The sonic benefits of an active crossover. A discussion.

LL21

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Dec 26, 2010
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Lloyd,

Andy Payor says lots of nice thinks about actives in the interview referred above, but he also is very clear:

" I make it a point to affirm that the active Arrakis has no “subwoofer” section, but that the extremely light, very compliant, low-distortion twin 15” woofers, each with approximately four cubic feet of volume, should be viewed as the foundation of the system, not merely an adjunct to extend bass. Because we are not attempting a lawless coercion of nature to extend the bass response, the integration is exceptional."

This is what you must look for in your system. A system and room with exceptional integration of bass, more than quantity and ultra-flat response. Just MHO.
Thanks, Micro. Agree...was just wondering if this active crossover was part of designing that. Having spoken with a few more people, i am starting to think even if well executed (which i feel confident it would be by the Wilson guys)...it could be a very nice step forward but not a quantum leap.
 

Groucho

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There are various levels of active speaker: some simply replace the passive crossover with line-level analogue filters and separate amps, and this gives great benefits (in my opionion). More ambitious perhaps is DSP with linear phase filters, individual impulse response-based driver correction, and time alignment between the drivers. Once you know it's possible, doesn't the prospect of correcting a speaker to this level make you at least curious as to how it could sound? I hear the objections to the extra boxes it involves, the evils of digital and so on, but for me the Rubicon has been crossed: the idea that my so-called reference speaker is smearing the sound in the time domain is something that I can no longer live with; it has to be the full DSP-based correction from now on.
 

LL21

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Hi Gents,

Still investigating this Active Crossover offer from my dealer for my X1s. I went back to the 1994 article by Martin Colloms and found the following which was interesting (to a non-techie like me)...and I wonder if it suggests that this suggestion of using the Wilson ACtive Crossover with dedicated amp for the woofers may be more worthwhile than (at least I) was originally expecting:
Wx1fig07.jpg
MARTIN COLLOMS: "Fig.7 [above] also shows the port response with its maximum output at 24Hz. There's also a strong return in output at around 220Hz which, in an ideal world, would be better controlled. The curve marked "B" shows the output of the bass drivers alone, revealing their gentle rolloff above 150Hz, which results in significant acoustic output to 1kHz. Interesting."

Wx1fig10.jpg
The in-room response in the Distributor's room (above) was quite flat, but nevertheless, I am curious about whether or not people think that the fact that the woofers appear to operate thru 1khz according to Martin Colloms, may reveal that my Dealer's suggestion of using an Active Crossover and dedicated Bass Amp to 'improve control' may actually be a good one...because this 'control' would be across a much wider spectrum of the music than I originally thought (ie, iwas thinking 'bass only'...below 250hz)

Thanks for your guidance on this.
 
Last edited:

zztop7

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opus111 inductor question

Hi Opus111,
On 04-28-2013, 08:03 AM Post #125 opus111 wrote:
“ the bass air-cored inductor's been wound with such thick wire to reduce the DCR that proximity effect hugely increases its losses at the turnover frequency.”

??? That loss [high frequency attenuation] is what you want as the frequency increases; that is how the low-pass happens /// or are you writing about some other "losses"?

Respectfully,
zz

P.S. All the contributors to this thread have done a Great Job by throwing the questions & info back & forth.
 
Feb 11, 2012
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You're correct that you want HF attenuation as the frequency increases, however this is not achieved by losses in the components, rather not taking the signal at all from the amp. In other words a low-pass filter functions not by turning the amp's signal into heat rather by presenting a high impedance to the signal so it flows less and less into the XO as the frequency rises.

The loss i'm talking about is indeed loss meaning its resistive and turns amplifier signal into heat. It's not rising inductive impedance and it also has a different frequency characteristic from the inductive impedance - it rises at 3dB/octave, not 6dB.
 

Groucho

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In this article, the author talks about non-linear distortion induced within passive crossovers by back EMF from the driver (not something I've seen emphasised overly elsewhere):

http://www.tnt-audio.com/casse/active_speakers_intro1_e.html

If cone-mass momentum continues to move the coil in the magnet gap for a few microseconds after the signal changes, there will be a voltage generated, because the coil is connected at both ends. This voltage will be generated and be divided in proportion to the resistances in the local circuit. The local circuit is the passive crossover, where the back EMF, if it exists, will combine with the input signal in a completely non-linear relationship. This is not even-order distortion, this is not odd-order distortion, this is not even high-order distortion, this is DISORDER DISTORTION (to coin a phrase). Passive bi-amping (using separate amplifier channels for each drive unit, but with a passive-crossover leg to each driver) would be a half-way measure, but spurious back EMF (voltage) even then adds to the amplifier signal voltage across the inductors, resistors & capacitors of the passive crossover causing non linear distortion. It will only be eliminated by eliminating the passive crossover.
Is this correct? If so, what sort of levels are we talking about?
 

