When installing a modern stereo system in a car, the owner must choose the right crossover. This choice is quite simple if you know and understand what it is and what it is intended for, as well as in which system this device will work. So let's see what a crossover is for acoustics.
Feature, purpose
A crossover is a special equipment included in the kit, the main function of which is to prepare the desired frequency range for each speaker. As you know, any is designed for a specific operating frequency range. Out-of-range signal input to the speaker may result in sound distortion.
So, if you apply a frequency that is too low for the speaker, then the sound picture will turn out to be distorted. If the frequency is too high, then the owner of the system will be able to face not only distorted sound, but also with the failure of the high-frequency speaker. The latter simply cannot withstand such a mode of operation.
Under normal conditions, the tweeter function is to reproduce sounds only at high frequencies. The low-frequency ones work separately. Sometimes they are even installed in different places of the cabin. The same goes for midrange sounds. They are fed only to the midrange speaker.
Therefore, for high-quality reproduction of music tracks in a car, it is necessary to allocate certain frequencies and feed them strictly to specific speakers. This is what a crossover for acoustics is for.
How it works
The design of the device is quite simple. These are two frequency filters working according to the following principle. So, when the crossover frequency is 1000 Hz, one of the two filters will select frequencies that are below this value. The second filter will work with a frequency band above the mark. Filters have their own names. Low pass is designed to work with low frequencies up to 1000 Hz. The high pass will only handle frequencies above 1000 Hz.
Two-way devices operate on this principle. However, there is also a three-way crossover on the market today. The main difference here is another filter capable of handling mid frequencies in the 600 to 1000 Hz range.
More channels for filtering audio frequency and feeding them to the speakers corresponding to these frequencies leads to better sound in the car.
Technical features of crossovers
Most modern devices are inductors and capacitors. Depending on the number and quality of manufacture of these elements, the cost of the product is formed.
Why does the speaker crossover include a capacitor and a coil? These are the most basic reactive parts. They are able to process different audio frequencies without special cost.
The capacitor can isolate and process high frequencies while the inductor operates at low frequencies. Manufacturers make good use of these properties and make constructively simple, but quite effective devices.
The number of reactive parts affects the capacity of the filters: 1 - one element is used, 2 - two elements. Depending on the number of reactive parts, as well as the crossover scheme, the system filters in different ways those frequencies that are not suitable for specific channels. It can be assumed that the more reactive elements in the circuit, the better the speaker crossovers will filter the signal. Filtration schemes have a certain characteristic. This is the so-called "slope". In other words, it is sensitivity. Depending on the level of "slope steepness", all products on the market can be divided into models of the first, second, third and fourth class.
Active and passive equipment
A passive speaker crossover is the most common solution. It can often be found in the modern market. As the name implies, this device does not need additional power to operate. Therefore, it will be much faster and easier for the car owner to complete the installation of sound equipment. The disadvantage of this group of devices is that simplicity is not always a guarantee of quality.
Due to the passive circuit, the system takes part of the energy to ensure the operation of the filter. At the same time, the reactive parts change the phase shift. Naturally, this is far from the most serious drawback. However, it will not be possible to perform frequency equalization as finely as possible.
The active crossover does not have this disadvantage. The fact is that, despite the more complex design, the stream of audio frequencies in them is filtered much better. Due to the presence in the circuit of not only several coils and capacitors, but also semiconductors, the developers create high-quality devices with more compact dimensions. An active crossover is rarely found as a stand-alone module. However, any amplifier has such active filters.
How do I set up my device correctly?
In order to get the highest quality sound in the car, it is necessary to choose the right frequency at which all unnecessary will be cut off. In the case of an active device designed for three strips, you need to find two cutoff points. The first will mark the line in the range between low and mid frequencies. The second is the difference between medium and high frequency.
How to calculate with your own hands?
The crossover calculation for acoustics is an important process. Not a single manufacturer has yet been able to produce an ideal one that could reproduce sound quality in different ranges. For low frequencies, subwoofers are used. For mids, mid-range speakers are used. But when this whole complex starts to sound, then a certain confusion can arise. This is what a crossover is for in acoustics - so that a signal of only a certain frequency goes to a specific speaker system.
