EF MAX
10-08-2003, 01:07 PM
This is a frequently asked question.
To start with you have the choice of either passive cross-overs or active cross-over.
Crossovers hold a very important role in loudspeaker design and operation. Each loudspeaker transducer, whether it is dynamic, electrostatic, planar magnetic, piezo, ribbon, or plasma is capable of producing only a limited range of frequencies.
Even if a specific transducer is capable of producing sound at a given range, the efficiency, power, and response may be severely limited. In response, speaker systems are built using multiple transducers optimized for a specific frequency range, such that the system as a whole covers the entire audio spectrum.
Passive Crossovers direct the power from the incoming signal to the appropriate transducer by its frequency response characteristics. By nature, the crossover is a set of electrical filters. Crossovers also serve to protect loudspeaker transducers.
A high power bass note can destroy an expensive silk dome dynamic tweeter, even if it is not capable of reproducing the sound. The crossover prevents unwanted signals from reaching the transducer.
In audio, there are a couple of different passive cross-over designs/theories. A passive cross-over is a one using only passive components to filter out signals. These passive components are capacitors and coils.
The two most common design/theories are Butterworth and Linkwitz/Riley. By constructing cross-over networks using a number of coils and capacitors, we can achieve different cross-over slopes. The slope is at what rate we filter out the signal. The most common slopes are 6, 12, 18 or 24dBs/octave. A high slope means the signal is quickly reduced in amplitude and therefore will be more quickly inaudible.
http://www.mmxpress.com/technical/images/6db.gif
A two-way crossover will split one full range signal into two signals, one that contains low frequencies with filtered out high frequencies; we call it low-pass signal.
http://www.mmxpress.com/technical/images/12db.gif
For two speakers that will both need filtered signals, we design two-way cross-overs.
The other contains higher frequencies with filtered out low frequencies, called high-pass signal. (Well, in fact, nothing can be filtered out, it can only be reduced in amplitude at a selected ratio/slope).
Of course there are also three way and four way passive cross-overs, and these are more complex.
Because the crossover is after the power amplifier, the crossover dissipates a great deal of power.
The greater cutoff slope, the higher number of components needed and therefore even more power is wasted. Also, the tolerances of these large, high power parts are very high, which means the values are not very accurate. This leads to improperly tuned filters.
Once constructed and installed, these filters are almost impossible to change, and therefore, the system cannot be tweaked. Sometimes, L-pads are included which allow attenuation of high frequency drivers, but this just leads to more wasted power.
So many components also lead somewhat harsh and non-linear phase distortion that varies by frequency. The biggest problem with passive crossovers is that they isolate the drivers from the amplifier.
This causes two problems.
First, the impedance of the loudspeaker varies with frequency because it is a reactive device (dynamic speakers are inductors, ESL's are capacitors).
A passive crossover is designed for a resistive load that doesn't change properties due to frequency. With the load impedance changing, the response of the crossover changes, and falls rapidly away from the designed specification. The only remedy is to include an impedance-correcting network across the loudspeaker element, which serves only to waste more power and possibly introduce another source of distortion.
The second problem with isolating the loudspeaker element from the amplifier is the dampening effects of the amplifier are lost.
When a dynamic speaker cone moves, it creates inertia. This inertia will cause the cone to continue moving, even after the signal has reversed direction in a distortion causing condition called overshoot.
In order the correct this, the amplifier must supply more immediate power to keep the voltage tracking.
However, the inductors and capacitors in the crossover can store energy and effectively decouple the loudspeaker element from the amplifier. Now the amplifier does not know if the loudspeaker is accurately tracking.
This destroys transient response in dynamic speakers and has sever effects on ESL efficiency and quality.
Active Crossovers are inserted between your head unit and your amplifiers. They split the signal, sending high frequencies to one amplifier, and the low frequencies to another amplifier.
An active crossover is called active because it needs it's own power supply. An Active Crossover can be the solution to almost every problem that passive crossovers possess. Active crossovers are filters, like passive crossovers. However, active crossovers work at audio line levels before the main power amplifiers.
