In article , Chris Morriss
wrote:
In message , Jim Lesurf
writes
However you may not actually want that. :-)

Personally, for active filtering, I'd tend to prefer using a LPF, then
creating a HPF output by subtracting the LPF output from the input. The
result if you keep the levels matched is a LP and HP pair of signals
whose vector sum always equals the input. Thus the combined result
shows no phase errors due to the filtering.

For the actual filters I tend to lift the basic designs from the Active

Oh yes, I quite agree, a complex phase-compensated crossover has only
one advantage: it does help keep down vertical lobing problems.

I would put this slightly differently. The 'lobing' problem arises as a
result of having an 'array' of speakers in operation in the crossover
frequency region. There will always tend to be a frequency region where the
two units are radiating similar powers. If the speakers are not very close
(in wavelength terms) lobing is then inevitable. The phasing in this region
won't prevent lobing, it will just displace the maxima and minima in
angular terms w.r.t. the line through the speakers and the speaker plane.

As Arnie has also said, it does also depend on the inherent amplitude
and phase response of the drivers.

The key point here for me is the phase responses of the two drivers in the
frequency region where they are tending to radiate similar powers.

If they are 'in phase' at this point, then ensuring the vector sum is
unchanged should mean that the 'far field' power sent normal to the line
through the speaker units (i.e. towards the nominal listener) will be
correct.

However the above makes assumptions about what is the case. So, for
example, if the speakers have phase delays that differ when they are
radiating similar amounts, you'd need to change what you are giving them.
We also have to worry about where the listener may be and the room
acoustic.

All of this is another reason why I'm not really a fan of 'dynamic'
speakers. :-)

The advantage of the method I prefer is that it ensures both constant
amplitude sum (for the correct unit phase behaviour) and constant total
power. Does this by ensuring the vector sum gain from the filtering is
frequency independent. However this may not be what a specific speaker
requires.

Above said, for electronic crossovers, I'd tend to do it this way, then add
any required modifiers to 'pre-correct' the split signals before delivering
them to the power amps and units...

I use constant-voltage subtraction crossovers, but without any phase
compensation they do force one of the outputs to only roll off at 6db
per octave.

The advantage of higher orders is they can cut down to size of the region
where we have an (unwanted) array effect. However you can do this using my
approach, and it saves money as you only need one high-order LPF and then
get the HPF that matches it 'for free'. :-)

Slainte,

Jim

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