Crossover 102 - Electronic Crossovers - Page 6

According to the AES (audio engineering society) standard, a loudspeaker that can handle 500 Watts of continuous signal, can handle 1,000 of program material (average music), and 2,000 Watts of peak signal level. Now the tricky qualifier here is that the amplifier must have +3 dB of remaining headroom, above the Peak level. This would require an amplifier of 4,000 Watts. The average uncertified system operator would probably blow this loudspeaker up with a 4,000 Watt amplifier. A 2,000 Watt amplifier with a "soft limiting" circuit, such as DDT, would probably suffice, in the hands of an experienced operator. But, I would probably give an entry-level system operator an amplifier between 600 and 1200 Watts.

For our scenario, I am going to suggest that we have a 2,000 Watt amplifier for the lows, 1200 Watts for the mids, and 350 watts for the highs. Even though the compression driver is only rated at 80 Watts, because of the needed headroom to accommodate the constant directivity horn EQ (nearly +15 dB at 15 kHz), we often use an amplifier rated at 300 watts or more. The horn is padded down or attenuated -12 dB in the octave around 2.5 kHz. For the most part this means the driver would get about 20 watts (10 log 300/20 = -11.76 dB) while producing these mid-band frequencies. So, for headroom reasons, 350 Watts of power is not out of the question. However I would certainly recommend that the chosen amplifier have some sort of "soft limiter", such as the Peavey DDT circuitry. For this example we are going to assume that each amplifier has the same input sensitivity. When you have amplifiers with differences in input sensitivity (how many volts does it take to reach full power), you would have to figure the differences into the gain structure calculations.

A good choice of crossover frequencies for our example active three-way system would be 150 Hz and 2 kHz. Up to a 2/3 of an octave lower for both the Low/Mid crossover and the Mid/High crossover point, would also be within a reasonable range. None of the devices would be driven above nor below their cutoff frequency or -3 dB points. We will assume that our horn has a low frequency cutoff of 800 Hz. When it comes to high frequency horns, standard practice today is to cross them over at least an octave above their respective -3 dB cutoff point on the low end. This protects the compression driver diaphragm even further, as well as minimizing distortion that can occur within the horn itself, if crossed over lower in frequency.

If we choose to use the low pass device as our reference for gain, and set the output drive level for the low pass at 0 dB, then the mid frequency output level should be set to -3 dB to match the mid to the low's sensitivity. Likewise, the high frequency output level would be set at -12 dB in order to attenuate the high frequency horn to match the sensitivity to the low and mid bands. On many variable electronic crossovers with constant directivity horn equalization, you only attenuate the mid-band energy, and the CD horn EQ then compensates for the inherent high frequency roll off.

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