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Straightforward Approach To Setting An Optimized Audio System Gain Structure

Now that the amplifier size has been determined, the next thing to look at is processing level inputs and outputs.

Most sound consoles can comfortably handle an output level between +18 dBu and +24 dBu.

This, in turn, will feed the processing equipment. Analog processors can usually handle +18 dBu input and output signals. This is the first place in line where attenuation or a pad may be required.

If you are using a console that can output +24 dBu, you will want 6 dB of attenuation at the input of the audio processor. This can usually be achieved by the input attenuators on the signal processor.

The outputs of the signal processors require a bit more discussion. Many DSP devices have either output switch settings or output jumper settings that can select between 0, +6 dB, or +12 dB, so the obvious questions are Why are there different options? and When do you use them?

To answer this, we must first continue our discussion about amplifiers.

Many manufacturers have input selection settings than can choose between 0.775V, 1.4V, X20 (or 26 dB), or X40 (or 32 dB). For the purpose of discussion, the table below indicates the input level that 200-, 400-, and 800-watt amplifiers will accept before the amp clips.

The 0.775V and the 1.4V input level settings indicate that all amplifiers will clip at the same input level. For the X20 (26dB) or the X40 (32dB) selection settings, the size of the amplifier and the load on the amplifier will determine the level at which the amp will clip. It is very important to be able to understand the clip levels and gains of the amplifiers in both dB and in voltage.

For 0.775V or 1.4V input sensitivity

(Equation 5a) Gain (volts) = sqrt [Max power rating * load (ohms)] / input sensitivity
(Equation 5b) (dB) = 20 log[Gain (voltage)]
(Equation 5c) Clip Level (volts) = input sensitivity (0.775V or 1.4V)
(Equation 5d) Clip Level (dB) = 20 log [clip level (volts)]

For X20 (26 dB) or X40 (32 dB) gain

(Equation 6a) Clip level (volts) = sqrt [Max power rating * load (ohms)] / gain (20 or 40)
(Equation 6b) Clip level (dB) = 20 log[Clip level (volts) / 0.775V]
(Equation 6c) Gain (volts) = gain (20 or 40)
(Equation 6d) Gain (dB) = 20 log[gain(volts)]

Amplifier Input Clip Levels
———————-200 Watts————————400 Watts————————800 Watts
X20 (26 dB)———-8.2 dB———————————11.2 dB—————————-14.2 dB
X40 (32 dB)———-2.2 dB———————————5.2 dB——————————-8.2 dB
0.75V——————0 dB————————————-0 dB——————————-0 dB
1.4V——————-+5 dB———————————-+5 dB——————————+5 dB

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