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Properly Setting Sound System Gain Structure

Analyzing each device in relation to the others -- and in relation to the entire signal path...

By Chuck McGregor May 21, 2019

Image courtesy of Alexander Stein

Doing It

You start the whole procedure by inputting the pink noise test signal into to mixing console. Set it so that it’s output just clips as seen on the oscilloscope.

Make sure it is the output of the mixing console that is clipping. Determine this by reducing the master fader. The clipping should stop. If it doesn’t, you are clipping something before the output fader.

While you’re at this point, note the reading on the output meter. This is a good indication of what the meter will read when you have reached the system’s maximum output after you set its gain structure. If you are using sine waves, this will NOT be a reliable indication.

Once completed, if the system noise levels are low enough, you may want to increase the setting of the amplifier(s) input level control. This will make the mixer more sensitive for operation.

If you reduce the amplifier input level control, something in the front end of the system will clip first. This means the amplifier will not reach full output. But it WILL reproduce that clipped signal and possibly damage the loudspeakers. Either way—if you choose to increase or reduce the amplifier’s input sensitivity from the optimum gain structure setting—you really don’t gain (pun intended) anything.

There is possible exception to this: by reducing the amplifier’s input level control, the output meters on the console will indicate you have reached the system’s maximum output before the amplifier’s clip.

This is useful so that a less than capable mixing engineer will THINK he’s pushing things to the limit but there will still be something left in the amplifiers. This may help protect the loudspeakers but, bear in mind, it will limit the maximum output of the system to something less than it could be.

Note that to reach a system’s maximum output analog Vu meters on mixing consoles may “peg” before the system clips. If you can afford the reduction in dynamic range, operating the system so the meters don’t peg means you’ll never clip the system. Generally, this means you won’t ever blow the loudspeakers assuming the amplifiers are chosen not to exceed the loudspeaker’s maximum ratings.

More Complex Situations

Up to now we’ve looked at a simple systems. Here is where gain structure gets more complicated. However the ideas are exactly the same. You just have to think about what specific pieces of equipment do and/or about more signal paths.

Devices with Gain/Loss and EQs: Parts 1 – 3 assumed devices in the signal chain have no gain (unity gain devices). However, a device may have gain or loss, or you may want to allow for boosts in an EQ, which may be needed to tune the system.

EQ boosts are like adding overall gain to the device. In such cases, as illustrated in Figure 4, input of the device’s window is shifted down below the output of the device’s window. The distance will be the gain in dB for this device or the desirable dB boost you choose for the EQ.

In this case, it is assumed the boosts will be limited to a maximum of 6 dB. Match the top of the window of the preceding device to this line. On the output side you still use the top of the window to match it to the next device.

When adjusting gain in an actual system, first set up the system with the EQ set to flat. Then make any EQ adjustments.

If all of your EQ is cut only, you can usually leave everything as is. However, if you add ANY EQ boosts, you will then have to redo the gain structure starting from the input to the EQ by finding the new maximum level it can accept without clipping.

This will of course require attenuation at the input to the EQ input. In some instances a device might introduce a loss in signal level.

The procedure is similar except that the output side of the device’s window is shifted to below the input of the device’s window a distance equal to the loss in dB.

Use this new output point to match the device to the top of the window of the following device. In the example it is assumed the limiter threshold is set so the maximum signal through the limiter is 6 dB below its maximum output.

However, on the input side you still use the top of the device’s window for matching to the preceding device’s window. The relative signal level prior to the limiter is 6 dB higher. As shown in Figure 4, everything, including the noise floor is raised 6 dB. The system dynamic range is still determined by the signal delay, because that is still the smallest “window” in the overall picture.

Because the maximum input of the amplifier (after its input attenuator) is still + 3 dB it has remained in the same position throughout

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