Tuesday, May 16, 2006

Gain Structure :|: Impedance Matching and Unity Gain

Today's Topic: The Basics

Our Tuesday topic for the last couple weeks has been gain structure. If you haven't read the last posts, check them out at "Intro to Gain Structure" and "Matching Signal Levels". Today we'll cover the last couple ideas related to setting good gain structure: impedance matching and unity gain.

Impedance "Matching"
Impedance is a measurement given the label "Z" and measured in Ohms (Ω). Impedance is similar to resistance, but it also includes some complexities related to AC signals which we don't need to understand for this discussion.

Impedance in audio equipment is either high (Hi-Z, 10KΩ) or low (Low-Z, 600Ω). While matching signal levels is essential to get correct gain structure, there is a little more flexibility on impedance levels.

The key to "matching" impedance levels is to make sure that the output impedance of the one piece of equipment is lower than or equal to the input impedance of the next piece of equipment. Professional audio equipment is always designed this way, with very low impedance outputs and very high impedance inputs. However, it is worth taking a look at your manuals to make sure that is the case for all your equipment.

Why does this matter? If a connection made so that the signal goes from Hi-Z to Low-Z (opposite the way it should go), the signal will distort and signal levels will be unpredictable. That introduces its own "noise" and could undo what you've already gotten right by setting signal levels correctly.

Adjusting Unity Gain
Ever wonder what that "U" means on the sliders on your mixer? The "U" stands for unity gain. That means when you place the slider there, the voltage that comes in is the voltage that comes out. Every piece of audio gear has a "U" on its knobs, though the manufacurer may not have been nice enough to label it that way -- some of you were saying, "My mixer doesn't have a 'U' on it!" It probably says "0dB" instead on yours. Anyway, even the knobs that go from "0" to "10" around the dial have a place that is unity.

To get the best gain structure, you need to make sure you're passing unity signal through all the components in your sound system. This means that no voltage is being cut (lowering your clipping level) and no unnecessary amplification is happening (raising your noise floor). Therefore, you should always set the input and output levels of all your gear at "unity" in order to get the best dynamic range. If you find that you have to crank a knob beyond unity or cut it way back below unity in order to keep control of the sound levels in your system, that probably means you have a gain structure problem.

But how do you know what unity is on your piece of equipment? You got it: read the manual. Everyone does it differently (especially those 0-10 kind of knobs), so please please please check out the manual. Then set your knobs to unity, and see the difference it makes!
More Posts on The Basics

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