The output from the crossovers are sent to individual amplifiers, which amplify each frequency range independently and then connect directly to the speaker elements.
Active crossovers can be constructed without expensive and inaccurate inductors, and all the parts are small, low power, cheap and most importantly, accurate.
The advantages over passive crossovers are enormous. Because they have power, they can compensate for insertion loss.
Also, since the circuit treats all frequencies the same, there is no delay or phasing issues associated with them.
Since the output is to an amplifier, the impedance is constant. This is important also because it is very easy to create a multiple pole filter with very sharp and precise response by using multiple op-amps in a chain while maintaining a steady input and output impedance.
The properties of active crossovers can be easily changed, and depending on design, the change can be while listening to the system. This offers much greater control and customization.
Because the loudspeaker elements are directly connected to the amplifiers, the dampening factor of the amplifiers is not lost, and no power is wasted.
Even the amplifiers have an easier load because any single amplifier only has to amplify a specific range of frequencies, offering even more customization. The only tradeoff is that more than one stereo amplifier is needed or at least one amplifier channel per frequency range per channel.
Digital Options. Active crossovers still use reactive components, and therefore non-linearities in phase and/or frequency response exist. Digital technology comes to the rescue with Digital Signal Processors, capable of filtering and processing audio signals in the digital domain. The proliferation of digital technology has created a new way to process audio signals.
A Digital Signal Processor is a very specialized computer with the single task of performing rapid real time calculations on a signal. Because the processor relies on a software algorithm to work it is almost infinitely flexible.
The two issues of concern are digital conversion quality and processing delay. Although digital audio is claimed to be superior to analog systems, this is not always true.
A digital audio signal alone does not degrade in quality, but conversion to and from analog and the use of improper mathematical operators can seriously effect the quality.
Also the processor does not render an output immediately, there is an associative lag, which is increased by the time it takes to move signals to and from the digital domain.
Therefore, the use of digital technology must be questioned on four aspects: use, effects, efficiency, ease.
It’s a personal choice in respect to which road you walk down, but once you understand the differences you also need to understand where you start in respect to setting these things up.
I work from a simple starting point. Take your midrange speaker as this starting point. This speaker is important when it comes to creating a good soundstage and a good image within your stereo sound.
This speaker needs to cover as wide a range of frequencies as possible, be light and fast in it’s response and be as clear as possible when it is working hard.
I have chosen a midrange speaker in my car that is on a cast alloy chassis, has a “Hexidiamond” type cone that is 100mm in size and handles 50wrms. It operates from 55Hz – 5KHz.
The trick in choosing a midrange speaker is to pick the best you can afford,, in my car it is one of the smallest speakers in the set-up but it is also the most expensive set of speakers in my set-up, more expensive than my subs.
In choosing a midrange speaker it’s off axis reproduction is an important consideration. I prefer to work within one octave, if possible, of the quoted frequency range.
So in my case I have set my cross-over points at 125Hz and 3.5KHz. I do this as I know that the speaker can operate within this range with no problems.
I believe that this “band-passed” range of frequencies is the most important decision, and that it can be kept simple by working to the above. And the above can be done either passively or actively though active is more easily reached from a simplicity point of view.
Now that the midrange is sorted, finding a set of tweeters that can go down one octave below the mid range cross-over point,, and in my case this is a cross-over point of 3.5KHz, should be easy as most tweeters go down to about 2.5KHz or less.
If your midrange unit will play higher then this is also cool,, you cab balance this with tweeters that do not have to come down so low.
The same can be said of your mid-bass unit, covering the lower stuff should be important at least down to about 60Hz,, most mid-bass units will play high up into the midrange frequencies but this is not needed within this equationa as collectively you have all the frequencies covered and each set of speakers is working within their own ideal range.
The integration issues that so many people refer to should then be sorted and a smooth sounding front stage is now easier to reach..
If a set-up is active and if there are are gain settings for each channel, then with the help of an RTA or SPL meter, each speakers dB output can be set so as to be equal to the micophone from a centralised position within your car.
This last point is critical if you want to reach a near perfect image with most types of music.. Sometimes a little bit of tweaking in the EQ department is required as a final touch.
