What is a crossover and why is it needed?

Before answering this question, it is first necessary to take a short detour into loudspeaker theory and outline the problem. As is known, almost any speaker currently produced is capable of effectively reproducing only a narrow frequency band, limited from below by the resonant frequency of its moving system, and from above by the mechanical properties of the diffuser (weight, rigidity). Beyond the boundaries of this frequency band, the sound pressure created by the speaker decreases significantly and the level of distortion increases. We can't talk about high-quality sound here. Therefore, to transmit the full spectrum of audio signals (20-20,000 Hz), it is necessary to use several speakers. Long ago, acousticians realized this need, and today in all areas of audio technology, be it home or automotive systems, the vast majority of speaker systems are implemented exclusively using a multi-driver design.

In relation to car audio systems, two fairly typical construction schemes can be distinguished, with which even more or less informed readers are familiar. The first and most common consists of three speakers: a subwoofer aimed exclusively at bass (approximately 20-100 Hz), a woofer/midrange speaker for the upper bass and mid-frequency ranges (100-3000 Hz), and a tweeter responsible for high frequencies (from 3000 Hz). Hz and higher). In more complex designs, such as those presented by professionals in car audio competitions, the number of speakers increases. Here, separate speakers are responsible for each frequency range: lower bass, mid/upper bass, mids and highs. But, despite the obvious differences, both schemes are subject to the same requirement: each speaker included in the speaker system must reproduce only its own frequency band and not affect neighboring ones. To fulfill this requirement, electrical filters are included in the audio path, which are responsible for highlighting some frequency bands and suppressing others. Obviously, if an acoustic system uses several speakers - a subwoofer, a bass/midrange driver, a midrange driver and a tweeter, there is a need to use several electrical filters. The combination of several such filters is called a crossover.

Filters

To a first approximation, any electrical filter is a combination of several elements that have the property of selectively transmitting signals of certain frequencies. The simplest circuits that have similar properties can be built using inductors and capacitors. The principle of operation of these circuits is based on the dependence of the resistance of the elements included in their composition on frequency: for inductors, the resistance increases with increasing frequency of the signal, and for capacitors, on the contrary, it decreases. Therefore, inductors pass low frequencies well, and capacitors pass high frequencies. These properties are used to construct filters - low-pass (LPF) and high-pass (HPF). In addition to low-pass filters and high-pass filters, there are other types of filters, for example, bandpass - in other words, bandpass. From the name it is clear that such filters transmit well only a certain frequency band, and everything that is beyond it is suppressed by a bandpass filter (BF). The usual role of such filters is to isolate the mid-frequency range for subsequent feeding of the filtered signal to the midrange speaker. According to the task being performed, the next type of filter is a notch filter (RF) - the exact opposite of PF. The band of frequencies that the PF passes through without changes is suppressed by the notch filter, opening free access to signals outside this frequency range. Somewhat different from all the above types of filters are infra-low frequency suppression filters (FINCH); in essence, these are the same high-pass filters, but with an extremely low cutoff frequency (10-30 Hz). The purpose of the FINCH is to protect the low-frequency head (subwoofer) from infra-low frequency signals, which can lead to overload of the subwoofer, and sometimes to its failure.

Each filter is characterized by several parameters. The first parameter of the filter is its order. The filter order corresponds to the number of reactive elements in the circuit (inductors, capacitors). A first order filter, as the name suggests, contains only one reactive element. A second-order filter contains two elements, etc. Another filter indicator is directly dependent on the order - the slope of the amplitude-frequency response. This parameter shows how sharply the filter attenuates signals outside the passband, that is, signals of those frequencies that should not overcome the filter barrier and reach the speaker. The slope is measured in decibels per octave (dB/oct). An octave is a frequency band in which the upper limit frequency is twice the lower frequency. For example, an octave can be considered frequency intervals from 100 to 200 Hz or from 200 to 400 Hz. It is easy to calculate that the entire range of sound signals (20-20,000 Hz) contains approximately ten octaves. The second unit of measurement is the decibel, named after the inventor of the telephone, A. G. Bell; this is the logarithm of the ratio of quantities (in this case, the filter's transmission coefficients at the cutoff frequencies of the octave), showing the relative difference between these quantities. A difference of 6 dB means that the levels differ by a factor of two, 12 dB by a factor of four, 20 dB by a factor of ten, etc. Now, returning to the slope of the amplitude-frequency response, we note that numerically it is directly proportional to the order filter and equals 6*N, where N is the order of the filter. Obviously, the slope of the first-order filter is 6 dB/oct, the second - 12 dB/oct, the third - 18 dB/oct, etc., and the higher it is, the more effectively the filters are able to suppress unnecessary signals. When choosing a filter order, along with the shape of the amplitude-frequency characteristic, it is necessary to take into account the phase-frequency characteristic. An ideally operating crossover should provide a uniform total frequency response in terms of sound pressure, which is summed up from the vibrations created by all heads of the speaker system. When summing, both amplitude and phase relationships appear, as well as the location of the heads in relation to the listener. The optimal result is ensured by using filters of a very specific order. Interested readers can find more detailed information on this matter, for example, in the book “Radio Broadcasting and Electroacoustics” edited by Yu. A. Kovalgin, published by the publishing house “Radio and Communications” in 1999.

