Talk:Audio crossover

message: It seems that there's a little confusion here. AFAIK Bessel filters don't have a -6dB gain at cutoff, and their sum is not flat, it has a little dip instead. The -6dB cutoff freq and flat summed response is the characteristic of Linkwitz-Riley filters. Refer to: http://www.rane.com/note147.html scoofy 10:18, 7 May 2006 (UTC)
replies: text: The sallen-key unity gain filter has a Q of 0.5 and this is LR2...however Bessel filters also have a Q of 0.5... Rohitbd 12:34, 7 May 2006 (UTC)
replies: text: You're wrong. Bessel filter has a Q of approximately 0.58. References:Active Lowpass Filter Design (look at Table 2.), Another reference Third reference Fourth reference . This page says Bessel filters have a Q factor of 0.5 to 0.7, with a remark that a 'true' Bessel filter has a Q of 0.57. So we might say that a filter with a Q of 0.5 is a Bessel-like filter. You're right in the sense that the LR filter has a character very similar to the Bessel character, which have almost the same gain at and the sum of the outputs is also nearly flat, and both filters are linear phase in the passband. Regards scoofy 12:56, 7 May 2006 (UTC)
replies: text: Yes, I did mix up LR2 & Bessel - I have made this change in the article. Rohitbd 08:25, 8 May 2006 (UTC)
replies: text: I think, we should not confuse LR2 with Bessel. A Bessel filter is a filter with a transfer function H(s)=K0/Ben(s/ω0), where Ben is the nth Bessel polynomial, meaning that a 2nd order Bessel filter has a transfer function H(s)=3/((s^2)+3s+3) (reference1) (reference2), which can be realized by a two pole filter with a Q of 0.577. This, and only this is a Bessel filter, anything else is not a Bessel filter, in the strictest sense. Besides this, a common definiton found on the net (which may be the source of confusion) is that a Bessel filter is filter with a linear phase resonse and a constant group delay. However, this is only partially true, Bessel filters are filters which at the same time have the maximally flat amplitude response retaining a linear phase response and a constant group delay. (ref) This is not true of LR2 filters, which have a softer "knee" at the cutoff frequency. I suggest that we don't mix up these two, because this is an encyclopedia, and many people will use it as a reference, so we shouldn't include an information that is not true. (Of course, we can include a remark that in common practice,  sometimes all linear phase filters are called Bessel filters, but we shouldn't mix them up in the article.) It is also a very wrong idea to say that a filter with a Q of 0.7 is a Bessel filter, because practically it is a a Butterworth filter (Butterworths have a Q of 0.707), so it doesn't have any of the typical Bessel character (it has a peak in the group delay, it's phase response is non-linear). Regarding the peak/dip question: The summed output of the Bessel filters can have a peak, a dip or a ripple depending on how you mix the two, whether you invert the phase of one output or not, and what kind of frequency normalization you use. Normalisation means you scale the cutoff frequency of the filters by a certain factor, so the cutoff freq of the LP and HP filter gets closer or further from each other. Possible summed outputs can look like: graph1 graph2 graph3 graph4. The last graph shows what happens if you use different normalisation values, which can be shown on a 3d graph: graph5. Probably the "phase match" graph is the most appropriate regarding crossovers, which - truly - has a dip. You should note that none of these graphs look flat, which is another reason not to say this graph depicts a Bessel filter. Regards scoofy 11:00, 9 May 2006 (UTC)
replies: text: Yes, differentiation should be made between generalised bessel, true bessel & LR types, but going into too much detail may be counter-productive especially since the filters have their own articles and the links in the references section should provide ample material for those who want to delve deeper into it. I would also recommend some simplification/shortening of the article where we just touch on the filters (Butterworth/Bessel/LR and link to the respective articles) and give a brief account of their salient characteristics from the point-of-view of use in audio and leave it at that. Rohitbd 11:15, 9 May 2006 (UTC)
replies: text: Agree with that. I'll look at it later and think of how could it be reorganized. scoofy 11:42, 9 May 2006 (UTC)
replies: text: Reorganized article. Removed Classification by filter type, put the relevant information into the appropriate sections. Also moved a lot of stuff from the Overview. I think the Overview is still a little bit fuzzy, we could possibly have an overview where first: we discuss what does a crossover network consist of (lowpass & highpass filters) (not going into details about filters and filter characteristics) and then second: get some overall info on filter characteristics and what kind of filters are best as crossovers. I think some info could be moved to the latter sections (eg info on 2nd order Butterworth filters could be moved tho the Second order crossovers section. Probably this would clean up the article a little bit. scoofy 23:20, 9 May 2006 (UTC)
replies: text: Thanks Scoofy for all your efforts, it is highly appreciated !! :-) Rohitbd 08:32, 10 May 2006 (UTC)
thread_title:  Bessel is not L-R 