To obtain a two-pole system or any other, a signal dividing device is connected to the first channel of the amplifier. This is the filter. Included with the speakers are passive crossovers, manufactured and calculated by the manufacturers.
But what if you need to divide sound into frequencies according to a different principle? You don't have to count anything manually - in our high-tech time, there is software for even the simplest operations. For these calculations, there is a program, for example, Crossover Elements Calculator.
First of all, the impedance indicator of the LF and HF speakers is introduced into the program, which is often 4 Ohms. Next, enter the frequency that the device should split. The order of the crossover is immediately introduced. Then they press the button and wait until the program gives the result. As a result, it will display a diagram where the required capacitors and coils for the entered parameters will be indicated.
Features of choice
The market offers a large selection of devices that differ in quality, cost, and specific manufacturers. Choosing a crossover for acoustics is not easy - you can't just go and buy what you like. The choice is made for certain
Imagine that your subwoofer produces a low frequency in the range from 18 to 200 Hz, a mid-range speaker reproduces frequencies from 200 to 1000 Hz, and a high-frequency speaker reproduces frequencies from 1000 to 16,000 Hz. The amplifier does not have a built-in filter and reproduces frequencies in the range from 18 to 20,000 Hz. In this particular case, you need a three-way crossover capable of filtering in these ranges.
Also, when choosing, pay attention to the number of stripes. Another important parameter is the frequency range. Be sure to consider the bandwidth. Multilevel devices with high sensitivity can significantly improve sound quality.
Conclusion
So, we found out what the crossover is and what functions it performs. As you can see, this is quite an important element in the car's acoustic system.
When installing a modern stereo system in a vehicle, the owner must choose the right crossover. This is easy to do if you first familiarize yourself with what it is, what it is intended for, and in which speaker system it will work.
Purpose
A crossover is a special device in the structure of a speaker system designed to prepare the required private range for each of the installed speakers. The latter are designed to operate within certain frequency ranges. The frequency of the signal supplied to the speaker outside the range can lead, at a minimum, to distortion of the reproduced sound, for example:
- if the frequency is too low, the sound picture will be distorted;
- if the frequency is too high, the owner of the stereo system will face not only sound distortion, but also with the failure of the tweeter (tweeter). He may simply not be able to withstand this mode of operation.
Under normal conditions, the tweeter's task is to reproduce only high frequency sound, and low frequency sound, respectively, low. The midrange band is fed to the midwoofer - a speaker responsible for the sounding of midrange frequencies.
Based on the foregoing, in order to reproduce high-quality car audio, it is necessary to select the appropriate frequency bands and apply them to specific speakers. To solve this problem, a crossover is used.
Crossover device
Structurally, the crossover includes a pair of frequency filters that work as follows: for example, if the crossover frequency is set to 1000 Hz, one of the filters will select frequencies below this value. And the second is to process only the frequency band exceeding the specified mark. The filters have their own names: low-pass - for processing frequencies below a thousand hertz; high pass - for processing frequencies above a thousand hertz.
So, above was presented the principle by which a two-way crossover works. There are also three-lane type products on the market. The main difference, as the name implies, is the third filter, which processes the middle frequency band, from six hundred to five thousand hertz.
In fact, increasing the filtering channels of the sound strip, and their subsequent supply to the corresponding speakers, leads to a better and more natural sound reproduction inside the car.
Technical features
Most modern crossovers have inductors and capacitors. Depending on the quantity and quality of manufacture of these reactive elements, the cost of the finished product is determined. 
Why are coils and capacitors in a bandpass crossover? The reason is that these are the simplest reactive elements. They process different frequencies of the audio signal without much difficulty.
Capacitors can extract and process high frequencies, while coils are needed to regulate low frequencies. Using these properties wisely, the result is the simplest frequency filter. It makes no sense to delve into the complex laws of physics and cite formulas as examples. Anyone who wants to get acquainted with the theoretical foundations in more detail can easily find information in textbooks or on the Internet. It is enough for specialized specialists to refresh their memory on the principle of operation of LC-CL type networks.
