Active Crossover Design Help

[Frosty_47]

Hello, I would like to know a design approach to multiple-order crossover for a 3 way speaker system. I have no problem understanding second order systems. All I want is suggestions on how to implement multiple-order filters to make a high-fidelity active crossover. I have no problem designing 10th order filters if necessary.

Oh and this may sound crazy, but I do not have a specific set of speakers at the moment. I just want to know the "topologies" used in high fidelity active crossover designs.

-Thanks

 

[MikeMl]

Isn't it as simple as high pass for the tweeter, bandbass for the midrange, and low pass for the woofer?

 

[crutschow]

The authority on active crossover speaker filters is Siegfried Linkwitz. See Crossovers or **broken link removed**.

 

[Frosty_47]

Isn't it as simple as high pass for the tweeter, bandbass for the midrange, and low pass for the woofer?

Yes that is what comes to mind.

But what should the cutoff frequencies be? And what Bandwidth for Bandpass filter? Is there a specific standard? Or do should I select the cutoff according to a specific set of speakers ?

 

[Nigel Goodwin]

But what should the cutoff frequencies be? And what Bandwidth for Bandpass filter? Is there a specific standard? Or do should I select the cutoff according to a specific set of speakers ?

It varies depending on the speakers, the cabinets, and the rooms they are used in - it's as much an art as a science.

 

[Frosty_47]

It varies depending on the speakers, the cabinets, and the rooms they are used in - it's as much an art as a science.

So experimentation is the best way to go about this?

Also, what should I look for in:

Woofer
Mid-range driver
Tweeter

Thanks

 

[crutschow]

The crossover points and rolloff rate need to be carefully selected for the particular speakers' characteristics so that there is not a peak or gap at the crossover. Sometimes an all-pass filter is added to shift the phase of one of the crossover filters so that the two speakers stay in phase during the crossover (you don't want them out-of-phase and canceling each other's output).

It's not a trivial task to do it correctly. Linkwitz's articles covers that in more detail. As he notes, using active filters at an amplifier input with separate amps for each speaker gives much better crossover control than doing it with passive filters at the speaker inputs as is normally done.

Edit: To check out your results you need an accurate sound source and sound meter under anechoic conditions (outside open space works well).

 

[Frosty_47]

The crossover points and rolloff rate need to be carefully selected for the particular speakers' characteristics so that there is not a peak or gap at the crossover. Sometimes an all-pass filter is added to shift the phase of one of the crossover filters so that the two speakers stay in phase during the crossover (you don't want them out-of-phase and canceling each other's output).

It's not a trivial task to do it correctly. Linkwitz's articles covers that in more detail. As he notes, using active filters at an amplifier input with separate amps for each speaker gives much better crossover control than doing it with passive filters at the speaker inputs as is normally done.

Edit: To check out your results you need an accurate sound source and sound meter under anechoic conditions (outside open space works well).

Thanks. I will defiantly be adding all-pass filter for phase compensation. I can try and adjust the phase using an oscilloscope.

 

[crutschow]

Also, what should I look for in:

Woofer
Mid-range driver
Tweeter

All speakers should have as flat a frequency response as possible and low distortion. You can usually find information on the first parameter, but not much on the second. Thus it often means just buying a more expensive speaker from a known brand since sound quality is generally, (but not exactly), related to speaker cost.

Woofers should be designed for good fidelity, not necessarily for maximum output. For example the woofers designed for car boom-box systems give very high output levels, but their frequency response is usually rather peaky (all bass notes tend to sound the same).

For woofers you also have to decide whether to use a sealed box or a ported (reflex) design. The ported designs have higher output level but the sealed box designs generally have better transient control (have a "tighter" sound) and are often preferred for music listening. The reflex designs tend to be favored for home theater systems where rattle-the-walls sound levels are considered by some to be more important than music accuracy.

Edit: An additional refinement you can do with active circuits is compensate for dips or valleys in the speaker frequency response. You add a network that has the inverse response to the speaker's output, the result being a flat output from the speaker. One way to achieve this is with some type of notch filter (can be inverted to give a peak) with the width ,depth, and frequency of the peak adjusted to match the inverse of the speaker response.

You can also extend the low end of a subwoofer's frequency response by adding a filter that provides a boost in the frequency response below the normal rolloff of the subwoofer (see **broken link removed** for example).