Gain structure - yet another part of the Force that can either lift you into superstardom or kick your butt into the dark galaxy of oblivion. What is it? Why should you care? Let's find out...
The explosion of affordable PA gear that started in the ‘90s has been a double edged sword. On one side of the blade, bands could take gigs that would have been impossible before. The economics of renting a sound system meant that on a lower paying gig, the amount of money to rent a system and pay a sound tech left little or nothing for the band. The other side of the blade is a sharp rise in the number of bad-sounding shows.
It’s that old hated question heard regularly by every sound tech out there--usually coming from a drunken audience member. “Do you know what all of those knobs and buttons do?” We know. You’re a songwriter. You’re a performer. You’re a rock star. The technical details are left to others. But if you are going to own a PA system, you need to understand what everything does. You don’t have to be an expert or get a degree in audio engineering but you do need a basic level of knowledge about what knob does what thing and how different controls interact to affect the overall sound.
Our goal here is to help you understand some basics. To that end, as you read this story, you will come across terms that you may not be familiar with. When you see a highlighted term, just click on it and you will get a definition of that term. We live to serve...
One of the few things in audio that remains constant in spite of technological change is the concept of gain structure. Gain structure refers to the manner in which signal levels are set in (and between) various components of any audio system. Poor gain structure can cause noise of the hissing type (as opposed to hum or buzzes), distortion, lack of headroom, and grossly mismatched meter readings between different devices. You might also find that your PA system does not play as loud as it should. Let’s examine this in detail.
Putting the Pieces Together
Think of audio signals like water flowing through the pipes in a building. When water enters a building there is a certain amount of pressure (gain). If the pressure is too low then the water might not be able to provide a comfortable shower on the upper floors. This is akin to insufficient gain. On the other hand, if we force the water into the building with too much pressure we might cause a pipe to burst. This is akin to distortion. The trick is to use enough pressure to get the water (signal) where you want it to go without causing damage.
Proper gain structure starts with interfacing the various components. Even before we think about setting ‘gain’ or ‘trim’ we must properly match our devices. You should already know that you need to connect microphones to microphone inputs, and line-level signal to line inputs. I’m alarmed at how many people connect outboard processors running at –10 to a console insert or line input designed to accept a signal at +4 (or vice-versa). This happens most often when introducing consumer products like a CD player into a professional system but it can happen between pro components as well. For example the Drawmer DL241 compressor can be switched between –10 and +4. This setting should match the operating level of the console’s insert or output bus. There’s no danger in a mismatch but it keeps the compressor from working correctly. If the operating level of the comp is set to -10, and the console insert is at +4 you may find that you are always getting too much compression, and you are overloading the audio output of the comp. That’s because the comp is receiving a signal that is too high for its operating level. A similar issue crops up when interfacing a console with outboard reverb or delay. Suppose your console is running an aux send at –10 and the input level on the processor is at +4. Even when you crank the aux way up, the processor receives too little signal. As a result, you can’t hear the effect. To compensate you turn the aux return up, introducing noise into your mix.
More serious problems arise when you have a drive processor mismatched with the operating level of the master L/R output bus. A +4 console driving a –10 processor eats up headroom too fast and often produces audible distortion, even though the master meters on the desk look OK. Conversely running a +4 limiter on the master bus insert with a console operating at –10 means you won’t get the system protection you are seeking because the console output may never reached the threshold of the limiter. While you’re at it pay attention to the outputs of your wireless receivers (some deliver mic level, others line level) and use direct boxes (not line inputs) for keyboards or other instrument-level devices such as samplers.
OK, we don’t want to overload anyone here. So we going to take a bit of a break. This installment gives you the smarts you need to hook up a system properly. Next time out, we will look at how signal flows through the actual console and address a couple of different schools of thought about setting levels between various parts of the system. Until next time...