The number of reactive elements affects the crossover bit rate. Number 1 denotes one element, 2 - respectively, two. Depending on the number and connection diagram of the elements, the system filters inappropriate frequencies for a specific channel in different ways.
It makes sense to assume that more reactive elements used make the filtration process better. The filtering scheme for unnecessary frequencies for a particular channel has its own characteristic, called the slope.
Filters have the inherent property of cutting off unnecessary frequencies gradually, rather than instantly.
It is called sensitivity. Depending on this indicator, products are divided into four categories:
- first order models;
- second order models;
- third order models;
- fourth-order models.
Differences between active and passive crossovers
Let's start the comparison with a passive crossover. It is known from practice that the passive crossover is the most widespread and most common type on the market. As the name suggests, the passive does not need additional power. Accordingly, it is easier and faster for the vehicle owner to install the equipment in his car. Unfortunately, speed does not always guarantee quality.
Due to the passive principle of the circuit, the system needs to take part of the energy from the filter to ensure its operation. In this case, reactive elements tend to change the phase shift. Of course, this is not the most serious drawback, but the owner will not be able to fine-tune the frequencies.
They are rarely found as separate equipment, but in any car amplifier, an active filter is present as an integral part. Due to the passive principle of the circuit, the system needs to take part of the energy from the filter to ensure its operation. In this case, reactive elements tend to change the phase shift. Of course, this is not the most serious drawback, but the owner will not be able to fine-tune the frequencies. 
Active crossovers allow you to get rid of this disadvantage. The fact is that, although they are much more complicated than passive ones, they filter the audio stream much better. Thanks to the presence of not only coils and capacities, but also additional semiconductor elements, the developers were able to significantly reduce the size of the device.
They are rarely found as separate equipment, but in any car amplifier, an active filter is present as an integral part.
We also invite you to familiarize yourself with the related topic "".
Customization features
To get a high-quality auto sound as a result, you need to choose the right cutoff frequency. When using an active three-way crossover, two cutoff frequencies should be defined. The first point will mark the border between low and medium frequency, the second - the border between medium and high. Before connecting the crossover, the car owner must always remember that it is necessary to correctly select the frequency characteristics of the speaker.
In no case should you apply frequencies to them, at which they simply will not be able to work normally. Otherwise, this will lead not only to a deterioration in sound quality, but also to a decrease in the service life.
Passive crossover wiring diagram
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Crossovers are called crossover filters for car acoustics. These filters separate the signal in multiband speakers into low, mid and high frequencies. It must be borne in mind before repairing or replacing the built-in crossover in the radio, that this is one of the key components of the car audio system along with the amplifier.
What is it and how it works
Due to their design features, the dynamics of the car radio are not able to reproduce the full spectrum of frequencies. Several speakers are installed in cars, each of which works in a limited range. The crossover in music is the main component of a multi-way audio system. Thanks to this detail, speakers designed to reproduce high frequencies receive the signal intended for them.
If such a device were not available, the tweeters could receive the signal that is intended for the subwoofer. The more channels the device supports, the more speakers you can install.
The speaker and subwoofer crossover is made of 2 main components: a capacitor and an inductor. These simplest reactive elements differentiate the signal.
Varieties and features
Crossovers that are part of car audio systems fall into 2 main categories: active and passive. The passive element is a more common variety and is more commonly found on the market. The passive device does not need additional power supply, it is easier and faster for the car owner to install it.
The disadvantage of such an element is poor quality. Due to the passive principle of operation, the device takes up part of the filter's energy in order to maintain its performance.
In this case, the reactive elements of the device operating at the limit change the phase shift, due to which the user of the audio system cannot adjust the frequencies as accurately as possible.
Active elements are free from this disadvantage. They have a more complex internal structure, and due to this, streaming audio is better filtered. In addition to coils and containers, such devices contain additional semiconductors, but at the same time they have more compact dimensions. Active elements are rarely sold as standalone equipment, but are always installed in car amplifiers.
