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= Introduction = <Paul Spencer> A crossover is a series of filters which seek to optimise the performance of a loudspeaker. One of the main functions of a crossover is to divide the signal to be reproduced by each driver. While this part alone is more involved than it may sound, there is a lot more to crossovers than this. There are two main types of crossover: 1. Passive crossovers: This is the most common type. After the signal has been amplified, the signal is divided by passive components to the drivers. A passive crossover consists of resistors, capacitors and inductors. 2. Active crossovers: An active crossover is inserted in-between the pre and power amps and uses active circuits. Each output from the crossover needs to then be amplified, hence a 2 way active crossover requires two power amps. If you have an integrated amp with pre-out and main-in connections, you merely need to add on another power amplifier. === Active vs passive crossovers: which is better? === As with many issues in audio, there isn't a single approach which is embraced by all as the best solution. I'll present three common views: 1. Passive is best: When cost is considered in the equation, a well designed passive speaker driven, the best you can afford with good quality amplification will provide the best solution. Those that hold to this view are likely to focus more on high end amps and source components, hence the cost of buying more power amplifiers is prohibitive. Further, there are those that believe the active circuits are more intrusive than passive components. 2. Active is best: An active system will optimse a speaker system far better, so that the full capabilities of the amplifiers and speakers can be experienced. While more amplifiers are needed, the available power is used more efficiently and not wasted, hence you are not paying for power that is being wasted. Some speakers can't be properly optimised with passive crossovers. 3. Combination: There are some which view active as the ideal, however, a compromise can often be more cost effective. The simplest and most cost-effective active implementation is to add a 2 way active crossover to a 3 way speaker, where the passive crossover between the tweeter and mid drivers is retained, but the mid crosses actively to the woofer. This is also a good upgrade to a 2 way passive speaker, which many will find more musically satisfying than adding a subwoofer. My own position on this is that an active system if done right should always perform better. However, an active system will often be beyond the skills of a beginner to design properly. In this case, it may be wise to start with a 2 way passive speaker that someone else has already designed. From there you can experiment with active crossovers and filters, however, it's best to have something that works simply first, then move on from there. === Digital active crossovers: the way of the future? === A very appealing new option has arrived - digital active crossovers. For the beginner, there is the appeal of being able to experiment, and not committ to a design which requires parts to be chosen and soldered together. Consider a popular diy unit, Behringer Ultradrive DCX2496: [Behringer Ultradrive DCX2496] Product website It has a number of features which aren't possible on any other type of crossover: * adjustable delays * manual/auto correction for room temperature, phase and arrival time differences * digital delay for each channel * different settings can be changed in a quick A/B comparison Other main features: * with selectable roll-off characteristics from 6 to 48 dB/octave * several types of EQs (LP/BP/HP) for each input and output * dynamic EQs for level-dependent equalization * parametric EQs A nice feature of such a unit for the diy enthusiast is that you can reconfigure for future projects. When you are finished with a passive speaker, your investment in passive components may be lost, unless you are able to sell for a good price. There are other units which are considerably more sophisticated, at greater cost. At this price, this unit is cheaper than most software programs you would need to design a passive speaker, unless you opt to navigate through the maze of working out how to operate Speaker Workshop! === So what is so good about active crossovers? === There is a very good article on this topic at Elliott Sound Products: [Benefits of Bi-amplification] The article sums up some of the main advantages of active crossovers: * Effectively (up to) twice the "real" power of the amplifiers themselves * Reduced intermodulation distortion * Accurate crossover points more readily achieved * Greater linearity * Easier load for amplifiers * No padding is required to align the driver sensitivities, so we are not simply wasting power * The damping factor is greatly improved for both the low and midrange loudspeakers * The flexibility to choose amplifiers which are at their best within a defined frequency range More exotic speaker types such as horns and dipoles are very difficult to optimse with passive crossovers, and in many cases not practical. === Crossover basics === There are some very good articles on the topic on the ESP website: *[Passive crossover design] *[Effects of impedance on loudspeaker drivers] *[Baffle step compensation] There are three types of filters: *high pass (HP): passes high