First Order Crossovers: Pros and Cons

Not sure if anyone is still paying attention here but I was just reading a review of the PMC OB1 loudspeaker in which the reviewer states that the crossover is wonderfully designed and produced a plot of phase coherence that is the best he has ever seen. According to the review, the crossover maintains the drivers in perfect phase with each other except for a small abberation at one point in the frequency spectrum. Given all that has been stated above by the first order crowd, how can this be given the use of 24db fourth order slopes? I'm sure I'm missing something here as this discussion lost me a long time ago. I'm just wondering how a steep slope design like the one in the OB1 can produce such perfect phase relationships as measured in the review.

Dodgealum...There is no magic! No matter who designs the crossover each section, which is worth 6 dB, causes a 90 degree phase difference between Hi and Lo. Both Hi and Lo phase vary with frequency, but the difference remains 90 degrees. Of course, as you go away from the filter break frequency (crossover frequency) the amplitude of the Hi and Lo signals diminish so that phase is not a big issue.

So, a 12dB crossover has a phase difference of 180 degrees, and if you hook the tweeter up with reversed phase the audio output will be back in phase. With the 24dB crossover that you mention the phase difference is 360 degrees, which is the same as zero, so the drivers, both connected with normal polarity, will be back in phase.

So why do people like 6dB crossovers? The audio output of the drivers may be phase shifted from the electrical signal, and, at a selected frequency, can also be adjusted by positioning of the tweeter with respect to the woofer. So, with tweeks, a 6dB crossover can avoid the 90 degree phase shift, at least at the crossover frequency which is where it matters most. FWIW, 6dB crossovers also require the minimum of crossover components.

A passive 24dB crossover is a tough nut to crack, and therefore rare, but is easily implemented in electronic crossovers, where it is very common.

A 2nd-order x-over with the tweeter's leads reversed gives you an in-phase output if you're considering _absolute_ phase (polarity). The 4th-order x-over is similar.

However, you're still seeing a phase shift. If you look at the electrical input and output with something other than a simple sine wave, it becomes obvious that the output is different from the input.

The classic statement I keep repeating is "a 1st-order x-over sums to a piece of wire." This is not true of other standard x-overs.

It's possible to add a delay in an active x-over to put things closer to normal, and this is even easier when the x-over is digital.

Technologies like DEQX hold a lot of promise. It's like having your cake and eating it too. :)

Skrivis...If, as you say "a 1st-order x-over sums to a piece of wire" it must follow that a second order crossover, which amounts to two first order filters in series, is the sum of two pieces of wire.

But I agree that digital crossovers can do wonderful things.

Hi Eldartford.
Thank you for your comments. Just a quick note, however. A second-order filter is not two first-order crossovers in series, nor "cascaded". The parts in a second-order filter, the capacitors and inductors, are of different values than those used in a first-order circuit.

Best regards,
Roy Johnson
Founder and Designer
Green Mountain Audio