Crossovers are devices in sound systems that create the desired operating frequency ranges for the speakers. The speakers are designed to work in a specific frequency range. They do not accept frequencies outside this range. If a low frequency is applied to the high-frequency speaker (tweeter), then the sound picture will deteriorate, and if the signal is also powerful, then the tweeter will "burn out". Tweeters should only work with high frequencies, and subwoofers should receive only the low frequency range from the overall sound signal. The remaining middle band goes to mid-range speakers (midwoofers). Therefore, the task of crossovers is to divide the audio signal into the desired (optimal) frequency bands for the respective speaker types.
Simply put, a crossover is a pair of electrical filters. Let's say the crossover has a cutoff frequency of 1000 Hz. This means that one of its filters cuts all frequencies below 1000 Hz and only passes frequencies above 1000 Hz. Such a filter is called a high-pass filter. Another filter that passes frequencies below 1000 Hz is called low-pass. The work of this crossover is graphically shown in the figure. The intersection of the two curves is the crossover cutoff frequency of 1000 Hz. Three-way crossovers also have a band-pass filter that only passes the midrange (approximately 600 Hz to 5000 Hz). The figure shows the frequency response of a three-way crossover.
The order of sensitivity is the ratio of the output signal intensity (dB) of the crossover to the frequency of the input signal, assuming the input signal intensity is constant. Usually, the sensitivity (slope) is characterized as a dB / octave ratio. For many mathematical reasons, crossover sensitivity is always a multiple of 6 dB / octave. The first order crossover has a sensitivity of 6 dB / octave. The second-order crossover has a sensitivity of 12 dB / octave, the third-order crossover has 18 dB / octave, and the fourth-order crossover sensitivity is 24 dB per octave.
Consider a third-order low-pass filter with a cutoff frequency of 100 Hz. As mentioned above, this crossover will only pass frequencies below 100 Hz, and will cut frequencies above 100 Hz. Frequencies will be cut in the following way: all frequencies above 100 Hz will lose their intensity at the output of the filter by a multiple of 18 dB, depending on the octave they enter. That is, the frequency of 200 Hz (the first octave above the cutoff frequency) will lose its intensity by 18 dB, the intensity of the frequency at 400 Hz (the second octave) will drop 36 Hz, and the third octave (800 Hz) will weaken by 54 dB. And so on, all subsequent octaves will weaken by a multiple of 18 dB. A less sensitive low-pass filter of the first order with a cutoff frequency of 100 Hz will do the same, only unnecessary octaves will be weakened not by 18 dB, but by 6 dB.
As you can see, the filters that make up the crossovers cannot immediately cut off unnecessary frequencies, but they do it gradually, with different sensitivity depending on their order.
First-order crossovers are the simplest passive crossover that consists of one capacitor and one inductor. The capacitor acts as a high-pass filter to protect the tweeter from unnecessary low and mid frequencies. The coil is used as a low-pass filter. The sensitivity of the first order crossovers is low - only 6 dB per octave. On the plus side of these crossovers, there is no phase shift between the tweeter and another speaker.
Second order crossovers. They are also called Butterworth crossovers, after the creator of the mathematical model of these crossovers. Structurally, they consist of one capacitor and coil on the tweeter and one capacitor and coil on the woofer. They have a higher sensitivity of 12 dB per octave, but give a phase shift of 180 degrees, which means that the tweeter membranes and the other speaker travel out of sync. To eliminate this problem, it is necessary to reverse the polarity of the wires on the tweeter.
Third order crossovers. These crossovers have one coil and two capacitors on the tweeter, while the opposite is true for the low frequency speaker. These crossovers have a sensitivity of 18 dB / octave and have good phase characteristics at any polarity. A negative feature of 3rd order crossovers is that it is unacceptable to use time delays to eliminate problems associated with speakers not radiating on the same vertical plane.
Fourth order crossovers. Fourth-order Butterworth crossovers have a high sensitivity of 24 dB per octave, which dramatically reduces the crossover of the speakers in the crossover region. The phase shift is 360 degrees, which effectively means no phase shift. However, the amount of phase shift in this case is not constant and can lead to unstable crossover operation. These crossovers are practically not used in practice.
