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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 For Real

To actually adjust a system you need to do exactly what you did on paper except you are now doing it for real. You start from the console output and find out what you need (gain or loss) to adjust its maximum output signal so that it just drives the next device into clipping. And so on.

You don’t need to know the specifications of the equipment. When you go through the system you’ll find out what those specifications are in terms of maximum output levels. As you should have understood by going through the exercise on paper, the noise floors of the equipment will take care of themselves.

Some device (like the signal delay in the above example) will be the weak link. There is nothing you can do to make this better except to replace it with a device with a better maximum output to noise floor window (better signal to noise ratio specification.

Because of production variations and possibly conservative specifications, you may be able to pick up a few more dB of dynamic range by adjusting the pads or gain values you determined on paper. If things are not reasonably close to your on-paper calculations, you have a problem such as bad wiring or a misadjusted or defective device.

What To Adjust

When you set gain in the system, the attenuation or gain needed between devices can be added externally or by using a device’s input level control, if it has one. DO NOT ADJUST THE OUTPUT LEVEL CONTROL ON ANY DEVICE—this should be left at maximum. This is because it is rarely the last thing in the internal circuitry before the output connector. Unlike some input level controls, it usually does NOT adjust actual gain.

Therefore using it will squash the dynamic range in that device’s output stage and you may end up making things worse, even though the signal level is matched up to the next device. Use an output control only if you KNOW ABSOLUTELY that it is a simple attenuator feeding its output connector. The reason it usually is not is that this topology would cause changes in the output impedance when the control is set for anything other than maximum.

Among other things this could would wreak havoc with – guess what – the gain structure. If the device has a noise floor below other devices when you have set the overall gain structure, you can use output gain.

But reduce it only by 3 dB less than the amount between the device’s noise floor and the device that determines the noise floor of the system. This is because if you bring its gain, and hence its noise floor, up to the worst case device its noise will add to the worst case device and give you 3 dB less dynamic range.

The Tools You Need

To find the clip points in a system, you need to use an oscilloscope and a pink noise test signal. There is really no good substitute for this equipment to set gain structure. Sine wave signals are not recommended as they only show one frequency at a time and you can easily miss something.

The pink noise should be full-range (20 Hz – 20 kHz) and have at least a 6 dB peak to average ratio. If you can find one with a 10 dB peak to average ratio, you will more closely simulate real audio signals.

If you must use sine wave signals, you will have to check each and every EQ boost frequency or range of frequencies very carefully. When measuring electronic crossovers or other frequency response limiting devices, only a full-range pink noise signal will allow you to see full-range signal energy losses easily. (See sections on crossovers and band limited devices.)

If using sine waves you must set the frequency to the center point of each frequency band of the crossover or the center of the band pass for a band limited device.

For simple systems (e.g., no electronic crossover), there is “poor man’s” method where you use a Piezoelectric tweeter and a 400 Hz sine wave to find clip levels.

Basically, you connect the tweeter directly to the output of each device. When the device hits clipping, the tweeter will emit a very noticeable buzzing sound due to the harmonics in the clipped signal.

For high-powered amplifiers, a resistive pad should be used to avoid burning out the tweeter. This method is detailed more rigorously by Pat Brown of Syn-Aud-Con. You can find this information here.

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