Archived from groups: rec.audio.tech (

More info?)

"Dick Pierce" <dpierce@cartchunk.org> wrote in message

news

c02c02f.0404141014.2252a762@posting.google.com...

> "Chris Berry" <christoforos@Notmail.com> wrote in message

news:<c5jiu5$efh$00$1@news.t-online.com>...

> > Can someone correct me if I've got this wrong please.

> >

> > Phase shift in crossovers works like this in 2-way systems:

> > For every capacitor in series with the tweeter followed by an inductive

load

> > to earth you add 90 degrees to the phase shift.

> > For every capacitor in parallel with the woofer you subtract 90 degrees

from

> > the woofer's phase (or add 90 degrees to the tweeter phase.

> >

> > Hence:

> > first order high pass + 3rd order low pass = wire drivers out of phase.

> >

> > Is that right?

>

> No, not exactly. You're confusing a couple of issues.

>

> First, it is the order of the filter that, given the common use

> in loudspeaker crossovers, that determines the ultimate STOP BAND

> phase shift. Within the pass band, the phase shift approaches 0.

>

> For example, well below the crossover point, the phase shift of

> the woofer crossover is 0. Above it, it approaches -90 degrees

> times the number of orders. On the other hand, well below the

> crossover, the phase shift to the tweeter is 90 degrees times

> the number of orders, while well above the crossover point, it's

> approaching 0 degrees.

>

> It's what's happen AT the crossover network that's interesting,

> because it's at an around the crossover where the amplitudes of

> the two are close enough for it to start to really matter. And

> at that point, you're rule of thumb is wrong.

>

> Let's take the simplest-to-understand case: assume that both the

> woofer and tweeter crossover filters have Butterworth characteristics,

> and that their -3dB points are the same frequency. Butterworth filters

> conventiently have a simple to understand behavior: at their cutoff

> frequency, they have half the passband amplitude, half the ultimate

> stopband rolloff rate, and half the stopband phase shift.

>

> THat means, for a simple Butterworth-aligned 2nd order network,

> that while the ultimate stop-band pahse shift of the woofer is

> -180 degrees and that of the tweeter is +180 degrees, at the

> crossover point, the phase shifts are -90 and +90 respectively.

> That's why, in 2nd order networks, you flip the phase of the

> tweeter, because the woofer is at -90, the tweeter is at +90,

> and the difference is 180 degrees, or complete cancellation.

>

> In your case, you have a 3rd order low pass and a 1st order high

> pass. The 3rd order low pass will have a stop-band phase shift

> of -270 degrees, but at the crossover, it will be -135 degrees.

> The 1st order high pass will have a stop-band phase shift of +90,

> but will be +45. So the difference AT the crossover point will

> be -135 - +45 or -180 degrees. On that basis alone, yes, you'd

> want to flip the phase of the tweeter to prevent cancellation.

>

> But because you're going with a non-symmetric topology, you run

> into a difficulty. IN the symmetrical 2nd order case I illustrated

> above, if you were to plot the phase difference vs frequency, you'd

> find that the rate at which the phase of each changes with frequency

> is the same: they are ALWAYS 180 degrees out of phase. This means

> that flipping the phase will correct for the phase difference across

> a wide bandwidth.

>

> However, your situation is not so neat: the rate of change of phase

> of the first-order high pass is 1/3 that of the 3rd-order low pass,

> so that you meet the necessary condition at only one frequency. In

> your particular situation, you may well find that the DIFFERENCE in

> the rate of phase change is enough to cause problems above and below

> the crossover point, ESPECIALLY considering that the 1st order low-

> pass has poor stop-band attenuation for wuite a significant range of

> frequency. Consider that an octave below the crossover, the tweeter

> will be singing away only 6 dB down. With it's phase flipped 180

> degrees, it will be in phase at the crossover, but will be approaching

> -90 degrees only an octave down.

My mistake... It's actually the other way round - 3rd order high pass, 1st

order low pass.

The theory would mean pretty much the same thing though - above the x-over

point.

Luckily, the phase plot of the woofer is between 0 and 10 degrees at the

x-over point but I have no phase data for the tweeter...

>

> And, of course, you've neglected entirely the fact that the drivers

> themselves add significantly to the total phase response of the system.

> You need to concern yourself with the TOTAL phase response, not just

> that of the crossover.

Just when I thought I had it sorted out, I find out that there's another

surprise round the bend...

I guess I'll just have to try using a 3rd order low and high pass and see if

it's worth the extra bit...

Thanks Dick. Appreciated.

cb