Linkwitz and Riley were able to optimize the design of the fourth-order crossover. This crossover consists of two second-order Butterworth crossovers in series for the tweeter, and the same for the woofer. Their sensitivity is also 24 dB per octave, but the output signal level at each filter is 6 dB less than the output signal level of the crossover. The Linkwitz-Riley crossover has no phase shifts and allows time correction for speakers that do not operate in the same physical plane. These crossovers offer the best acoustic performance compared to other designs.
Passive Crossover Design
As mentioned above, a passive crossover consists of capacitors and inductors. In order to assemble a first-order passive crossover, it is necessary to have one capacitor and one inductor. The capacitor is installed in series on the tweeter (high-pass filter) and the coil in series on the woofer (low-pass filter). The nominal inductance values for coil ((H - microhenry) and capacitance ((F - microfarads)) are given in the table depending on the desired crossover frequency and speaker impedance.
1st order crossover (6 dB / octave)
For example, let's select the capacitance and inductance for a crossover with a cutoff frequency of 4000 Hz and a speaker impedance of 4 ohms. From the above table, we find that the capacitance of the first order capacitor should be 10 mF, and the inductance of the coil 0.2 mH.
To determine the nominal values of the components for the second order crossover (12 dB / octave), the values from the same table for the capacitor must be multiplied by a factor of 0.7, and the value for the inductor must be multiplied by the factor 1.414. Remember that a second order crossover requires two capacitors and two inductors. Let's compose a second order crossover for a cutoff frequency of 4000 Hz. To determine the values for both capacitors, multiply the value from the 10 mF table by a factor of 0.7 and get 7 mF. Next, multiply the inductance value of 0.2 mH by a factor of 1.414 to obtain an inductance value for each coil of 0.28 mH. One of these capacitors is installed in series on the tweeter, and the other in parallel on the woofer. One coil in parallel to the tweeter, and the other in series to the woofer.
Passive and active crossovers
The difference between these two types of crossovers is very simple. An active crossover requires external power supply, but a passive crossover does not. Because of this, the active crossover takes place in the sound system before the amplifier, processing the audio signal from the preamplifier of the head unit (for example, a car radio). Further, after the active crossover, two or three power amplifiers are installed. In this case, one amplifier is not installed, since it makes no sense to combine the signals separated by the active crossover in the amplifier into a single signal. Separated signals must be amplified separately. As you can see, active crossovers are used in expensive high quality sound systems.
Passive crossovers process the already amplified signal and are installed in front of the speakers. The possibilities of passive crossovers are limited compared to active ones, but their correct application can give good results with minimal financial costs. Passive crossovers work well with sensitivity orders of less than 18 dB / octave required. Above this limit, only active crossovers work well.
Passive crossovers are mainly used for signal processing of tweeters and midrange speakers. For woofers, these crossovers can be used, but the demand for the quality of capacitors and inductors is sharply increasing, which leads to their rise in price and increase in size. Passive crossovers do not tolerate overload well. The peak signal intensities from the amplifier can change the cutoff frequency of the filters. In addition, an overdriven filter attenuates the audio signal (damping). Therefore, when choosing passive crossovers, pay attention to their ability to withstand the peak loads generated by the amplifier.
Active (or electronic) crossovers are many active filters that you can control and easily change the cutoff frequency of any channel. Active crossovers can be in any order of sensitivity, from 6 dB to 72 dB per octave (or higher). In general, active crossovers for car audio systems have a sensitivity of 24 dB per octave. With this sensitivity, the exchange of frequencies between the speakers is practically impossible. The sound picture is very high quality. The only drawback of active crossovers is that they are expensive compared to passive ones.
Phase shift
Now let's talk about the phase shifts that can occur in sound systems that use crossovers. Phase shift is an inevitable phenomenon resulting from the design features of high-pass, low-pass and band-pass filters.
A phase is a temporary connection between two signals. The phase is measured in degrees from 0 to 360. If two identical speakers emit sound waves in the opposite phase (phase shift 180 degrees), then sound attenuation occurs. The problem is corrected by reversing the polarity on one of the speakers.
