Minimum phase design "recipe"

[dabbler]

This is a theoretical question about designing a minimum phase passive crossover. If you could put together a simple “recipe” for such a design, would it go something like this:

1.pick two drivers with wide overlap and good frequency response
2.Use first order network to connect the drivers to achieve 6db slope through the critical region (inductor on woofer, capacitor on tweeter)
3. Flatten impedance curves of both drivers with zobels. (Do the drivers need to have the same impedance once flattened?)
4. physically align driver acoustic centres relative to listening position

Is this the right direction to achieve minimal phase shift over a wide range? How closely does the driver rolloff have to follow a 6db slope to achieve this?

[sreten]

Hi,

Its a nebulous task with real drivers.

If you want something simple :

Use a bass driver with a full range mid/treble unit e.g. FR88EX.
( http://zaphaudio.com/smalltest/ )

Arrange the bass driver sensitivity to be higher than the FR by the
amount of BSC (baffle step compensation) you think you will need.

Design the box for the bass driver, ideally sealed with a Q of around 0.6.

Cross them over with a 1st order series network at the baffle step frequency.
(This should be quite a bit higher than the FR Fs ....)

Google and investigate all terms above not understood.

Sod Zobels and such guff, the above does not need them.

The above will get you as near to minimum phase issues as is sensibly possible.

rgds, sreten.

[planet10]

Originally Posted by sreten

Use a bass driver with a full range mid/treble unit e.g. FR88EX.
( Zaph|Audio )

Arrange the bass driver sensitivity to be higher than the FR by the
amount of BSC (baffle step compensation) you think you will need.

Design the box for the bass driver, ideally sealed with a Q of around 0.6.

Cross them over with a 1st order series network at the baffle step frequency.
(This should be quite a bit higher than the FR Fs ....)

Except for the possibility of different bass alignments i agree totally with Sreten.

Sreten's bass alignment but active instead of passive series (and a bit of fiddling to deal with baffle step), Tysen fits the recipe.

Except for the use of ML-TL loading, this EL166 MTM fits the receipe.

One of the reasons these will work, but a mid-woof/dome is VERY difficult, is because the acoustic centre of a driver moves with frequency & with dynamics. By XOing quite low you can get the drivers essentially co-incident.

dave

[dabbler]

Thanks guys, ill do a bit more digging through those links

[Loren42]

Originally Posted by sreten

Hi,

Its a nebulous task with real drivers.

If you want something simple :

Use a bass driver with a full range mid/treble unit e.g. FR88EX.
( Zaph|Audio )

Arrange the bass driver sensitivity to be higher than the FR by the
amount of BSC (baffle step compensation) you think you will need.

Design the box for the bass driver, ideally sealed with a Q of around 0.6.

Cross them over with a 1st order series network at the baffle step frequency.
(This should be quite a bit higher than the FR Fs ....)

Google and investigate all terms above not understood.

Sod Zobels and such guff, the above does not need them.

The above will get you as near to minimum phase issues as is sensibly possible.

rgds, sreten.

And why is a Zobel unneeded in this case?

[speaker dave]

Lets not forget, that if you want a minimum phase system (not "minimum phase crossover") you will have to allign the acoustic centers of the drivers! This means recessing the tweeter voice coil to approx. the depth of the woofer voice coil.

David S.

[sreten]

Originally Posted by Loren42

And why is a Zobel unneeded in this case?

Hi,

Zobels are "needed" for 1st order parallel crossovers, but for a
first order series c/o they are more of a hindrance than a help.
Simply don't do any useful for series so simply leave them out.

rgds, sreten.

[doug20]

Why not just go active with linear phase filters?

[speaker dave]

Originally Posted by doug20

Why not just go active with linear phase filters?

Linear phase filters will not equate to a linear phase system. Driver phase will still be there. I'm also not sure if a linear phase highpass and linear phase lowpass would have the same group delay (and linear phase networks will have group delay). A first order total response (driver and network) can add to flat response and flat phase.

Still, we are fooling ourselves if we ignore the different group delays due to the relative driver depths. What was the objective here? Linear phase system or linear phase crossover?

David S.

[sreten]

Hi,

When c/o at the baffle step frequency voice coil position error is minimal.
The whole thing is an approximation, and near as you can sensibly get.

Whether the whole is laudible or not is a different matter, but its an approach.

Any frequency response errors will introduce phase errors by definition.
Getting BSC right also fixes its phase error as well as the response error.

rgds, sreten.