First Order Crossovers: Pros and Cons

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

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).

The output of a crossover network is a vector sum with real and imaginary components in polar coordinates. What you have in a 1st order at the crossover freq is one vector at .707, +45, and the other at .707, -45, which adds to unity in vector space with a combined phase shift of zero. But because the vectors rotate together with frequency, they are always 90 degrees apart, and they always add to unity voltage and zero phase in vector space, no matter what the frequency.

To say it differently, the combined output of the two drivers is always unity at zero phase, even though the two vectors are always 90 degrees apart. This is difficult to conceptualize, but the math behind it is relatively simple.

For obvious reasons, this is called a constant-voltage minimum-phase transfer function, and the first-order is the only crossover type that has this characteristic. I should note that this presumes identical drivers mounted very closely together and resistive loading, which is hard to achieve in the real world. But with some effort, one can come close, and the effort is well rewarded in the listening.

The exchange between Roy and Golix above leads me to bring up something I have been thinking about for a long time – the phasing of the recorded music before it even reaches your stereo system.

I have read enough about 1st order crossovers and phasing and to convince myself that this design philosophy is valid and this approach is the only one that can achieved a time coherent design. To me the $20,000 question is – does it matter enough to outweigh all the other compromises that speaker designers must make?

Others have asked if time coherence, or lack there of, audible. Let’s assume the answer is yes. The next question, is it preferable? Obviously not to everyone. Many have auditioned Vandersteens, Theils, and Meadowlarks and still chosen other brands.

Time coherence designs with first order crossovers are the only design that can come close to preserving the original waveform, most easily seen by measuring its step response. I totally agree with this. But music is not a square wave, so tracking something that doesn’t exist in natural music doesn’t prove you can make better music. Transients can be sharp, but they are not infinite slop step functions.

Going back to my first paragraph, what about the phasing of the music recorded on that CD or LP? Even if a stereo system from source to speaker was perfectly able to preserve the original waveform, what are we trying to preserve? I know very little about the recording process, but I can bet many an album has been processed by recording engineers in ways that destroys the phasing of the instruments used to make the music. Some genre’s may be better than others. Some studios may be better than others. But how are we, the music buyer, supposed to know if the waveform we are buying is worth preserving? I would love to see some discussion on this HUGE factor. I have read Richard Vandersteen tests his speakers with his own recordings. I am sure those recording have their phasing preserved. But doesn’t this say something about the phasing of recorded music in general?

Finally I would also like to see discussions on the real world trade-offs of using 1st order crossovers. Like small sweet spots. Is that inherent to all 1st order designs or do all good imaging direct radiating speakers have that? And is the sound outside the sweet spot worse for time coherent designs? If so why? I have only listened to Vandersteens (of the 1st order designs) and the difference in sound from sitting in the time coherence zone to standing up is quite alarming. There is something special going on in the sweet spot, but the Vandersteens sound flat when I stand up – treble drops right off. I know they just lost their time coherence, but when I perform the stand/sit test with my home speakers, the difference is far less dramatic. Do 1st order designs sound extra good in the sweet spot and extra bad everywhere else? Comments please.

Other inherent trade-offs of 1st order designs? Thanks in advance.

It seems Roy is correct, as usual.

But I do want to chime in on one point: There are companies which design and advertise phase coherent speakers without making claims of time alignment, such as VMPS or Fried. There is nothing wrong with doing this, although it's understood that Roy Johnson would not pursue or endorse this design decision.

Then there are companies whose ads specifically claim time coherence, when their multi driver speakers do not have first order crossovers and the baffles are not stepped back in any way. These ads are lying consciously, or someone in the engineering or marketing department is confused. Dali is the most recent and eggregious example of this phenomenon, which unlike VMPS or Fried, amounts to snake oil.