At the same time, the filter function is characterized not only by the order and steepness of the frequency response decline. The approximation method on the basis of which its transfer function is determined can tell a lot about the nature of the filter. There are a lot of such methods today, and they all bear the names of their creators: Butterworth, Bessel, Linkwitz-Ralley and many others. It would seem that the large number of methods means many design differences in the implementations of filters even of the same order, but nothing like that. The reactive elements that can be seen on the electrical circuits of Butterworth, Bessel, Linkwitz-Ralley filters of the same order are the same, but the ratings of these elements are significantly different, which means different behavior of the amplitude and phase-frequency characteristics of the filters. As a result, the timing characteristics are also different.

In general, all types of filters are divided into two more fairly broad classes - active and passive, and accordingly, the crossovers that include these filters can be passive and active.

Passive crossovers consist solely of reactive elements - inductors and capacitors, and do not require power. They are very undemanding, and under certain conditions they can be included in any part of the path, both before and after the power amplifier. But most often, passive crossovers are allocated a strictly defined area - between the power amplifier and the speakers. Using a crossover, it is possible to connect several heads operating in adjacent frequency bands to one amplifier. Cheap and cheerful! But there are also shadow sides. The presence of a crossover on the path between the power amplifier and the loudspeaker leads to the fact that up to ten percent of the useful energy is dissipated on the reactive elements and matching resistors. However, this is far from the only drawback of passive crossovers. We should also not forget that they have very limited capabilities for adjusting the sound, most often limited to level controls for individual frequency bands. The characteristics of passive filters depend significantly on the load resistance, which is the electrical resistance of the loudspeaker. In the operating frequency range it is very unstable, therefore optimal matching conditions cannot be ensured, and the frequency response of the filters differs from the calculated one. This also cannot be attributed to the advantages of passive crossovers.

Active crossovers in the service of automotive power amplifiers

If all the filter circuits currently used in audio equipment were built on passive elements, then most likely, over time, the copper reserves on planet Earth would be under threat. Why? Yes, because the manufacture of even the simplest first-order low-pass filter with a low cutoff frequency (100 Hz) based on an inductor requires a lot of copper wire, and not just simple, but the most serious: large cross-section, with low losses and high quality. It is unknown what we would be faced with today if several decades ago electronics specialists had not invented active filters, where bulky inductors and capacitors were replaced by electronic elements - transistors and operational amplifiers, which, when turned on in a certain way, in combination with resistors and capacitors have the same properties as LC circuits - identical phase shift between current and voltage and dependence of the transmission coefficient on frequency.

The appearance of fundamentally new filter circuits, like any other innovation in audio technology, immediately caused a lot of controversy. The main wave of criticism arose from the ranks of real audiophiles, who unanimously argued that active filters that require supply voltage are a serious obstacle to natural, natural sound. In this they were partly right, but the wide list of advantages of the newly introduced filters became a powerful argument in their favor. And soon these filters began to be actively used in built-in crossovers of car amplifiers. Such crossovers are usually located inside the amplifier housing, and their place in the signal path is at the input, immediately after the input sensitivity control circuits, before the pre-amplification circuits. It must be said that an important role in this transformation was played by the possibility of implementing active filters in minimalist dimensions, which remains a utopia for passive analogues to this day.

