First Order Crossovers: Pros and Cons

I cannot really add to the science, but I would like to add to the voice of several others regarding the Green Mountain Audio Europas I own (I previously owned Spica Angelus, and prior to that non-coherent designs).

The Spicas and the Europas both possess great imaging. Absolutely pinpoint images seem to hang in the air, as if not coming from the speakers at all. On orchestral CDs the room behind the speakers appears to melt away and one is left with a sense of scale that I never had from previous non-coherent designs.

The Spicas were not true coherent designs, but an approximation, with a 4th order Bessel low pass filter on the LF driver (to the best of my knowledge). The Europas are true first order on both drivers.

The Europas outclass the Spicas in their ability to define transients. Macro and micro dynamics are much more defined on the Europas. Depth of image is also better on the Europas. Strangely the Europas also load the room slightly better than the spicas sounding uniformly good around the room, and with less of a tendency for the image to collapse once you move away from the sweet spot.

Both the Europas and Spicas are extremely fussy of placement if they are to give optimum performance. You really must arrange the room around the speakers, and be prepared to have them well out into the room. If you can live with this then you are rewarded with music that fills the room, not sounding as if it is emanating from two little boxes.

If there is a downside to these two speakers it is high frequency response. The Spicas did not do high-frequency, with a rolloff above 14kHz. The GMA Europas are more extended, but the treble sounds a little less defined than in previous speakers that I have owned that were 3-ways with a ribbon tweeter. I suspect the treble performance of the Europas could be bettered at the same price, but only by losing some of the dynamics and imaging due to higher order filtering.

I'm not sure that it's all due to the phase coherency, or also a side benefit of having fewer, higher quality crossover components, but I am sold on the result.

Placing the speakers well out in the room should reduce early reflections. (More distance from a reflective surface equals more delay and more attenuation.) That fits in with the idea of wanting to separate the reflection (echo) from the original event so that your ears/brain will hear them as two separate events and not one muddled-up event.

There are definitely people who don't agree with this approach. Some feel that you're going to have reflections, so you should get your speakers close to the walls so that the direct and reflected events blend together. I think Ted Jordan claims that it's best to have speakers mounted in the wall, and I think this is why he says that.

I'm not positive that phase-coherency leads absolutely to pinpoint imaging. The source to microphone to loudspeaker process has some inherent flaws. Binaural and transaural methods attempt to correct this. (With some success IMO.)

I think it's quite possible that phase distortions could result in the type of soundstage that all the reviewers love to describe. "I could hear that the violinist had a rash on his left elbow. Not the right, but the left..." :-)

I have more to learn about this before trying to come to conclusions.

I do remember hearing a remarkable soundstage from a pair of Acoustats a long time ago. But panel speakers are hardly phase-coherent (nor very linear), so there's something more going on here.

I also once heard a demo of signal processing, played on a boombox, where a helicopter moved toward you from the right front toward your rear left, and seemed to pass overhead as it went! It opens up intriguing possibilities, such as listening to a performance in a "virtual" concert hall and being able to choose where in the hall you "sit."

"I'm not positive that phase-coherency leads absolutely to pinpoint imaging"
I think coherency may be one of the more important of many factors that lead to pinpoint imaging. Inert cabinets, stiff, lightweight drivers, minimal crossovers all help ... in short pinpoint imaging is the result of an excellent transducer.

"I think it's quite possible that phase distortions could result in the type of soundstage that all the reviewers love to describe".

I don't think ANY distortion will improve pinpoint imaging, but I suspect it might help to create an artificially broad soundstage. I remember once demoing a large pair of Martin Logans. The soundstage was huge, and almost sounded like 180 degrees wrapped around in front of you. However within that "wall of sound" the placement of individual performers was very vague ... nothing like the spicas or the europas. It was impressive, and I'm sure some people would love it, but ultimately everything sounded a bit too "huge" to be accurate.

>The lobing is actually due to having 2 spaced sources of the same signal at the same frequency. It isn't really caused by the crossover, but by the drivers. (Just being pedantic here.)

This is news to me, as stated. Of course, any two or more driver units will have interference effects, some configurations less than others. (It's even controversial whether a line array of dynamic drivers is truly a line array, for example.)

But in addition to these driver interference effects, the crossover topology certainly does influence off axis lobing. And you could demonstrate this by using the same pair of drivers and swapping a first order crossover and an "infinite slope" crossover in turn.

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.