message: I uploaded an SVG image of a comparison of Butterworh and L-R crossovers, which does not appear correctly. However, when I download the  imageit appears correctly in my browser. Any suggestions? scoofy 11:33, 7 May 2006 (UTC)
replies: text: Even I cannot see it. Perhaps changing the format to GIF/PNG will work. Rohitbd 12:37, 7 May 2006 (UTC)
replies: text: I uploaded it in PNG. It is fine now.scoofy 13:20, 7 May 2006 (UTC)
thread_title:  SVG image does not appear correcty 

message: I'm by no means an expert in this area, but this statement strikes me as incorrect (or at the very least, misleading):
:This means that an active crossover based system will often cost more than a passive crossover based system, although no one of the amplifiers needs to provide as high an output as a single full-frequency, power amplifier.
Isn't it true that the vast majority of power consumed by a loudspeaker is in the reproduction of low frequencies? If so, it seems to me that this statement is more or less incorrect.
Think about it this way...you have one loudspeaker with a passive crossover powered by a 120W amp. 99% of the power is consumed by the woofers while less than 1% of the power is consumed by the tweeter. If you were to replace the passive crossover with an active crossover containing two amplifiers, the low frequency amp would have to very, very close to 120W...so close it may as well be a 120W amp.
Yes? No? Do I have this totally wrong?
:I think the statement in the article is technically correct and that you are more or less correct in a practical way, if you restrict the fact situation to your example, which implies a loud bass signal compared to the remainder of the musical program.  However, in other situations, there may be no comparatively large bass component in the signal.  In such cases, the ratio of power consumed between the woofer and, say, the mid range speaker will be more equal. Anoneditor 16:58, 31 January 2007 (UTC)
thread_title:  Error in Active Crossover Section? 

message: I deleted the last paragraph of this section (as of April 4) for two reasons:
*First, it offers a point of view that is highly disputed in audiophile circles, i.e. that passive crossovers are preferred. In fact, the author's recounting of how difficult it is to properly mate a passive crossover to a specific speaker testifies against the practicality of the notion.
*Second, I think it's wrong in its statement that it is impossible for an active Butterworth filter to be designed without ripple. What's the source for this statement?  Also, why does the author believe that in practice it is possible to create a passive Butterworth crossover that hits the theoretical transfer function exactly.  This seems problematical given the inevitable component and measurement tolerances and the problem of mating the crossover to the speaker? Anoneditor 23:37, 4 April 2007 (UTC)
replies: text: Theoretically, odd order Butterworth filters (no matter active or passive) sum flat without any ripple. For this reason, 3rd order Butterworth is used in crossovers. For even orders, flat summing is problematic, so L-R is preferred. scoofy 14:36, 7 April 2007 (UTC)
thread_title: Passive Crossovers

message: I think we should add some more information about the comparsion of different filters (analog or IIR). The short graph at the introduction gives only a view information about the most common L-R and butterworth filters.
I think about something like
*http://www.maazl.de/electronic/LS/images/lowpass-response.png - Amplitude response of different lowpass-filters and
*http://www.maazl.de/electronic/LS/images/lowpass-delay.png - normalized group delay of the filters. The real group delay is the value from the graph divided by the crossover frequency (exactly the -3dB frequency).
Of course, even more information should be supplied, especially about filter matching (Phase, Amplitude, Power). This part is related to the above discussion about the relation of drivers and crossovers.
I think this article is the right place for informations like these. If there is nothing to be said against it, I would start to add some information about different low and high pass filters. If it grows too much it might be separated in it's own page. (Marcel Müller)  —Preceding unsigned comment added by Marcel Müller (talk • contribs) 10:37, 14 September 2007 (UTC)
thread_title: Filter Comparsion

