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.

"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.

A previous poster has made the statement that using a 1st order crossover causes the LF and HF drivers to be 90degrees out of phase and this confuses me. At what frequency ?

If the LF driver uses a 1st order butterworth type low pass filter, and the HF driver uses a 1st order butterworth type high pass filter, with the -3dB frequencies at the same point (to give a flat amplitude response) doesn't this automatically put the two drivers in phase at the crossover frequency ? I must admit it's a long time since I studied filter design (since I went over to the dark side of software).

That would be a phase-coherent time for me ... the two drivers in phase at the crossover frequency, with linear phase response in the low pass and high pass filters as you move away from the crossover point ensuring phase response continuity elsewhere in the frequency response.

Such a speaker should have a good step response, because linear phase filters offer the least distortion of the original waveform.

I am assuming that this is what the Green Mountain Audio designs attempt to approximate (given that everything in real life is an approximation).

A previous post asked whether preserving the waveform was more important than other aspects of speaker design (I am guessing that other aspects are flat frequency response, radiation pattern, and input impedance curve, dynamics and ability to handle high SPLs).

This led me to wonder whether the real catalyst for the increasing number of 1st order designs is that the newer generation of drivers is allowing speaker designers to offer 1st order designs, without having to make great sacrifices elsewhere. I remember reading an interview with Jon Bau of Spica fame where he said he would have liked a stronger bass response from the Angelus, and would have liked a design to handle higher SPL but that drivers to achieve that and also achieve his other design goals were not available at the time within his price constraints.

Looking at the drivers on the green mountain speakers, the Morel HF unit and the Aurasound LF unit I did a little research on the units and found that they appear to offer very high performance for relatively little money. The Morel tweeter is able to reproduce relatively low frequencies, and the aurasound woofer has a very lightweight, but quite rigid cone, allowing it to produce quite high frequencies before it breaks up. These low(ish) cost wideband, high sensitivity drivers are the enabler for a first order 2 way design. Perhaps they just didn't exist 10 years ago, and perhaps that is why 1st order designs have become more popular of late.

That's not to take away from the skills of designers like Roy, but it does seem that he has some great raw materials to work with now that Jon Bau and others may not previously have had access to.

I'm not convinced that amplifiers have made great strides in the last 20 years, but I am convinced that speaker technology has.

"At the crossover point, for example, one driver is at .707, +45, and the other is at .707, -45, as I stated previously. Due to the fact that this is vector addition, they sum to unity at zero phase. And they do this not only at the crossover frequency, but at every single point from DC to infinity. The first-order is the only crossover that does this."

Karls, thanks for that explanation ... now I completely understand why the first order crossover can work in amplitude and phase terms through the region where both drivers are contributing to the sound. The power output of both driver is 3dB down at this point (amplitude is reduced by 1/(square root of 2), and they are 90 degrees out of phase, but the vector addition of these two waves results in a sine wave that is in phase and 0dB down in amplitude.

Is it audible ? I think the best answer is for everyone to demo for themselves and answer for themselves.