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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

Multiple Signal Paths, Arrays and Delays

Another variation in this procedure is when a system has several branches, such as a mixer feeding multiple sub-systems. You have to separately analyze each branch and include in each analysis the source common to all branches (the mixer in the example Figure 5).

This will automatically optimize the system so that the common source and all the branches clip at once. To do this, the mixer In Figure 5 must feed each branch through a separate pad. Note that the dynamic range is different in each branch.

To balance the multiple branch systems acoustically in the actual system, you will probably need different operating levels in the branches than what the optimized electronic gain structure provides.

An example would be a central cluster with delayed balcony speakers. To balance operating levels in these instances, use the branch that is lowest in acoustic level as your reference branch (i.e. the one you are itching to turn up because it isn’t loud enough – but don’t touch that dial). Use the input attenuation on the amplifiers for each of the OTHER branches.

This will reduce their output levels and achieve proper acoustic balance with the reference branch. This will also have the effect of lowering the noise levels and reducing the maximum capability of the other branches. In this case, less capability is acceptable because you have determined that the maximum capability can’t be used in these branches unless you drive the reference branch into clipping.

However, if you find, for example, that you have to significantly reduce the maximum output capability of the central cluster so you won’t clip the balcony system, then your balcony system is under-powered. Instead of attenuating the central cluster, you could add gain prior to the balcony system amplifiers (or “unattenuate” the amplifier input).

While this will balance the system, the balcony amplifiers will be driven into clipping before the central cluster amplifiers.

In this situation, the only way you can have your cake and eat it too, is to increase the size of the balcony amplifier, which translates to more voltage (power) capability for the balcony speakers.

You will not spot this problem by analyzing the electronic gain structure.

This could only have been spotted on paper with proper analysis of the acoustic output for each branch based on loudspeaker sensitivities and listening distances.

Electronic Crossovers

Electronic crossovers require special attention. Consider a full-range signal with equal energy per octave (e.g. pink noise). A crossover will divide the total energy of such a signal among two or more frequency bands. This causes an inherent signal loss at each band-limited output, compared to the full-range crossover input signal.

In effect, crossovers are NOT unity gain devices when fed a full-range signal. You can approximate these losses by calculating how much of the total energy is in each frequency band by using the following procedure:

Example: A 3-way crossover with frequency bands of 50 Hz – 125 Hz, 125 Hz – 500 Hz, 500 Hz -10 kHz.

1) Multiply the lowest frequency in each band by 2 until you get to the highest frequency for that band. The number of times you multiplied = the number of octaves. Round off the results for each band to the nearest whole octave [= 1, 2, 4].

2) Add up the total octaves from all bands [= 7].

3) Divide the octaves in each band by the total octaves [= 0.14, 0.29, 0.57]

4) Push the LOG key for each result [= -0.9, -0.6, -0.2].

5) Multiply each result by 10 to find the approximate losses [= -9 dB, -6 dB, -2 dB].

Note the low frequency output is down almost 10 dB. That is why many systems have problems achieving enough drive levels for the subwoofers.

Now you must draw horizontal lines on the output side of the crossover’s window. Draw these lines at a distance below the top of the window equal to the loss in dB for each output as found above. This line for each crossover output is used to match up the crossover window to the top of the window of the device it feeds (usually an amplifier).

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