To start with you have the choice of either passive cross-overs or active cross-over.
Crossovers hold a very important role in loudspeaker design and operation. Each loudspeaker transducer, whether it is dynamic, electrostatic, planar magnetic, piezo, ribbon, or plasma is capable of producing only a limited range of frequencies.
Even if a specific transducer is capable of producing sound at a given range, the efficiency, power, and response may be severely limited. In response, speaker systems are built using multiple transducers optimized for a specific frequency range, such that the system as a whole covers the entire audio spectrum.
Passive Crossovers direct the power from the incoming signal to the appropriate transducer by its frequency response characteristics. By nature, the crossover is a set of electrical filters. Crossovers also serve to protect loudspeaker transducers.
A high power bass note can destroy an expensive silk dome dynamic tweeter, even if it is not capable of reproducing the sound. The crossover prevents unwanted signals from reaching the transducer.
In audio, there are a couple of different passive cross-over designs/theories. A passive cross-over is a one using only passive components to filter out signals. These passive components are capacitors and coils.
The two most common design/theories are Butterworth and Linkwitz/Riley. By constructing cross-over networks using a number of coils and capacitors, we can achieve different cross-over slopes. The slope is at what rate we filter out the signal. The most common slopes are 6, 12, 18 or 24dBs/octave. A high slope means the signal is quickly reduced in amplitude and therefore will be more quickly inaudible.
http://www.mmxpress.com/technical/images/6db.gif
A two-way crossover will split one full range signal into two signals, one that contains low frequencies with filtered out high frequencies; we call it low-pass signal.
http://www.mmxpress.com/technical/images/12db.gif
For two speakers that will both need filtered signals, we design two-way cross-overs.
The other contains higher frequencies with filtered out low frequencies, called high-pass signal. (Well, in fact, nothing can be filtered out, it can only be reduced in amplitude at a selected ratio/slope).
Of course there are also three way and four way passive cross-overs, and these are more complex.
Because the crossover is after the power amplifier, the crossover dissipates a great deal of power.
The greater cutoff slope, the higher number of components needed and therefore even more power is wasted. Also, the tolerances of these large, high power parts are very high, which means the values are not very accurate. This leads to improperly tuned filters.
Once constructed and installed, these filters are almost impossible to change, and therefore, the system cannot be tweaked. Sometimes, L-pads are included which allow attenuation of high frequency drivers, but this just leads to more wasted power.
So many components also lead somewhat harsh and non-linear phase distortion that varies by frequency. The biggest problem with passive crossovers is that they isolate the drivers from the amplifier.
This causes two problems.
First, the impedance of the loudspeaker varies with frequency because it is a reactive device (dynamic speakers are inductors, ESL's are capacitors).
A passive crossover is designed for a resistive load that doesn't change properties due to frequency. With the load impedance changing, the response of the crossover changes, and falls rapidly away from the designed specification. The only remedy is to include an impedance-correcting network across the loudspeaker element, which serves only to waste more power and possibly introduce another source of distortion.
The second problem with isolating the loudspeaker element from the amplifier is the dampening effects of the amplifier are lost.
When a dynamic speaker cone moves, it creates inertia. This inertia will cause the cone to continue moving, even after the signal has reversed direction in a distortion causing condition called overshoot.
In order the correct this, the amplifier must supply more immediate power to keep the voltage tracking.
However, the inductors and capacitors in the crossover can store energy and effectively decouple the loudspeaker element from the amplifier. Now the amplifier does not know if the loudspeaker is accurately tracking.
This destroys transient response in dynamic speakers and has sever effects on ESL efficiency and quality.
Active Crossovers are inserted between your head unit and your amplifiers. They split the signal, sending high frequencies to one amplifier, and the low frequencies to another amplifier.
An active crossover is called active because it needs it's own power supply. An Active Crossover can be the solution to almost every problem that passive crossovers possess. Active crossovers are filters, like passive crossovers. However, active crossovers work at audio line levels before the main power amplifiers.
The output from the crossovers are sent to individual amplifiers, which amplify each frequency range independently and then connect directly to the speaker elements.
