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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