frequencies, but filters out anything below the corner frequency (fc) *low pass (LP): passes low frequencies, but filters out above fc *band pass (BP): passes frequencies in between the HP and LP filters grouped together All filters have a centre or corner frequency (fc). They may have a positive or negative gain. They also have a slope which may be described an order: *1st order filter: 6db/octave *2nd order filter: 12db/octave *3rd order filter: 18db/octave *4th order filter: 24db/octave Other common filter orders are 6 & 8. Filters with steeper slopes are rare, although some active digital filters go as high as 300db/octave Crossovers are also classified by different alignments: ||Filter||Characteristics||Q||Comments|| ||Butterworth||Maximally flat amplitude||0.707|| || ||Bessel||Maximally flat phase||0.5-0.7||Fastest Settling time|| ||Chebyshev||Fastest rolloff||0.8-1.2||Slight peaks/dips|| === Linkwitz Riley 24db/octave active crossovers === This is popular for active crossovers because if used with suitable drivers, the result is a flat frequency response with a phase coherent crossover. However, you can't simply replace your passive crossover with an off the shelf system like this and expect it to sound better than a decent passive crossover. The main reason for this is that the drivers must each have two octaves either side of the crossover point in order for it to be phase coherent. Consider a three way speaker with crossover points at 200 Hz and 2.4kHz. The tweeter must be flat down to 600 Hz and the mid must be flat up to 9.6kHz! Further, it must be flat down to 50 Hz and the woofer must be flat up to 800 Hz! A crossover like this could only work with a midrange driver with a very smooth and extended response and it could not be used in a sealed chamber. The tweeter would need to be able to operate lower than virtually any tweeter can handle. The alternative approach is to design filters to actually work with the acoustic roll-off of the drivers. Suppose you want to integrate a woofer with a 2 way passive speaker. The simplest approach would be to use a sealed box for the main speaker. If it has a typical 6.5" midbass hifi driver, it will most likely achieve an F3 (-3db point) at 80 Hz with a 2nd order (12db/octave) rolloff. You could then add a low pass filter to the woofer with a slope and corner frequency to match the acoustic rolloff. A slightly more complicated but better performing alternative would be to put a 2nd order high pass filter on the midrange driver with an fc at 80 Hz. The combined roll-off will then be 4th order. Then you put a 4th order Linkwitz Riley low pass filter on the woofer. The result is a correctly implemented Linkwitz Riley crossover. If you wanted to achieve this with the textbook approach, the mid would have to be flat down to 20 Hz! You would have to select a higher crossover point of say 200 Hz and you would need some kind of loading to the mid so that it will be flat down low enough. Procedure: Design crossover (2 way) Choose mid-bass driver Obtain T/S parameters Calculate box size Calculate port size Calculate BSC frequency Obtain frequency response graph Choose crossover point Decide crossover order Obtain impedance curve Decide whether Zobel required Calculate crossover parts values Choose suitable tweeter Obtain frequency response graph Decide crossover order Obtain impedance curve Decide whether Zobel required Calculate crossover parts values Calculate L pad values Procedure: Design crossover (3 way) Choose mid Obtain T/S parameters Calculate Box size Obtain frequency response graph Choose crossover points Decide crossover order Obtain impedance curve Decide whether Zobel required Calculate crossover parts values (Calculate L pad values – can be avoided if sensitivity between Woofer & mid chosen carefully) Choose suitable tweeter Obtain frequency response graph Decide crossover order Obtain impedance curve Decide whether Zobel required Calculate crossover parts values Calculate L pad values Choose suitable Woofer Obtain T/S parameters Calculate box size Calculate port size Calculate BSC frequency Obtain frequency response graph Decide crossover order Obtain impedance curve Decide whether Zobel required Calculate crossover parts values Getting started in Crossover Design The basics (You've got to understand the rules before you can break them :-) https://sound.westhost.com/lr-passive.htm choosing xover point: https://www.speakerbuilder.net/web_files/Articles/xover%20article/xpointmain.htm Baffle Step Compensation: https://sound.westhost.com/bafflestep.htm Getting it done: Jay has a page on designing xovers using manufacturers specs without measuring: https://www.geocities.com/woove99/Spkrbldg/DesigningXO.htm Calculators: https://ccs.exl.info/calc_cr.html#second https://www.sengpielaudio.com/calculator-Lpad.htm box calculator: https://www.linearteam.dk/default.aspx?pageid=winisdpro Measurement freeware: Synrta - https://libinst.com/SynRTA.htm ARTA https://www.fesb.hr/~mateljan/arta/download.htm ARTA Jig - https://zobsky.blogspot.com/2008/01/simple-loudspeaker-measurement-jig-for.html Examples from the designs of others can be quite instructive: https://www.zaphaudio.com/ https://www.troelsgravesen.dk/Diy_Loudspeaker_Projects.htm https://www.rjbaudio.com/projects.html https://www.humblehomemadehifi.com/ = Related DIYAudio Wiki Pages = (Crossover Table)? = Books = (Include recommended books here) = Links = |