Active crossovers can be constructed without expensive and inaccurate inductors, and all the parts are small, low power, cheap and most importantly, accurate.
The advantages over passive crossovers are enormous. Because they have power, they can compensate for insertion loss.
Also, since the circuit treats all frequencies the same, there is no delay or phasing issues associated with them.
Since the output is to an amplifier, the impedance is constant. This is important also because it is very easy to create a multiple pole filter with very sharp and precise response by using multiple op-amps in a chain while maintaining a steady input and output impedance.
The properties of active crossovers can be easily changed, and depending on design, the change can be while listening to the system. This offers much greater control and customization.
Because the loudspeaker elements are directly connected to the amplifiers, the dampening factor of the amplifiers is not lost, and no power is wasted.
Even the amplifiers have an easier load because any single amplifier only has to amplify a specific range of frequencies, offering even more customization. The only tradeoff is that more than one stereo amplifier is needed or at least one amplifier channel per frequency range per channel.
Digital Options. Active crossovers still use reactive components, and therefore non-linearities in phase and/or frequency response exist. Digital technology comes to the rescue with Digital Signal Processors, capable of filtering and processing audio signals in the digital domain. The proliferation of digital technology has created a new way to process audio signals.
A Digital Signal Processor is a very specialized computer with the single task of performing rapid real time calculations on a signal. Because the processor relies on a software algorithm to work it is almost infinitely flexible.
The two issues of concern are digital conversion quality and processing delay. Although digital audio is claimed to be superior to analog systems, this is not always true.
A digital audio signal alone does not degrade in quality, but conversion to and from analog and the use of improper mathematical operators can seriously effect the quality.
Also the processor does not render an output immediately, there is an associative lag, which is increased by the time it takes to move signals to and from the digital domain.
Therefore, the use of digital technology must be questioned on four aspects: use, effects, efficiency, ease.
It’s a personal choice in respect to which road you walk down, but once you understand the differences you also need to understand where you start in respect to setting these things up.
I work from a simple starting point. Take your midrange speaker as this starting point. This speaker is important when it comes to creating a good soundstage and a good image within your stereo sound.
This speaker needs to cover as wide a range of frequencies as possible, be light and fast in it’s response and be as clear as possible when it is working hard.
I have chosen a midrange speaker in my car that is on a cast alloy chassis, has a “Hexidiamond” type cone that is 100mm in size and handles 50wrms. It operates from 55Hz – 5KHz.
The trick in choosing a midrange speaker is to pick the best you can afford,, in my car it is one of the smallest speakers in the set-up but it is also the most expensive set of speakers in my set-up, more expensive than my subs.
In choosing a midrange speaker it’s off axis reproduction is an important consideration. I prefer to work within one octave, if possible, of the quoted frequency range.
So in my case I have set my cross-over points at 125Hz and 3.5KHz. I do this as I know that the speaker can operate within this range with no problems.
I believe that this “band-passed” range of frequencies is the most important decision, and that it can be kept simple by working to the above. And the above can be done either passively or actively though active is more easily reached from a simplicity point of view.
Now that the midrange is sorted, finding a set of tweeters that can go down one octave below the mid range cross-over point,, and in my case this is a cross-over point of 3.5KHz, should be easy as most tweeters go down to about 2.5KHz or less.
If your midrange unit will play higher then this is also cool,, you cab balance this with tweeters that do not have to come down so low.
The same can be said of your mid-bass unit, covering the lower stuff should be important at least down to about 60Hz,, most mid-bass units will play high up into the midrange frequencies but this is not needed within this equationa as collectively you have all the frequencies covered and each set of speakers is working within their own ideal range.
The integration issues that so many people refer to should then be sorted and a smooth sounding front stage is now easier to reach..
If a set-up is active and if there are are gain settings for each channel, then with the help of an RTA or SPL meter, each speakers dB output can be set so as to be equal to the micophone from a centralised position within your car.
This last point is critical if you want to reach a near perfect image with most types of music.. Sometimes a little bit of tweaking in the EQ department is required as a final touch.