First Order Crossovers: Pros and Cons

Every step up in xover order shifts the phase between tweeter and woofer by an additional 90deg. This means for maximum phase-coherence the obvious choices are second order (180deg shift so you just invert the connections to one driver to get it all back in phase) or fourth order (360deg shift: back in phase but not in time, a simple delay circuit will fix this). The ones to avoid are first and third order as a 90deg (270 for the third order) is very dificult to fix.
One advantage of a first order xover is a small number of parts which results in a very good transient response. The other is their immunity to electrical resonance ("ringing"). The higher the order the xover the easier it is to induce resonance with a high enough power input. When such a filter resonates it actually turns into a sinewave generator and produces an output at the xover frequency! Almost all analog synthsizers use this effect very succesfully but you don't really want that happening in your speakers, to this end most higher order xovers contain a damping circuit to avoid this.

The only truly phase-coherent dynamic multiway speaker in existence (Tannoy DualConcentric) uses a second order xover and inverts the tweeter.
It is phase-coherent to within 18deg. The only way to get better than that is a full-range driver, be it dynamic or planar. The result of this is a stereo image
that is far more precise than any other dynamic speaker I have ever heard.
And, being a true point source, you are always in the sweet spot as long as you are somewhere between the speakers and, where ever you are in the room, the sound never changes! After having spent a fair bit of time in recording studios I can say I have never heard a speaker as close to the real thing as a Tannoy! None, at all. Electrostats have better micro resolution and a Klipshorn better dynamics but, apart from those and overall, theres just nothing that comes even close. I put this down to phase-coherence and the point source characteristic. There is also a marked difference if I invert the connection (on both speakers at the same time): the stereo image pretty much collapses ie it goes flat in depth and height and the bass goes soft.
With other speakers I've never heard a difference.

Dear Golix,

We have exchanged some thoughts on the Time Coherence thread- thank you for sharing those. I greatly respect what Tannoy has done for sound quality, as I can tell you do. Certainly working in a studio qualifies you to know when a speaker's distortions are minimized.

Some corrections, please, to your information above, in the spirit of technical accuracy in our comments:

As I described in the recent Time Coherence thread, the phase shifts by 90 degrees, yes, for each additional order of crossover, as you state above.

But do know that the correct expression to use is that the phase DIFFERENCE is changed by 90 degrees for each additional order- at the CROSSOVER FREQUENCY. Those are important distinctions.

You see, the problem caused by higher-order crossover circuits is that this phase difference is ALWAYS CHANGING as one moves away from the crossover point- a DIFFERENT time delay is being imposed on each and every frequency.

This ever-changing phase difference (ever-changing time delay) CANNOT be corrected with any additional circuit.

The math also shows, without question, that this ever-changing phase difference/time-delay distortion cannot be "fixed" by inverting the tweeter's polarity.

Flipping a tweeter's wires from + to - serves only to flatten the frequency response, when one measures using continuous sine-wave test tones or continuous pink noise.

Those are both unchanging signals, without beginning nor end (and therefore carry no information). These test signals do not indicate anything about WHEN things happen- about the time-delay distortions that are occuring, frequency-by-frequency. When the tweeter's wires are reversed, the resulting transient response has only a "different kind" of inaccuracy, even though one will like "the tone balance better".

Only a first-order crossover has a CONSTANT PHASE DIFFERENCE at every frequency above, at, and below the crossver point. That means there is Zero change in phase between them, so the signals out of the high- and low-pass sections are "in Phase", always, on every frequency.

And all of that means, finally, that first-order circuits are the only ones that pass all their signals through with the SAME time delay, so all of those emerge in the same time-order in which they entered. This is called time-coherent behaviour. So you get the original transient, one not smeared out in time.

A speaker that is designed to deliver a time-coherent output is automatically "phase coherent". The converse is not true, as you may know: A phase-coherent speaker is not necessarily a time-coherent speaker. In fact, if you see the advertisement claim phase coherency, you can bet that the speaker is not time coherent. One has to put something in an ad!

On the Tannoys, their "phase distortions" (time-delay distortions) are rather mild. In my experience, once those varying delays are removed (which cannot be in Tannoys), the difference you'd hear is at least the difference between an average mic and an outstanding one. By the way, neither a phase- or time-coherent behaviour can be directly inferred by looking at the electrical phase of a speaker's impedance curve.

