Preamp low pass filter
 

In message , Old Fart at Play
writes
You can keep the low-pass and high-pass outputs in phase
with a 4th-order filter.


Not without an additional all-pass filter you can't. And with an added
all-pass you can make a second order crossover have no phase difference
between the LP and HP sections if you want.
--
Chris Morriss

Preamp low pass filter
 

In article , Chris Morriss
wrote:
In message , Old Fart at Play
writes
You can keep the low-pass and high-pass outputs in phase with a
4th-order filter.


Not without an additional all-pass filter you can't. And with an added
all-pass you can make a second order crossover have no phase difference
between the LP and HP sections if you want.

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
Filter Cookbook by Don Lancaster.

Slainte,

Jim

--
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Audio Misc http://www.st-and.demon.co.uk/AudioMisc/index.html
Armstrong Audio http://www.st-and.demon.co.uk/Audio/armstrong.html
Barbirolli Soc. http://www.st-and.demon.co.uk/JBSoc/JBSoc.html

Preamp low pass filter
 

Chris Morriss wrote:

In message , Old Fart at Play
writes
You can keep the low-pass and high-pass outputs in phase
with a 4th-order filter.

Not without an additional all-pass filter you can't. And with an added
all-pass you can make a second order crossover have no phase difference
between the LP and HP sections if you want.

Perhaps you would like to refer to the Loudspeaker Design Cookbook
which has graphs of amplitude and phase for various filters
including the fourth order Linkwitz-Riley filter.

--
Roger.

Preamp low pass filter
 

Old Fart at Play wrote:
Chris Morriss wrote:

In message , Old Fart at Play
writes
You can keep the low-pass and high-pass outputs in phase
with a 4th-order filter.

Not without an additional all-pass filter you can't. And with an
added all-pass you can make a second order crossover have no phase
difference between the LP and HP sections if you want.

Perhaps you would like to refer to the Loudspeaker Design Cookbook
which has graphs of amplitude and phase for various filters
including the fourth order Linkwitz-Riley filter.

Remember that it's the acoustical 4th order Linkwitz-Riley that has the
so-called zero phase difference, not some speaker implemented with 4th order
crossovers. The acoustical 4th order is implemented with second-order
crossovers. The other second-order filters are the roll-offs of the speakers
themselves.

Preamp low pass filter
 

In message , Old Fart at Play
writes
Chris Morriss wrote:

In message , Old Fart at Play
writes
You can keep the low-pass and high-pass outputs in phase
with a 4th-order filter.

Not without an additional all-pass filter you can't. And with an added
all-pass you can make a second order crossover have no phase difference
between the LP and HP sections if you want.

Perhaps you would like to refer to the Loudspeaker Design Cookbook
which has graphs of amplitude and phase for various filters
including the fourth order Linkwitz-Riley filter.


OK, I've looked at that, and it doesn't support what you say at all. I
think you are getting confused between a crossover that keeps the
outputs of all the sections in phase at all frequencies (which can only
be done with even-ordered networks, and then only in conjunction with
all-pass phase-correction networks) and the family of crossovers that
attempt to sum to a flat frequency and phase response, even though the
individual outputs have phase differences between them.

(By the way, you can't get a passive filter to have LP and HP outputs in
phase with each other at all frequencies, as it's not possible to
produce the right sort of all-pass network with passive components.)
You can of course make passive networks that have a summed flat
magnitude and phase response, but this is a different thing entirely.

Although all-pass phase corrected active crossovers can be made, they
are not universally liked, as the extra group-delay added by the
phase-compensating all-pass networks mean that the total variation in
phase across the whole audio band can be very considerable. (Whether or
not this is audible on music is debatable).

--
Chris Morriss

Preamp low pass filter
 

Jim Lesurf wrote:

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.

That's a neat trick. (Maybe tri-amping isn't such a bad idea...)


For the actual filters I tend to lift the basic designs from the
Active Filter Cookbook by Don Lancaster.

taking notes


--
Wally
www.artbywally.com
www.wally.myby.co.uk/music

Preamp low pass filter
 

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

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.
--
Chris Morriss

Preamp low pass filter
 

Chris Morriss wrote:

In message , Old Fart at Play
writes
Chris Morriss wrote:

In message , Old Fart at Play
writes
You can keep the low-pass and high-pass outputs in phase
with a 4th-order filter.

Not without an additional all-pass filter you can't. And with an added
all-pass you can make a second order crossover have no phase difference
between the LP and HP sections if you want.

Perhaps you would like to refer to the Loudspeaker Design Cookbook
which has graphs of amplitude and phase for various filters
including the fourth order Linkwitz-Riley filter.


OK, I've looked at that, and it doesn't support what you say at all. I
think you are getting confused between a crossover that keeps the
outputs of all the sections in phase at all frequencies (which can only
be done with even-ordered networks, and then only in conjunction with
all-pass phase-correction networks) and the family of crossovers that
attempt to sum to a flat frequency and phase response, even though the
individual outputs have phase differences between them.

Have the laws of physics changed since my LDC4 was published?

Section 7.21:Combined response of two-way crossovers
"....exhibit a high-pass and low-pass phase relationship which
is in-phase."

Graphs 7.58 and 7.59 show what I mean.

