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Distortion and summing.

Q: “I’m mixing live sound for a band and we all hear distortion through the PA, even though nothing on my board indicates clipping. What’s the cause?”

A: First, check your power amplifiers and make certain they aren’t overdriving your speakers. The test is simple – turn down the amp; if the distortion goes away, there’s your cause. But life is rarely that simple, is it? Next, let’s look at your mixer (for this example we’ll pretend that your mixer has no auxiliary or effects sends – it’s just signal in straight to the master out).

Think of what a mixer does in its simplest form: it collects input signals from all the microphones, instruments, direct boxes, and other audio equipment that is plugged into it, combines those signals and sends the result down one (or two, if it’s stereo) wire (which we’ll call the “bus wire”). In between the inputs and the bus wire is a summing (or combining) amplifier that makes up for any signal loss caused by the individual signals’ “send” controls and summing resistors.

It is possible for the combined signals to overload the summing amplifier even if no one signal itself is overloading. This is one of those instances in which setting inputs at unity gain doesn’t tell the whole story – it’s the combination of many signals that cause the problem. Further, if the master fader at the end of the bus wire is set sufficiently low, its meter may not register the overload as clipping. When the summing amplifier overloads, turning down the master fader won’t do away with the distortion although it will reduce the overall volume.

So you must reduce the levels of the individual input faders or sends. Remember, the more channels that are included in a mix, the more likely the chance of summing distortion. Sometimes you can track the problem down to one offending channel, but more often you’ll find you must reduce each channel’s level by 2 or 3dB to remove the overload. Then you can raise the master fader to make up the gain you’ve “lost” without encountering distortion.

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