That the Tannoy's phase distortion is tolerable for you, is because it is unconsciously ameliorated in those studios by their mic selection, mic technique, the type and tone of the echo/reverb mixed in, the settings of the tone controls on the mic's channel, and by the EQ applied to the monitor system. And in pop music, the phase shift problems are also "danced around" by the sounds created by compressors, limiters, de-essers, and other tools. I speak from many years of recording experience, and of designing speakers with all these different crossovers (and using many others that had all sorts of their own time-domain distortions).

Thank you for your input. I am glad you get to work in studios so much. There are not enough people with that background contributing to high-end home audio reproduction. I hope my information is of help.

Best regards,
Roy Johnson
Founder and Designer
Green Mountain Audio

So by "phase coherent" you really mean that a first order filter remains entirely true to its 90deg phase separeation between tweeter and woofer?
Thats a new definition to me, maybe we have to start with defining every term properly first.
Also a speaker which has the drivers on a vertical axis can only be phase coherent at one point in space. This of course is a purely geometrical problem independent of any electrical feature.

Last but not least a time coherent speaker can easily be not phase coherent if the crossover separates the phase between drivers (as they all do). On the other hand a phase coherent speaker has got to be time coherent as well as a slight time mis alignment MUST result in a phase shift. Indeed a phaser works by splitting the signal in two and then time delays one with respect to the other before merging the signal again. If you modulate the delay you get a phaser/flanger sound , if you don't its a chorus. I hope you get the idea: all phasing is done by playing in the time domain.
Basically a phase coherent speaker is one that is not only in time but also in phase; a time coherent speaker is one thats in time but not in phase.
And most speakers are neither.

Tannoy manages to get theirs phase coherent by fitting the treble coil exactly half a wavelenght (at crossover) behind the bass coil and then using crossover characteristics and a very simple delay circuit. Thus there is no phase separation between tweeter and woofer at all (except some small aberration at crossover). They are thus in phase acoustically and electrically AND independent from the listening position.
And no I am not trying to sell Tannoy, I don't actually like the new ones with their plastic cones.
Personally, I think the aforementioned geometrical phase problem is the reason that conventional speakers have a sweet spot where everything sounds better and the stereo image snaps into place. Tannoy don't really have that: they sound the same where ever you are although the imaging shifts if you stand to the side, like looking through a window at an angle.

Having had a look at your website I see you claim a phase error of 2deg!
A bold statement, care to back that up with some measurments?
I am sure you've got access to an anechoic chamber and the necessary equipment.

Cheers Golix

Step function tests seem to show all of this very well. Stereophile performs these tests on speakers, but they fall down in not drawing any conclusions from the results.

"This speaker simply can't pass a transient correctly, therefore it's crap, no matter how much a listener may like it." :-)

I will chime in here; Roy is correct as usual. It is not possible to fix the inherent phase problems in a second-order crossover, because they are frequency-dependent. This is simply a mathematical absolute, at least in the analog domain. And phase inversion of one driver is a Really Bad Thing, no matter what the reason for it, because it completely screws up the original signal. Now it may be that the Tannoys are good-sounding speakers for many reasons (not having heard them myself, I can't comment), but true time-and-phase coherence is certainly not among them.

Skrivis's comment about the Stereophile step response measurements is right-on and needed to be said. I still remember several years ago when they reviewed the Quad 988 or 989. JA published all the measurements, and then scratched his head in a rhetorical sense, saying something like the measurements were "enigmatic" because most of them looked horrible in comparison to average loudspeakers (as did the ESL-63 when Stereophile reviewed it decades ago.)

After all those years, he still literally couldn't comprehend how a speaker that measured so poorly in the "traditional" ways (i.e., in the frequency domain) could possibly sound as good and as "right" as the Quad does. And all the while the step response, that beautiful, glorious, near-perfect step response, was staring him right in the face.

Of course, this isn't to say that full-range electrostats are perfect; far from it, especially when they have to be put in a listening room. But it can't be argued that what they do well, they do spectacularly. That is, they reproduce the signal in the time domain more correctly than just about anything else ever made. And in so doing, they set a shining example for us all.

Best,
Karl
AudioMachina