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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