--
Roger.

Preamp low pass filter
 

In message , Old Fart at Play
writes
Chris Morriss wrote:

In message , Old Fart at Play
writes
Chris Morriss wrote:

In message , Old Fart at Play
writes
You can keep the low-pass and high-pass outputs in phase
with a 4th-order filter.

Not without an additional all-pass filter you can't. And with an added
all-pass you can make a second order crossover have no phase difference
between the LP and HP sections if you want.

Perhaps you would like to refer to the Loudspeaker Design Cookbook
which has graphs of amplitude and phase for various filters
including the fourth order Linkwitz-Riley filter.


OK, I've looked at that, and it doesn't support what you say at all. I
think you are getting confused between a crossover that keeps the
outputs of all the sections in phase at all frequencies (which can only
be done with even-ordered networks, and then only in conjunction with
all-pass phase-correction networks) and the family of crossovers that
attempt to sum to a flat frequency and phase response, even though the
individual outputs have phase differences between them.

Have the laws of physics changed since my LDC4 was published?

Section 7.21:Combined response of two-way crossovers
"....exhibit a high-pass and low-pass phase relationship which
is in-phase."

Graphs 7.58 and 7.59 show what I mean.


Ok, I can see where your confusion is coming from. No, the laws of
physics haven't changed, but the LS cookbook doesn't make things clear.
If you read on in the same section, you'll see that it says "the two
sections sum together flat when the level of both filters is down 6dB at
the crossover frequency".

This is the crux of the issue. To get a flat amplitude response from an
even order filter, the crossover frequency should be at the -6dB point,
but to get the two outputs to be in-phase (actually 180 out of phase,
but this is cured by turning the connections round on either the tweeter
or the bass unit), the crossover frequency needs to be at the -3dB
point.

Here's an example. It's for a second order Butterworth. (And remember
that a 4th order L_R is simply two identical 2nd order Butterworths in
series)

If the crossover is at the -3dB point, the phase is at 90 degrees at
that point, and the HP and LP will be consistently 180 degrees out of
phase, BUT the magnitude will sum to have a 3dB hump.

If the crossover is at the 6dB point, the magnitude will sum to be flat,
but as the phase shift at the -6dB point is 110 degrees (rather than 90)
the phase shifts of the two outputs will not track.

In reality a passive crossover is tweaked to give a compromise (and to
allow for the amplitude/phase characteristics of the drivers...if you've
got a competent design team that is.

An active crossover can be made to have perfect phase tracking between
the HP and LP outputs by judicious use of all-pass networks. (Though as
Jim says, that may not be what you want for best fidelity)
--
Chris Morriss

Preamp low pass filter
 

Chris Morriss wrote:

In message , Old Fart at Play
writes
Chris Morriss wrote:

In message , Old Fart at Play
writes
Chris Morriss wrote:

In message , Old Fart at Play
writes
You can keep the low-pass and high-pass outputs in phase
with a 4th-order filter.

Not without an additional all-pass filter you can't. And with an added
all-pass you can make a second order crossover have no phase difference
between the LP and HP sections if you want.

Perhaps you would like to refer to the Loudspeaker Design Cookbook
which has graphs of amplitude and phase for various filters
including the fourth order Linkwitz-Riley filter.


OK, I've looked at that, and it doesn't support what you say at all. I
think you are getting confused between a crossover that keeps the
outputs of all the sections in phase at all frequencies (which can only
be done with even-ordered networks, and then only in conjunction with
all-pass phase-correction networks) and the family of crossovers that
attempt to sum to a flat frequency and phase response, even though the
individual outputs have phase differences between them.

Have the laws of physics changed since my LDC4 was published?

Section 7.21:Combined response of two-way crossovers
"....exhibit a high-pass and low-pass phase relationship which
is in-phase."

Graphs 7.58 and 7.59 show what I mean.


Ok, I can see where your confusion is coming from. No, the laws of
physics haven't changed, but the LS cookbook doesn't make things clear.
If you read on in the same section, you'll see that it says "the two
sections sum together flat when the level of both filters is down 6dB at
the crossover frequency".

This is the crux of the issue. To get a flat amplitude response from an
even order filter, the crossover frequency should be at the -6dB point,
but to get the two outputs to be in-phase (actually 180 out of phase,
but this is cured by turning the connections round on either the tweeter
or the bass unit), the crossover frequency needs to be at the -3dB
point.

Here's an example. It's for a second order Butterworth. (And remember
that a 4th order L_R is simply two identical 2nd order Butterworths in
series)

If the crossover is at the -3dB point, the phase is at 90 degrees at
that point, and the HP and LP will be consistently 180 degrees out of
phase, BUT the magnitude will sum to have a 3dB hump.

If the crossover is at the 6dB point, the magnitude will sum to be flat,
but as the phase shift at the -6dB point is 110 degrees (rather than 90)
the phase shifts of the two outputs will not track.

Chris, you are still confused but nearly there.

As you say, a 4th order L-R is two 2nd order Butterworths.
At the crossover frequency a B2 filter is -3dB and 90 degrees phase
shift.
Therefore the 4LR is -6dB and 180 degrees.
The LP and HP outputs are in phase at all frequencies
and the voltage sum is constant.

HTH,

Roger.