Groucho

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Looks like a pile of nonsense to me. Possibly an acute case of audiophilia nervosa?
OK, thanks. So just to wrap that up, would it be correct to say that a finite impedance between amp and speaker does not give rise to any nonlinearity? It may give rise to various frequency and phase response effects, but it doesn't distort..?
 

LL21

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Dec 26, 2010
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So I just spoke with Wilson Audio about using the Wilson Active Crossover with my X1s. Synopsis:

- in theory an interesting idea
- recognize the benefits of putting amps directly with drivers without passive crossovers in the way (ie, using Active Crossover in between preamp and amp)
- HOWEVER, there are phase issues which have been accounted for in the Passive Crossover network which would not be adjustable with the Wilson Active Crossover...which would counteract theoretical benefits from directly connecting amp to driver

Any thoughts on what they say above as their reasoning for not recommending it?
 

zztop7

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Phase issues

To lloydelee21,
If the phase issue is 180 degrees with one or more drivers, just reverse the leads on the affected driver.
Some people reverse their tweeter leads to look for a likable or discernible difference even if there is not an issue.
Best to you,
zz.
 

DonH50

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Jun 23, 2010
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So I just spoke with Wilson Audio about using the Wilson Active Crossover with my X1s. Synopsis:

- in theory an interesting idea
- recognize the benefits of putting amps directly with drivers without passive crossovers in the way (ie, using Active Crossover in between preamp and amp)
- HOWEVER, there are phase issues which have been accounted for in the Passive Crossover network which would not be adjustable with the Wilson Active Crossover...which would counteract theoretical benefits from directly connecting amp to driver

Any thoughts on what they say above as their reasoning for not recommending it?
Hard to see how phase could not be compensated using a digital crossover network to match the pasive crossover and potentially improve the response. Or are they talking about a specific active crossover implementation (sorry, I have not been following closely)? Either way, if they can create a network at speaker levels, I do not see why the same network could not be designed at line level using analog or digital circuits. My experience has always been better performance with an active line-level crossover and amps directly connected to drivers.
 

DonH50

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OK, thanks. So just to wrap that up, would it be correct to say that a finite impedance between amp and speaker does not give rise to any nonlinearity? It may give rise to various frequency and phase response effects, but it doesn't distort..?
It can get complicated. The speaker (driver, voice coil) also acts like a generator, creating signals that can modulate the incoming signal, causing distortion. The degree to which this happens depends upon the electromechanical interface and acoustic properties of the design. That is, the crossover's impedance, amplifier's impedance, parameters of the cables connecting everything, speaker's electrical parameters, and the coupling to the air all impact the amount of modulation at a given frequency. (Yes, of course it varies with frequency.) All that influences the magnitude of the modulating signal from the speaker (the back-electromotive force, back-EMF) and it's impact upon the incoming signal. It can certainly cause distortion. The magnitude of that distortion, and its audibility, are the usual open-ended questions. It is one of the distortion effects active drive purportedly reduces since connecting the amp directly to the driver greatly reduces the effective impedance (or increases the control) of the amplifier as seen by the driver.

HTH - Don
 

Groucho

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Thanks Don. So you think there is something in what I quoted here?
http://www.whatsbestforum.com/showt...r-A-discussion&p=195963&viewfull=1#post195963

Intuitively I thought that it sounded as though an uncontrolled speaker cone should create some sort of distortion, but it's not something that people seem to worry about with valve amps etc. They only seem to worry about the frequency response effects.

Headphone people seem happy to stick a 47R or 120R resistor in series, too.
 

DonH50

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Hmmm... That paragraph starts off not too bad then gets pretty hard to follow.

The impedances are low enough in-band, even with a passive crossover, that I doubt you would hear the distortion caused by the effect, especially buried in a typical music or movies. I am not sure I have tried to assess its audibility -- little tough to isolate just that when switching from passive to active crossover designs. There are a lot of things that are readily defined, calculated, and measured and then used in marketing that have little practical basis in the real world of audibility.

