loudly or when a singer gets aggressive. Both of these problems are caused by improper gain structure. This discussion is going to teach you how to avoid that problem.

Depending upon the nature of the presentation, we can have several different types of mics being used in a performance. The band might have four dynamic vocal mics, one acoustic/electric guitar pickup with a built-in preamp, and one electronic keyboard going direct to the PA and a bass amp going direct to the PA. The drum kit has a combination of dynamic and condenser mics, and the guitar amps have dynamic mics stuck directly in front of the speakers.

All of these devices send different signal levels to the mixer. The dynamic vocal mics send a very tiny signal. The guitar preamp sends a larger signal powered by a 9v battery. The keyboard and bass amps send a strong line level signal since they are powered devices. The condenser mics are somewhere between the two. The guitar amps are putting out 105 decibels so those mics are sending a very strong signal to the mixer input.

Let's take a side path for a moment: All electronic devices have a "noise floor". Whether a $50 component or a $50,000 component, they all produce a certain amount of noise. The manufacturer attempts to increase the audio signal to its maximum and hold the noise floor to its minimum. This difference in level is the "signal to noise ratio" that you see in the manufacturer's specifications. Typically a 90 db ratio is considered to be studio quality.

Here's what you do to maintain the correct signal to noise ratio for each channel, thereby eliminating hiss and lessening the possibility of distortion in the channel:

1. Set the top control (usually called gain or trim) to the fully counter-clockwise (off) position.
2. Set the channel fader at the -0- position, indicated with + numbers above it and - numbers below it.
3. Set the master output fader at the -0- position. (On most mixers you will typically find the -0- position about ¾ of the way up its travel, but be aware that on some units the master output fader has the -0- position at the top of its travel).
4. Now have your speaker/performer address the mic or instrument exactly the same way they would during the performance. As they are giving you signal, slowly increase the gain clockwise until you believe that it is loud enough for the purposes of your performance.
5. Now, shut off that channel with the channel fader, and move to the next and the next and so on until all the channels are done.

Depending on what level signal each channel had coming in at the input, you will notice now that the gain controls are all over the place. Some are very high. Some are very low. Some are in between. In spite of the fact that we had many different signal levels coming into the mixer, as soon as they all pass through their respective gain stages, they are now all at the same level in the mixer. Things that once were at different levels are now all the same, hence the term "unity gain."

A wonderful side-note to having done this procedure is that now all your channels and your master faders are set to -0-. If you have the flu and can't be there, you can tell your buddy at the mixer "It's easy..set everything to -0- and you'll be fine". This is an over-simplification, but you get my point. You have now maintained the signal to noise ratio all the way through your mixer from input to output. The noise floor is low, the audio signal is high, and you probably have lower gain settings than you had previously, which decreases the possibility of overdriving the input thereby causing distortion. If your input gain settings tend to be near or past the 3:00 position on the rotary dial, that will be an indicator that you need to turn up the signal strength passing through the system EQ or at the power amp (be careful not to overpower your speakers!) and lower the gain setting a bit at the mixer inputs. If left at the higher position, a strong momentary signal peak (a singer gobbling the mic or a keyboard player really playing strongly) may push the input into distortion.

All of the pieces of equipment in the signal path have a relationship to each other and it is important to maximize signal and headroom and minimize noise and distortion along the way. Starting with the proper input gain is the most important step.

As always, you can contact me at jbaumgartner@westmusic.com with any questions you might have. I'll be delighted to help.

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Part 4