When a speaker system consists of different speakers operating in different frequency ranges (tweeter and midwoofer), phase shift elimination is not always solved by simply changing "+" to "-". The wavelength from the tweeter is shorter than from the midwoofer. Therefore, the front of the high-frequency wave can reach the listener later (or earlier) than the front of the mid-frequency (or low-frequency) wave. This time delay is due to the phase shift. In this case, you can optimize the sound picture by physically aligning the two speakers relative to each other in the vertical plane until the sound picture is improved. For example, at a wavelength of 1000 Hz, a time delay of one millisecond is eliminated by shifting the speakers 30 cm relative to each other.
Active crossover setting
The most important thing in crossover tuning is choosing the right cutoff frequency. If we have a three-way active crossover, then we are faced with the task of determining two points (frequencies) of the cutoff. The first point defines the cutoff frequency for the subwoofer (low-pass) and the beginning of the mid-range for the midwoofer (high-pass). The second point defines the frequency of the end of the mid-range (low-pass) and the starting frequency of the high-frequency range for the tweeter (high-pass). Most importantly, when setting the crossover cutoff frequencies, remember the frequency characteristics of the speaker and in no case do not load the speaker with frequencies that are not included in its operating range.
For example, if the subwoofer rattles a little or emits a hum, then it is overloaded with undesirable mid frequencies (above 100 Hz). Move the cutoff frequency (low-pass) to 75 Hz and / or set the sensitivity to 18 dB or 24 dB per octave if possible. Recall that increasing the order of the crossover sensitivity (dB / octave) cuts unnecessary frequencies better, preventing them from leaking through the filter. The order of sensitivity of the high-pass filters for the midwoofer can be left at 12 dB / octave (for “soft” mid-range speakers). This active crossover setting is called asymmetrical.
This table shows the initial values for the cutoff frequencies for various speaker types when setting the active crossovers.
What is a crossover and why is it needed?Before answering this question, you first need to take a little digression into loudspeaker theory and outline the problem in general terms. As you know, almost any of the currently produced speakers is able to effectively reproduce 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 (mass, stiffness). Outside this frequency band, the sound pressure generated by the speaker is significantly reduced, and the level of distortion increases. At the same time, it is impossible to talk about high-quality sound. Therefore, multiple speakers must be used to transmit the entire spectrum of audio signals (20-20,000 Hz). Long ago, acoustics specialists recognized this need, and today in all areas of audio technology, be it home or car systems, the vast majority of acoustic systems are implemented exclusively in a multi-emitter scheme.
With regard to car audio systems, two fairly typical construction schemes can be distinguished, with which even the slightest informed readers are well familiar. The first and most common consists of three speakers: a subwoofer aimed exclusively at the bass (approximately 20-100 Hz), a woofer / midrange for the upper bass and midrange ranges (100-3000 Hz), and a tweeter responsible for high frequencies (from 3000 Hz and above). In more complex layouts, such as those represented by professional car audio competitions, the number of speakers is increased. Here, for each frequency range: low bass, mid / high bass, mid frequencies and high, separate speakers are responsible. But, despite the obvious differences, both schemes obey the same requirement: each speaker included in the speaker system must reproduce only its own frequency band and not affect the neighboring ones. To fulfill this requirement, electrical filters are included in the audio path, which are precisely engaged in the selection of some frequency bands and suppression of others. Obviously, if the speaker system uses several speakers - subwoofer, bass / midrange, midrange and tweeter, it becomes necessary to use several electrical filters. The combination of several of these filters is called a crossover.
Filters
In the first approximation, any electrical filter is a collection of several elements that have the property of selective transmission of 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 their constituent elements on frequency: for inductors, the resistance increases with increasing signal frequency, while for capacitors, on the contrary, it decreases. Therefore, inductors pass low frequencies well, and capacitors - high frequencies. These properties are used to build filters - low pass (LPF) and high pass (HPF). In addition to low-pass and high-pass filters, there are other types of filters, for example, band-pass - in other words, band-pass. From the name it is clear that such filters pass well only a certain frequency band, and everything outside it is suppressed by the bandpass filter (BPF). The usual role of such filters is to isolate the mid-frequency range for the subsequent delivery of the filtered signal to the midrange speaker. According to the task being performed, the next type of filter - notch (RF) - is the complete opposite of the PF. The notch filter suppresses the frequency band that the PF passes without changes, opening up free access to signals outside this frequency range. The filters for suppression of infra-low frequencies (FINF) stand somewhat apart from all the types of filters listed above; in fact, 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 signals of infra-low frequencies, which can lead to overloading of the subwoofer, and sometimes to its failure.