Headphones are generally a much easier load than a loudspeaker, so the audibility argument applies even more so for headphones.

FWIWFM, charge kickback is a serious issue in high-speed (GHz) ADCs and DACs I have designed in the past. I have piddled with it in the audio realm, and am virtually certain I have measured it in the primordial past, but I have no recollection how bad it was. That probably says something about its audibility as back then I was concerned with all sorts of little things that might add up, and did a lot of testing with a group of "golden-eared audiophiles" around the state (mostly of MO, USA).
 

LL21

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Dec 26, 2010
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Hard to see how phase could not be compensated using a digital crossover network to match the pasive crossover and potentially improve the response. Or are they talking about a specific active crossover implementation (sorry, I have not been following closely)? Either way, if they can create a network at speaker levels, I do not see why the same network could not be designed at line level using analog or digital circuits. My experience has always been better performance with an active line-level crossover and amps directly connected to drivers.
Thank you...there was a reason I posted here to ask. I was not sure if that was a manufacturer simply saying they've designed it as well as could be...so no tweaking could improve it, or if in truth the active solution would not work. I know an engineer who knows Wilson well...very well. I'll ask him as well.

Don I ask Wlson about using the Wilson Active crossover machine, inputting the X1 passive crossover info...to bypass the passive bass crossover in the speaker...and this phase thing was their reason not to do it.
 

LL21

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Dec 26, 2010
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To lloydelee21,
If the phase issue is 180 degrees with one or more drivers, just reverse the leads on the affected driver.
Some people reverse their tweeter leads to look for a likable or discernible difference even if there is not an issue.
Best to you,
zz.


Thanks...I always thought phase angles could be different...90 degree, 180, etc? I am no techie, so when Wilson mentioned that the phasing had been deliberately considered in the passive crossover design, I wondered if phase might be something that is more complicated than switching leads. I honestly don't know, hence why I've posted their answer/recommendation here.
 

DonH50

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Jun 23, 2010
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Phasing has to be considered in any crossover design. It varies with frequency, and the phase response is an integral part of the overall design to optimize the time (aas well as frequency) response. Wilson probably uses a fairly complicated crossover network, but that does not mean it cannot be successfully implemented by other means. Unless there is something really funky in there, it is a linear network, after all, reproducible with RLC components, models of them in an analog filter transfer function, or standard IIR or FIR modeling techniques.
 

LL21

Active Member
Dec 26, 2010
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Phasing has to be considered in any crossover design. It varies with frequency, and the phase response is an integral part of the overall design to optimize the time (aas well as frequency) response. Wilson probably uses a fairly complicated crossover network, but that does not mean it cannot be successfully implemented by other means. Unless there is something really funky in there, it is a linear network, after all, reproducible with RLC components, models of them in an analog filter transfer function, or standard IIR or FIR modeling techniques.
Thanks, Don. Here is the seminal question for a non-techie like me. If I am working with a former speaker designer who has worked extensively on Wilson speakers over the last 15 years...and he is 'highly confident' that he can use the Wilson Active Crossover to bypass the passive bass network (ie, handle the technical issues of the crossover frequency and roll-off but also the phase issue you refer to above)
that it would not surprise you if connecting a great amp (perhaps a second Gryphon) directly to the bass drivers should result in a superior sound than using the current passive crossover network. Fair interpretation of your posts? If not, please correct me...only trying to understand, not to put words in anyone's mouth. Thank you for helping!
 
Feb 11, 2012
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OK, thanks. So just to wrap that up, would it be correct to say that a finite impedance between amp and speaker does not give rise to any nonlinearity? It may give rise to various frequency and phase response effects, but it doesn't distort..?
As DonH says, it gets complicated. My take on this is a little different to Don's.

There's no distortion due to 'back EMF' - that's a perfectly linear phenomenon, inductors give 'back EMF' it happens whenever energy is stored by reactive components. So that in itself isn't a problem.

Drive units distort though, and not just acoustically. A driver's inductance is modulated by its physical position - this is a non-linear phenomenon and as a result of this, it presents a non-linear impedance to the amp. With zero amp source impedance there would be zero distortion but as you probably know, no amp presents zero impedance and even if it did (feedback could be used to make it zero or even negative) the impedance of the speaker wires and intervening components would quickly make it non-zero. Given the finite source impedance then in practice, there's going to be distortion of the voltage measured at the driver's terminals as a result of its non-linear impedance characteristic. Whether this matters in practice is the subject of some debate because drivers are current driven.
 

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