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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

If you think about it, the only point you can put a limiter in a properly gain structured system that will truly work perfectly is at the output of the signal source. It would be set so that the input level to system would never allow the first device it feeds to clip. This is technically practical if only one signal source is used at a time (i.e. not mixed with others).

Therefore, if you are simply switching between multiple input sources, put your limiting device at the output of the switcher. The system sees one input source and really doesn’t care which one it is and no input signal can drive the system into clipping.

With multiple mixed sources and a properly set system gain structure, the next best place to put the device is at the output of the mixer. This is because any mixer output voltage at any frequency exceeding the system’s maximum output will clip the system somewhere.

With multiple branch systems you might think to use a limiter on each branch. But with proper gain structure they would either all work at once or compress some part(s) of the system and not others, thus upsetting the acoustic balance. Thus a single limiter for each main output that is controlled by the operator makes the most sense.

“Controlled by the operator” means outputs such as a separate sub-woofer output where the acoustic balance is actively “mixed” by the operator based on the input signal content. You should make sure the operator can see when a threshold is exceeded to avoid clipping the mixer.

If the noise floor of the mixer is low enough compared to the other devices in the system you can allow more than the 3 dB margin the mixer has above the limiter’s threshold. You can do this by reducing a pad between the mixer and the limiter. You can also lower the limiter’s threshold level (and increase the output of the limiter by the same amount) if the noise floor of the limiter allows this.

Because it is used as a “hard-line” device, you should set the compression ratio to maximum (10:1 or higher if available). As to any attack and release settings, they do not affect the gain structure. However, as the limiter is intended to function only as an emergency protection device, there is every reason to use the fastest attack and release times. You are not going for sound quality here; you are protecting the system from any overdrive.

So get into and out of protection as fast as possible. If you think sound quality IS important, then you are not thinking correctly. What you should have thought about is a more powerful system that would rarely be pushed into limiting. In other words, if the system is constantly pushed into limiting, it is under-designed.

Summary

Gain structure is only a problem because invariably we use equipment with different input/output capabilities and noise floors. There is no easy way to properly set system gain except to analyze each device in relation to the device that feeds it and in relation to the entire signal path.

Fortunately, the little information you need is readily available on equipment specification sheets. By working out proper gain structure on paper before you purchase and wire up the equipment, you can spot potential problems and make appropriate substitutions.

In any case, with the gain properly structured, you can make significant, or in some cases, spectacular improvements in the system’s dynamic range and its noise floor. In the example system, 18 dB is certainly a spectacular improvement.


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