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Crossover 102 - Electronic Crossovers - Page 3

Now because of the very strong demands for power made on a full range system by the low frequency content of the musical program material, when a full range system runs out of headroom, it's the high frequency information that suffers. Since the highs are modulated by the lows, and ride on the lower frequency fundamentals, when clipping occurs, the high frequencies are the first to clip. Therefore the intelligibility of the vocals is the first to go. Also it is the clipping or complete lack of headroom in a full range system that is the biggest reason for high frequency compression driver failure (can you say "toasted diaphragm"). Once the sound system is crossed over electronically, even if the low pass amp clips, the highs can remain clean because they are now being powered by their own dedicated high pass amplifier.

In part one we covered the roll of inductors and capacitors and how they act as low and high pass filters in passive crossovers. In active crossovers, inductors and capacitors can do the same frequency filtering, but in many adjustable electronic crossovers, a circuit called a state variable filter replaces the inductors, which emulate the performance of inductors. It is a combination of the component values of inductors and capacitors, along with the selected values of resistance's that form the circuitry of the frequency filtering networks. The combination of a specific inductor and a specific resistor create something called an RL circuit (low pass), and a specific capacitor and specific resistor are called a RC (high pass). Varying the value of the resistance (R) with a given value of L or C will determine the high or low pass cutoff frequency of the crossover network.

In part one we introduced the concept of various orders of filters (-6 dB/Octave, -12 dB/Octave, -18 dB, -24 dB, etc.). For each filter section (pole) or order, we introduce more resistors, inductors, or capacitors. The most accepted type of circuitry for audio is something called a Butterworth filter network. We have been listening to them for 60+ some years. There have been other possible design classes, but generally they all have their shortcomings. Less than 20 years ago, a Mr. Linkwitz and a Mr. Riley CO-wrote a paper that essentially trashed Butterworth filters and the 3-rd order Butterworth filter in particular. The reason for this has to do with the inherent phase shift of the output signal that we initially mentioned in part one: Crossovers 101. We will expand upon this subject at this time.

In the technology of filter circuitry, for each order of network components, we get -6 dB per octave roll off, and a 90-degree shift in phase. Here is the chart for review:

Filter Order Attenuation per Octave Phase Shift
1st -6 dB 90 Degrees
2nd -12 dB 180
3rd -18 dB 270
4th -24 dB 360
5th -30 dB 450
6th -36 dB 540
7th -42 dB 630
8th -48 dB 720

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