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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

Picturing Gain Structure

Before you get out your equipment and start setting gain structure you have to learn just what it is you are trying to accomplish. Go through the following “on paper” analysis of a typical system.

Only after you understand this can you appreciate where to actually set the controls on equipment to achieve optimum gain structure.

Figure 1 shows a simple system consisting of six pieces of equipment. The device clip level (maximum output) is listed for each device as published by the manufacturer. For this example, all devices between the mixer output and the amplifier input are set for unity gain and the amplifier input is set for maximum sensitivity.

Each device is represented by what looks like a bar. Rather than a bar, picture it as a tall, narrow window. The maximum output or clipping point from the specifications for each device defines the top of the window using the absolute dBu scale on the right.

The published noise floor (or signal-to-noise ratio) specification below maximum output determines the height of the window. The relative dB scale on the left is used to determine this height. All usable signals must pass between the top and bottom of the window.

However, remember that your low level signals won’t be near the noise floor. Realistically the minimum usable signal is one that is at least 30 dB above the noise floor.

Next, a horizontal line is drawn across the top of the lowest window (in this case the amplifier). This is the system clip level, and for the rest of the analysis this line stays in the same place. Another line is drawn across the highest bottom window sill (in this case the mixer).

The relative dB scale is used to measure the distance in dB between the 1st and 2nd lines. As you can see, it is only 72 dB for this set of devices and gain structure.

That’s equal to the performance of your average consumer cassette deck—and you thought that professional equipment automatically guaranteed a professional grade audio system. Oh well, live and learn!

Now subtract 30 dB to find the “true” dynamic range (30 dB above the noise floor to the clipping level). The result is 42 dB.

Measurements of the maximum dynamic ranges for acoustic instruments and voice yield maximum figures in excess of 40 dB. This means our system really doesn’t have enough dynamic range to reproduce them.

Most Common Approach

As seen in Figure 1 from the absolute scale on the right, the amplifier input sensitivity limits the maximum signal level in all the other devices to +3 dB.

Above +3 dB the amplifier will clip—period. It doesn’t matter how much “headroom” is in the mixer, you can’t use it without distorting the amplifier.

Well, you say, the obvious step is to put a pad (usually the amplifier input attenuator) so the amplifier will clip at about the same point as the next least capable device.

In this case it is the notch filter. Using a -12 dB pad, the notch filter and amplifier will both clip at once and the signal level will be 12 dB higher through the other devices at the amplifier’s maximum output.

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