Crossover 102 - Electronic Crossovers - Page 7

The Low Pass maximum SPL would be 133 dB (10 log 2000 + 100), Mid Pass maximum SPL would be 133.8 dB (10 log 1200 + 103), the High Pass would be based on the 80 Watt continuous rating of the driver, plus the sensitivity, for a max SPL of 131 dB (10 log 80 = 19 + 112).

Generally this would be a good starting for a Three-way or Tri-amped system. The actual optimum crossover frequencies would depend on the composition of the music being reinforced. Most of the energy in contemporary music is below 250 Hz, which is the most demanding bandpass. The second most demanding band of frequencies is between 250 Hz and 1200 Hz. Middle 'C' is 261.63 Hz and the 'D' above the melody staff is 1174.66 Hz. A Rock band with three electric guitar players may be more demanding on this system than another band with only one guitar player. It is now up to the system operator to determine which of these three bandpasses (low, mid, or high) are working hardest. This is easy of course if the amplifiers have an LED metering system. If the amplifiers do not have metering arrays, you would have to push this system until one of the three bandpasses' power amplifiers began showing limiting (or even clipping). Let's say the low pass amp is indicating that it is working the hardest. You could then lower the low/mid crossover frequency point, to allow more headroom in the lowpass amp, but now of course the midpass amp would be working harder. If you then got to a point where the mid pass amp began to show that it was now working harder, you may try lowering the mid/high crossover point a little. The idea here is to try to spread out the demand more evenly. A general rule of thumb, for fourth order and less analog crossovers, is that the crossover points should at the very least be more than 2 1/3 octaves apart. This minimizes something called out-of-bandpass by-product distortion, due to the crossover points overlapping too much.

Many uninformed system operators keep the crossover at the FOH mixer position, so that they can twiddle with it when they deem necessary. This can be a dangerous practice, particularly if you have no way of seeing how hard any of the amplifiers are working. A particularly bad practice is to raise or lower the level of an entire bandpass during a performance, as this actually changes the crossover point of the system. A low cut crossover frequency will move down when raising the gain, and a high cut will move up in frequency. In the example 3-way system below, crossed over at 200 Hz and 2 kHz, note how raising the level of the midpass changes the effective crossover points:

Raising the output drive level changes the crossover frequency


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