message: Posting explanations of the tweaks here...
"Networks are not only frequency specific, but also impedance specific."
yes
"Passive crossovers are not generally interchangeable between different speaker systems."
not true. If you want the best possible performance, the whole speaker system will be designed together. However generic crossovers still give good performance, and I've used many of them. Millions of generic crossovers have been made and sold, and plenty of people listen to the results happily enough.
"at high voltage and current levels."
usually high current only, high voltage speakers using crossovers are somewhat the exception, so changed phrasing. 
I removed the statement that crossover noise was only amplified in some cases, as it inevitably always is. There is no way to avoid that. As for noise figure (and any other specs) sometimes being unacceptable, surely thats true for all audio system specs, and I would think everyone and their dog knows that, its why there are (consumer) audio systems on the market with widely varying specs and prices.
"Generally, active crossovers require use of tweeter protection (typically a small capacitor) since the tweeter is now directly connected to an amplifier and may be damaged due to DC in that output, or a short thump produced when an amplifier turns on or off."
This one's a bit more complex. Caps are often used but not for the above reasons.
Tweeters require protection against dc no more or less than other speaker drivers. Biamping actually reduces the risk of tweeter frying from a faulty amp, since a 10w tweeter on a 10w amp is less susceptible than the same tweeter on a 50w amp. 
Thump is low frequency, and the crossover hf channel will reject it due to its frequency content. There is no more than trivial high frequency content in power-up conditions; if there were, end users would find the experience very unpleasant and consistently reject the equipment as faulty. If you've ever experienced a bad amp design that does hit the tweeter with large amplitude hf during start up you'll know what I mean!
The use of a series cap is primarily a question of amplifier design, many amps use them, many dont. A relatively small cap on the tweeter is a simple way to knock out hum and cut s/n figure by wiping out out of band noise.
"reduction in power amplifier cost and output requirement."
Use of 2 amps, one of which is of nearly as much power as the single amp would be, does not reduce cost, quite the opposite. This is precisely why its not popular. If it reduced costs, all mass market manufacturers would be biamping.
"while the whizzer, which is directly coupled to the voice coil, responds only to the rapid movements of the coil at higher frequencies."
The whizzer is hard coupled to the coil, thus it unavoidably responds to all frequencies the speaker handles. Useful output only occurs at higher frequencies, not because the whizzer doesnt respond, but because its small diameter makes its lf output of relatively low amplitude. (For a given spl a lf requires a much larger diaphragm than hf)
"argue that a single diaphragm that must produce both low and high frequencies does neither justice. "
Since around 99.9% of hifi speaker cabs avoid the inherently cheaper full range unit approach, it seems there are almost no designers that would diagree with this view. IOW its more accepted than POV, those that vote for full range units are a tiny fringe only, and not any part of mainstream design.
Tabby (talk) 19:40, 17 January 2008 (UTC)
thread_title:  Tweaks 

message: I've moved this here for discussion as its not really scientificaly valid, tho some do claim it as fact:
"For instance, use of non inductive resistors and special capacitor components adds significant cost, although they have some sonic benefits."
The inductance of wirewound Rs used in crossovers is truly miniscule:
* bear in mind such Rs are always counterwound to wipe almost all trace of inductance measurable at audio frequencies
* compared to the inductance tolerances elsewhere in the crossover, any R inductance is a non-event.
...Yes and no. Non-inductive resistors are usually of the CERMET type and thus offer very low noise and thermal stability. This was very relevant in the era of carbon and carbon film resistors (some of which were meander resistors with carbon film on them). But current SMD resistors are rare metal film types, so, if you don't need the power rating, any SMD resistor will do.
Re special capacitors, again there really is no science that shows any improvement for 'special' cap types. Yes one can go wrong, but small caps across long life electrolytics is about as good as one can get, and neither is a special component.
...yes and no. In the critical parts of any amplifier, a PS (polystyrene capacitor) was specified, as other types at that era had significantly higher dielectric absorption, which lead to hysteresis, and thus higher THD. Today, you can pick up inexpensive (relatively inexpensive, that is!) polypropylene capacitors of several types (metal film, foil film/foil) which have excellent dielectric characteristics and are used in critical parts of A/D samplers, timing, measuring, instrumentation and of course, audio.
Widespread shortage of technical knowledge and marketing spin makes a ready market for components with tall tale marketing spiel. ...yes, there are some unbelieveable stories out there, like folks selling "capacitor" to audiophiles who did not know that capacitor is the same as a condenser (local name). But in the area of precision electronics, precision components are and will be costly. It all depends on what the desired outcome should be and what the limiting components for such design are. But then, if you are looking for expensive components and can't mathematically reason why you need this or that, you are not likely fit to be the one doing the component selection anyway.
The one area in passive crossovers where component quality is significant is with inductors, not Cs or Rs.
Tabby (talk) 06:47, 21 January 2008 (UTC)
thread_title:  Components: fact or marketing 

message: I'm going to suggest including discussion of the single capacitor passive speaker filter. Although its not a crossover on its own, when combined with the falling response of the woofer you do end up with a crossover system, part electronic, part mechanical. So it does end up being part of an audio crossover, of a most basic (but popular) sort.
IMHO no discussion of passive LS crossovers is complete if it omits this very popular option.
Tabby (talk) 06:57, 21 January 2008 (UTC)
replies: text: I agree, but I think it should go further to include the inductor, too.  Simple crossovers marketed by Radio Shack and others in the 1970s, and perhaps later, were composed of a capacitor (to increase the impedance to the tweeter as frequencies declined) and a coil (to increase the impedance to the woofer as they rose). Anoneditor (talk) 17:33, 21 January 2008 (UTC)
thread_title:  Capacitor only 

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thread_title:  External links modified 

message: removed the External links section which may be appropriate. The sources may be useful as additional references. I reproduce the content here in case I or another editor is able to make use of them. ~Kvng (talk) 13:13, 14 December 2021 (UTC)
thread_title:  Former ELs, potential refs