Each filter has several parameters. The first filter parameter is its order. The order of the filter 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. In direct relation to the order, there is another filter indicator - the slope of the amplitude-frequency characteristic. This parameter shows how sharply the filter attenuates the signal outside the passband, that is, the signals of those frequencies that should not overcome the filter obstacle and get to the speaker. The slope is measured in decibels per octave (dB / oct). An octave is a band of frequencies in which the upper cutoff frequency is twice the lower one. 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 audio signals (20-20,000 Hz) contains about ten octaves. The second unit of measurement is the decibel, named after the inventor of the telephone, A. G. Bell; it is the logarithm of the ratio of the quantities (in this case, the filter gains at the cutoff frequencies of the octave), showing the relative difference between these quantities. A difference of 6 dB means that the levels differ twice, 12 dB - four times, 20 dB - ten times, etc. Now, returning to the slope of the frequency response, we note that it is numerically directly proportional to the order filter and equals 6 * N, where N is the filter order. Obviously, the slope of the first-order filter is 6 dB / oct, the second is 12 dB / oct, the third is 18 dB / oct, etc., and the higher it is, the more effectively the filters are able to suppress unnecessary signals. When choosing the order of the filter, along with the shape of the amplitude-frequency characteristic, it is necessary to take into account the phase-frequency characteristic. A perfectly working crossover should provide a uniform total frequency response in terms of sound pressure, which is the sum of the vibrations created by all the loudspeaker heads. When summing up, both amplitude and phase relationships appear, as well as the position of the heads in relation to the listener. The best result is provided by using filters of a well-defined order. Interested readers can find more detailed information on this subject, for example, in the book "Radio Broadcasting and Electroacoustics" edited by Yu. A. Kovalgin, published by the "Radio and Communication" publishing house in 1999.
At the same time, the filter function is characterized not only by the order and slope of the frequency response. 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 the creators: Butterworth, Bessel, Linkwitz-Relay and many others. It would seem that the large number of methods means many design differences in the filter implementations, even of the same order, but nothing like that. The reactive elements that can be seen on the electrical circuits of Butterworth, Bessel, Linkwitz-Rayleigh filters of the same order are the same, but the ratings of these elements differ significantly, which means different behavior of the amplitude and phase frequency characteristics of the filters. As a consequence, the timing is also different.
In general, all types of filters are further subdivided into two rather extensive classes - active and passive, and, accordingly, the crossovers that include these filters can be passive and active.
Passive crossovers consist exclusively of reactive elements - inductors and capacitors - and do not require power. They are very undemanding, and under some conditions they can be included in any section of the path, both before and after the power amplifier. But more often than not, passive crossovers are assigned a strictly defined section - between the power amplifier and the loudspeakers. Using a crossover to one amplifier, it is possible to connect several heads operating in adjacent frequency bands. Cheap and cheerful! But there are also shadow sides. The presence of a crossover in 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 terminating resistors. However, this is far from the only drawback of passive crossovers. Do not forget also that they have very modest possibilities for adjusting the sound, most often limited to level controls for individual frequency bands. The characteristics of passive filters significantly depend on the load impedance, which is the electrical impedance of the loudspeaker. In the operating frequency range, it is very variable, therefore, it is not possible to provide optimal matching conditions, and the frequency response of the filters differs from the calculated one. This also cannot be attributed to the merits of passive crossovers.
Active Crossovers Serving Automotive Power Amplifiers
If all filtering circuits currently used in audio technology were built on passive elements, then most likely, after some time, the reserves of copper 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 a simple one, but the most serious one: a large section, with low losses and high quality. It is not known what we would have faced 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 combination with resistors and capacitors have the same properties as LC-circuits - identical phase shift between current and voltage and the dependence of the transmission coefficient on frequency.
