Crossover Frequencies, Slopes etc.
Home What are they? Active vs Passive Adjustable Crossover Analog or Digital? Frequencies, Slopes Filter Topologies FR Correction   < Tech << T-Line Speakers		The first parameter which needs to be specified for any crossover is a crossover frequency (or frequencies) which is also known as a crossover point (or points). This parameter comes from the driver's specifications. When you decided to design a speaker system, you picked up drivers which conform to your requirements. The next step is to select crossover frequency (~cies) best for these particular drivers. Another important parameter is the slope or roll-off characteristics of the filters. This parameter is related to the type or alignment of the filters used in crossover. We will discuss this topic on a separate page because of it's extreme importance. For now, we will accept today's industry standard: 24 dB/octave slopes and Linkwitz-Riley (Butterworth Squared) alignment. In active crossovers, we can easily build filters with 36 dB per octave or even 48 dB per octave slopes, but in most cases this will be an overkill. Besides, high order filters have nasty phase response and group delays. 24 dB per octave filters will provide enough reduction of signal above or below crossover point and will have reasonably good phase response. In this type of filter, crossover point is at - 6 dB below signal level in a pass band. Well, we have now 3 major parameters which are needed for crossover design. The next step is much more complicated. We have to decide what circuit topology to choose for the crossover. Alignment of the filter can be implemented on different types of filter circuits. The same type of circuit (circuit topology) can be used for Butterworth, Linkwitz-Riley, Chebyshev or almost any other filter alignment with the same order. The only difference will be in component values. Today, most companies use State-Variable filters in their crossovers. The reason for using this circuit topology is simple: state variable circuits produce low and high frequency passbands simultaneously and crossover points can be made adjustable. Also, these filters are cheap to build because they use 5 or 6 OpAmps for complete high and low pass 4th order filter (24 dB per octave). Another circuit topology, the so called Sallen-Key circuit, is also very popular but it is used mostly in home made crossovers or for cutoff filters. This circuit can be built with just 1 or 2 OpAmps, but you can get only a high pass or a low pass filter. For that reason, it is much more difficult to make an adjustable filter using a Sallen-Key circuit. You will need a 4-gang potentiometer for each section, or an 8-gang potentiometer if you want to adjust crossover frequency simultaneously in both sections. For a state-variable filter circuit, you will need only one 4-gang potentiometer. Since we decided that an adjustable crossover is not the best choice, we can use much more sophisticated circuit topology in order to achieve better sound quality. We would like to avoid multiple feedback loops and build filters based on the so called Generalized Impedance Converter (GIC) also known as gyrator circuits. Two types of GIC are used in our filters. One is L-component or simulated inductor and the other is D-component or FDNR (Frequency Dependent Negative Resistor). By using these types of circuits, we can emulate filters based on buffers and avoid feedback loops altogether. These filters have very precise roll-off characteristics and excellent noise and distortion parameters. For complete high and low pass 24 dB per octave filters we will need 12 OpAmps, but only 2 of them really are in the signal path.