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Thread: The sonic benefits of an active crossover. A discussion.

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    Moderator Moderator treitz3's Avatar
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    The sonic benefits of an active crossover. A discussion.

    Good evening ladies and gentlemen of the WBF. Moving on to the next topic in the "Dissecting a speaker series", this discussion will be concentrated solely on the sonic benefits of using an active crossover, ways to implement the use thereof, different configurations and why one would want to do something like this in the first place. Some folks love passive, some like external but on the flip side, one might venture to say that a vast majority of those who have tried or heard an active speaker think that this is the way to go. Simply put, why?

    What would the sonic benefits be?

    What other benefits would those who are interested in active crossovers want to know?

    This discussion is not only limited to the sonic benefits and the positive things about an active crossover but some of the deficiencies or drawbacks, if they exist as well. Anything and everything having to do with an active crossover can be discussed here. What say you?

    Tom
    In search of accurate reproduction of music. Real sound is my reference and while perfection may not be attainable? If I chase it, I might just catch excellence.

    The best way to enjoy digital music reproduction is to never listen to a good analogue reproduction.

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    I wonder if 'active crossover' is the right descriptive term? How about 'line level crossover' (as opposed to 'speaker level crossover') because the option I currently prefer is building a passive crossover (using inductors and capacitors) but at signal levels. This stems from the hypothesis which I currently have that its active electronics' intermodulation distortion which is the limitation on SQ. I reckon passive components generate less IMD than active circuits.

    With this hypothesis, line level crossovers make life easier for poweramps because by bandlimiting the signals prior to being amplified, the amps generate less IMD by virtue of handling a narrower bandwidth. Their power supplies are also less stressed because the total output power is spread across more than one amplifier. Designing an amp to handle a narrower bandwidth is easier than one handling the whole audio spectrum, line level crossovers allow an amp to be optimized to handle driving only a tweeter (for example).

  3. #3
    Quote Originally Posted by opus111 View Post
    I wonder if 'active crossover' is the right descriptive term? How about 'line level crossover' (as opposed to 'speaker level crossover') because the option I currently prefer is building a passive crossover (using inductors and capacitors) but at signal levels. This stems from the hypothesis which I currently have that its active electronics' intermodulation distortion which is the limitation on SQ. I reckon passive components generate less IMD than active circuits.

    With this hypothesis, line level crossovers make life easier for poweramps because by bandlimiting the signals prior to being amplified, the amps generate less IMD by virtue of handling a narrower bandwidth. Their power supplies are also less stressed because the total output power is spread across more than one amplifier. Designing an amp to handle a narrower bandwidth is easier than one handling the whole audio spectrum, line level crossovers allow an amp to be optimized to handle driving only a tweeter (for example).
    I've done this inspired by a wonderful old article in "Audio" magazine. Do you know it or should start digging into my closets?
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    I don't know it - would be very interested if its not a whole lot of bother...

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    [WBF Founding Member] Addicted to Best! JackD201's Avatar
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    IMO

    Pros: Efficiency, flexibility

    Cons: Space, more signal and power wiring

    Thing to watch out for: sonic signature of the active crossover unit itself, obviously the less the better
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    Yes - agree that the signature of the active (I prefer to call it line-level) XO is a major issue. In the current way of building systems, its a separate box, with separate mains power supply, another set of cables to introduce degradation.

    As a designer my currently preferred way of building a no-compromise digital system is to have one DAC per drive unit, and implement the crossover passively right after the I/V stage of each DAC. This is because multibit DACs already need to have passive filters after them anyway, to cut off the OOB signals, so why have two filters there when one will do (for the bass/mid at least) ? I haven't built a whole system like this yet, but perhaps for the tweeter we'd do better to implement a digital high pass filter prior to the DAC and keep the anti-imaging low pass passive filter. Since to my ears NOS delivers the sweetest sound in future I'd like to play with fractional oversampling (aka undersampling) for bass/mid units - perhaps even running as low as 11.05kHz sample rate where the crossover frequency is 3kHz.

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    Addicted to Best! Keith_W's Avatar
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    If we are going to talk about crossovers, we should be quite clear about what we are talking about. This has already caused some confusion in the other thread and I see the beginnings of it here These are standard industry definitions:

    - PHLXO - Passive, high level crossover. These are speaker level crossovers installed in the majority of speakers.
    - PLLXO - Passive, line level crossover. These are preamp level crossovers and require multiple power amps. These are a rarity but can be found in some speakers, e.g. B&W Nautilus.
    - ALLXO - Active, line level crossover. Preamp level crossovers which are powered, and require multiple power amps. These can be either analog or digital.

    The confusion comes because some people refer to all systems with line level crossovers as "active speakers" or "active systems", because the speakers are directly powered by the amplifier. Active speakers are still enough of a rarity in the high end world that some people make the mistake of lumping them all in the same category with no consideration of the nuances between them. I much prefer the more precise terms as used by the industry.

    The advantages and disadvantages of PLLXO vs. ALLXO is similar to those between a passive preamp and an active preamp. Some might say that the simpler electronics of a PLLXO provide a more pure experience, others believe the better impedance matching and greater versatility of ALLXO's outweigh the negatives.

    Furthermore, ALLXO's come in analog or digital varieties. In addition to the simple crossover function, digital XO's are very easy to tune - you simply change the settings through the front knobs or through a laptop computer, and can correct phase, group delay, and compensate for frequency response anomalies in the speaker and the room. The downside is that the signal needs to be re-digitized if you are not already feeding it a digital signal directly off your transport. Some people think that this is completely inaudible, or argue that the sonic advantages outweigh the disadvantages. My position on this is that it is system dependent.

    The other downside is the learning curve - if you fail to take care when obtaining your measurements, you will be telling your crossover to do all sorts of horrible things to your sound. I know some people whose systems are in a constant state of flux because they can't help but keep fiddling with their digital crossovers. That might be OK if your hobby is tuning your system, but my hobby is listening to music and not fooling around with microphones and laptops.
    Classical music enthusiast. System photos here.

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    The original B&W Nautilus used an active line-level crossover, not passive.

    To add to the confusion over nomenclature, I've designed an active crossover using DACs which isn't digital in the sense it doesn't use a DSP, rather an array of DACs to build a hardware-based FIR filter, aka a transversal filter. This has a potential dynamic range advantage over the more traditional digital filters followed by DACs.

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    Oh boy. I don't know if I can do this again.

    Just to cast my vote, I'll say that active crossovers and multi-amping is the way to go.

    For the most part, I'll second what Keith says. Based on other threads, he and I seem to be pretty much on the same page with this topic.


    With regard to definitions, I like to use "active crossovers" to describe an active line level crossover between the preamp and power amps, "passive crossovers" are the common high level crossovers inside the speaker, and I prefer "POWERED speakers" to "active speakers" when the speaker contains the amplifiers, which may also contain an active line level crossover and multiple amplifier channels. Yeah, I know, people have their own preferences with definitions. We should try to get a consensus.
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    I second Keith's suggestions but would like to extend them a little. Rather than 'ALLXO' for active line level crossover, let's keep this monika to mean 'analog (meaning active analog) line level XO'. Then let's add 'DLLXO' for 'digital line level XO' and 'HLLXO' for hybrid (digital) line level XO (which is my transversal filter).

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