The appearance of fundamentally new filtering circuits, like any other innovation in audio technology, immediately caused a lot of controversy. The main wave of criticism originated in 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 appeared filters became a weighty argument in their favor. And soon these filters began to be actively used in built-in crossovers of car amplifiers. These crossovers are usually located inside the amplifier cabinet, and their place in the signal path is at the input, immediately after the input gain 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 for passive analogs remains a utopia to this day.
In budget models of amplifiers, built-in crossovers are based on the same filtering links. This type of filter is a simplified variation of the Bessel filter; it is very easy to manufacture, because, unlike the Linkwitz-Rayleigh, Bessel and Butterworth filters, it is built on the same nominal elements and is not particularly critical to the tolerances for the deviation of parameters, which can sometimes reach many tens of percent. Obviously, 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 found in the performance of budget-level built-in crossovers is related to the organization of the crossover frequency selection. To reduce the cost of the crossover, many manufacturers deliberately minimize the number of tuning elements, and as a result, only one section of the second-order filter is tuned in frequency. It is clear that in this case it is rather difficult to talk about the stability of the crossover characteristics in the entire range of settings.
In amplifiers of the middle and high price category, crossovers are most often implemented on the basis of Linkwitz-Relley, Butterworth and Bessel filters - of the second, third, 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 - phase response. In this class of amplifiers, in order to ensure precise adjustment of the cutoff frequency, many reputable manufacturers have adopted the so-called "click" method. Its essence lies in the fact that the cutoff frequency of the HPF and LPF is adjusted according to a special table of correspondence "click-frequency", where, for example, the extreme left position of the potentiometer can correspond to the cutoff frequency of 20 Hz, the next - 22 Hz, etc., and the last - five, and sometimes even ten kilohertz. This tuning method has a very high accuracy of the result, it is found in amplifiers "PPI" and "Orion" and others.
Amplifiers manufactured by the Italian firms "Steg", "Audiosystem", and also a number of other companies demonstrate a slightly different approach to tuning the cutoff frequency. Here, the desired cutoff frequency is selected by installing one or another resistive chip module. This method is less versatile than the one described above, but nevertheless 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. Many high end amplifiers from McIntosh are good examples. Here the cutoff frequency of both filters - HPF and LPF - is fixed and limited to two values - 80 and 120 Hz. By the way, using these amplifiers as an example, you can demonstrate the use of notch filters in the 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 the possible rise in the amplitude-frequency characteristic.
A special group is made up of crossovers, in which you can adjust not only the cutoff frequency of a particular filter, but also the slope of the amplitude-frequency characteristic. Such wide possibilities in themselves are a rarity, but they can boast of Japanese amplifiers "hDimension" from the "Forte" series, in which the maximum possible value of the attenuation slope reaches 48 dB / oct.
Sometimes in the low-frequency link of the built-in crossovers, you can find a high-pass filter (FINC) with an adjustable Q-factor, which allows you to get a rise in the frequency response near the cutoff frequency up to 10 dB (Hawkins circuit). Such a circuit design is often found in Soundstream amplifiers, it allows you to exclude a separate stage of the bass boosting circuit from the tuning path.
The implementation of infra-low pass filters in the built-in crossovers clearly demonstrates the benefits of active filtering. Such a filter on the board of many amplifiers occupies an insignificant area, but at the same time it allows you to adjust the cutoff frequency in the range from 15 to 50 Hz, and with an attenuation slope from 18 to 24 dB / oct. True, some manufacturers sometimes deliberately reduce the setting 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 quite reasonable question, is the use of a built-in crossover in car power amplifiers justified, or is it another way to take away "back-breaking" 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 class, then it will certainly be naive to hope that the built-in crossover will not make significant changes to the signal. It is another matter when the amplifier belongs to the middle and even elite class. Here manufacturers are already playing by different rules. The credibility of a firm is at stake, and the use of poor crossovers, as well as other elements, can damage its prestige. It is quite obvious that in this case you can already seriously think about using an amplifier crossover, especially since high-quality amplifiers have very good capabilities, as a rule. Naturally, such a solution will lead to the construction of an audio system based on the principle of multiband amplification (bi-amping), which does not help to save the budget, because at least four amplifying channels will be needed.
