Crossover 102 - Electronic Crossovers - Page 4

Odd order filter networks will always introduce a situation where the output of the network is either +/- 90 degrees out of phase with the input signal. For instance the 3-rd order filter will introduce 270 degrees of phase shift, which is still 90 degrees short of 360. If the output of an electrical circuit exhibits a 360-degree change in phase, you are essentially back "In-phase." However the signal does spend a finite amount of time passing through four high or low pass filter circuit networks. This is something called group delay (in my day group delay meant that the band was late). Even though it took a small amount of time to pass through the filter stages, since you ultimately made a 360-degree turn, you are now headed in the same direction; i.e. the output is in-phase with its input.

Even order filter networks will always give you a multiple of either a 180-degree or 360 degree shift in phase. It is accepted practice in professional audio that it is desirable to maintain a unity of phase through out the system, in other words, the output should be in-phase or headed in the same direction as the original input signal. When a 2nd order filter network is introduced, the outputs are 180 degrees out-of-phase. This is not really a problem, because we can switch or reverse either the outputs of the crossover circuit itself, or switch the leads to the loudspeaker itself (but not both), to restore phase unity. If you try reversing the outputs of an odd order crossover filter design, since they are going to introduce a shift in output phase that is some odd number multiple of 90, you are then still dealing with +/- 90 degrees of phase shift.

Even though Linkwitz and Riley were correct in the areas outlined in their paper, we continued to use 3 rd order crossover filter networks (for the most part) throughout the remainder of the 20th century, and into the 21st. The reason is that 2nd order filters do not offer enough protection for the high frequency compression driver, and variable fourth order filter circuitry can be very difficult to accomplish precisely or accurately. In the electronic technology, there are controls called potentiometers that are used as volume or tone controls, as well as to change a crossover filter frequency. Remember we said if you vary the R with a fixed value of L or C, you will change the turnover point or crossover frequency of the filter. To have multiple poles or orders of filter networks, we repeat the value of R, L, & C, in each subsequent filter stage or order. The precision of the filter design depends on the tolerances of these component values.

There is a special type of potentiometer called a ganged control. You can have two, three, or even four variable resistors that are stacked or staged so that you can change the resistance with a common control shaft that simultaneously moves two, three, or four wipers or variable resistor contacts. One of the technological challenges involving variable ganged controls, is the typical tolerances of each potentiometer stage is usually no better than 20%.

As a result of the research done by Linkwitz and Riley, we now have a filter circuit topology that bear their names. Now you will essentially find identical component values in each filter stage of both a Butterworth and a Linkwitz Riley filter network.

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