In budget amplifier models, built-in crossovers are based on identical filter units. This type of filter is a simplified variation of the Bessel filter; it is very simple to manufacture, because, unlike Linkwitz-Ralley, Bessel and Butterworth filters, it is built on elements of the same nominal value and is not particularly critical to tolerances for parameter deviations, which can sometimes reach many tens of percent. It is obvious that the amplitude and phase frequency characteristics of such filters are far from perfect, to say the least - they are the worst. The next pitfall that can be encountered in the design of budget-level built-in crossovers is related to the organization of the choice of crossover frequency. To reduce the cost of a crossover, many manufacturers deliberately minimize the number of tuning elements, and as a result, only one link of a second-order filter is tuned in frequency. It is clear that in this case it is quite difficult to talk about the stability of the crossover characteristics over the entire range of settings.

In mid- and high-price amplifiers, crossovers are most often implemented based on Linkwitz-Ralley, Butterworth and Bessel filters - second, third, and less often fourth order. Each of them has its own advantages and disadvantages, but, other things being equal, it is generally accepted that Butterworth filters have minimal frequency response unevenness, and Bessel filters have a phase response. In this class of amplifiers, to ensure precise adjustment of the cutoff frequency, many reputable manufacturers have adopted the so-called “click” method. Its essence is that the cutoff frequency of the high-pass filter and low-pass filter is adjusted according to a special “click-frequency” correspondence table, where, for example, the extreme left position of the potentiometer can correspond to a cutoff frequency of 20 Hz, the next - 22 Hz, etc., and the latter - five, and sometimes ten kilohertz. This tuning method is distinguished by very high accuracy of the result; it is found in amplifiers "PPI" and "Orion", etc.

A slightly different approach to setting the cutoff frequency is demonstrated by amplifiers produced by the Italian companies "Steg", "Audiosystem", as well as a number of other companies. Here, the desired cutoff frequency is selected by installing one or another resistive module-chip. This method is less universal than the one described above, however, it promises good results. A logical continuation of this approach are crossovers, in which the cutoff frequency is limited to a few fixed values. This is a fairly common solution, often found in high-end amplifiers. A good example is the many high end amplifiers from McIntosh. Here, the cutoff frequency of both filters - the high-pass filter and the low-pass filter - is fixed, and is limited to two values ​​- 80 and 120 Hz. By the way, using these amplifiers as an example, we can demonstrate the use of notch filters in built-in crossovers. In them, the notch filter is tuned to the average resonance frequency of the car interior (150 Hz) and, to some extent, allows you to correct a possible rise in the amplitude-frequency response.

A special group consists of crossovers, in which you can adjust not only the cutoff frequency of a particular filter, but in addition also the slope of the amplitude-frequency response. Such broad capabilities are in themselves a rarity, but the Japanese “hDimension” amplifiers from the “Forte” series can boast of them, in which the maximum possible value of the slope of the attenuation characteristic reaches 48 dB/oct.

Sometimes in the low-frequency section of built-in crossovers you can find a high-pass filter with an adjustable quality factor, which allows you to increase the frequency response near the cutoff frequency up to 10 dB (Hawkins circuit). This circuit design is often found in Soundstream amplifiers; it allows you to exclude a separate stage of the bass boost circuit from the settings path.

The implementation of infra-low frequency suppression filters in built-in crossovers clearly demonstrates the advantages of active filtering. Such a filter on the board of many amplifiers takes up an insignificant area, but at the same time allows you to adjust the cutoff frequency in the range from 15 to 50 Hz, and with a slope of the attenuation characteristic from 18 to 24 dB/oct. True, some manufacturers sometimes deliberately reduce customization options, limiting themselves to a few fixed, typical values. As practice shows, this is more than enough.

Conclusion

After reading this review, many readers will probably want to ask a very reasonable question: is the use of a built-in crossover in car power amplifiers justified or is it just another way of withdrawing “hard-earned” funds? In many ways, the answer to this question depends on the level of the amplifier. If the device belongs to the budget or entry-level class, then it would certainly be naive to hope that the built-in crossover will not make significant changes to the signal. It's another matter when the amplifier belongs to the middle, or even elite class. Here manufacturers are playing by different rules. The credibility of the company is at stake, and the use of poor separation filters, as well as other elements, can damage its prestige. It is quite obvious that in this case you can seriously think about using an amplifier crossover, especially since high-end amplifiers usually have very good capabilities. Naturally, such a solution will lead to the construction of an audio system based on the principle of multi-band amplification (bi-amping), which does not help save the budget, because at least four amplification channels will be needed.