Active Crossovers & Baffle Step Compensation

Mikett · 28th April 2005, 02:10 PM

Why do designers of crossovers ignore this? OK, I can understand it's a crossover.
But then builders using these on real world boxes need an extra stage to compensate for this. Why do they choose to ignore this after they have gone to the trouble of multi-amps?

Who offers a ready built PCB that can add this?

tiroth · 28th April 2005, 02:35 PM

It might be ignored because you CAN ignore it -- if you set the crossover frequency at the baffle step transition. Then you need only adjust the level to correct for baffle step, something which is rather easier to do actively.

Mikett · 28th April 2005, 03:22 PM

But many if not most people are favoring a 4th order LR slope. Isn't this rather steep to accomodate baffle step? What about phase issues?

Furthermore if one uses a state variable filter, like the Marchand's one cannot separate the crossover points.

Davey · 28th April 2005, 03:37 PM

The MT1 board from Siegfried Linkwitz can address all of these issues.

http://www.linkwitzlab.com/pcb.htm#MT1

Davey.

tiroth · 28th April 2005, 03:45 PM

Baffle step is normally in the 3-4dB range due to room effects. Although you are correct that a 4th-order transition won't be correct, as long as there is a filter of about the right order in the right spot the ripple that results will probably be no worse than the ripples due to diffraction, driver FR variation, etc. I don't think that many people are using 4th order LR crossovers -- most people that are going for 4th order slopes use 2nd or 3rd order crossovers.

You are correct about the limitations of state variable crossovers, but that is really irrelevant -- there is no need for a asymmetric crossover to correct for baffle step.

Nothing you say is wrong, but when designing real world speakers you have to keep things in perspective. Nothing is flat, and the transition points and magnitude are going to have to be determined experimentally if you want them to be really accurate. If at the end of the day you can get a response that is +-3dB, you are doing great. There is no need to sweat an error of a dB or two, unless of course it is compounding another error. By the same token, an "error" in your baffle step filter may even serendipitously correct for another anomaly! This is why designing loudspeakers is an iterative process no matter how much modeling you do.

RHosch · 28th April 2005, 06:14 PM

State variable designs do allow for separation of crossover points in a two way design if you plan ahead. The full SV circuit (as opposed to the abridged version) gives low pass, band pass, and high pass. The transitions from low to band and from band to high pass are independently variable. In a two way, if you ignore the band pass tap and use only the low and high pass taps then you can independently set the crossover point of each. It is true however that many SV designs use a somewhat consolidated topology that effectively has no band pass (the output tap is there in spirit, but its bandwidth is zero I believe) and the high/band and low/band transition points are locked in step. I use this simplified version myself. There is also an even simpler SV version that only provides either high or low pass output (can't remember which), which is nice when you need only one side of an XO to be continuously variable, but otherwise isn't of that much usefulness.

planet10 · 28th April 2005, 08:47 PM

Sometimes, thru use of different terminolgy, you can miss the BSC. ie Marchand has had this option all along.

dave

Mikett · 28th April 2005, 09:54 PM

What feature is on the Marchand? There is a the variable resistor to vary how the two bands sum, but I don't see anything that can shelve a bit.

If you implement the correction via staggering the crossover point, in practice that is difficult because in a two way, it would require that the tweeter cross at a very low point wouldn't it?

planet10 · 28th April 2005, 10:02 PM

Quote:

Originally posted by Mikett
What feature is on the Marchand?

I think it is called the Bassis.

dave

catapult · 28th April 2005, 10:36 PM

The Bassis is a similar but somewhat different critter. It's a pole shifter or "Linkwitz Transform" circuit to extend and shape the bass response of sealed woofers. It's a 12dB/octave shelving filter plus Q adjustment at each end of the slope. Baffle step is usually only about 3dB/octave.

http://www.linkwitzlab.com/filters.htm#9

28th April 2005, 02:10 PM	#1
Mikett diyAudio Member Join Date: Aug 2003 Location: Toronto	Active Crossovers & Baffle Step Compensation Why do designers of crossovers ignore this? OK, I can understand it's a crossover. But then builders using these on real world boxes need an extra stage to compensate for this. Why do they choose to ignore this after they have gone to the trouble of multi-amps? Who offers a ready built PCB that can add this?

28th April 2005, 08:47 PM	#7
planet10 frugal-phile(tm) diyAudio Moderator Join Date: Oct 2001 Location: Victoria, BC, NA, Sol III Blog Entries: 4	Sometimes, thru use of different terminolgy, you can miss the BSC. ie Marchand has had this option all along. dave __________________ community sites t-linespeakers.org, frugal-horn.com ........ commercial site planet10-HiFi p10-hifi forum here at diyA

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28th April 2005, 02:35 PM	#2
tiroth diyAudio Member Join Date: Dec 2001 Location: Pittsburgh, PA, USA	It might be ignored because you CAN ignore it -- if you set the crossover frequency at the baffle step transition. Then you need only adjust the level to correct for baffle step, something which is rather easier to do actively.

28th April 2005, 03:22 PM	#3
Mikett diyAudio Member Join Date: Aug 2003 Location: Toronto	But many if not most people are favoring a 4th order LR slope. Isn't this rather steep to accomodate baffle step? What about phase issues? Furthermore if one uses a state variable filter, like the Marchand's one cannot separate the crossover points.

28th April 2005, 03:37 PM	#4
Davey diyAudio Member Join Date: Dec 2001 Location: Bremerton, WA.	The MT1 board from Siegfried Linkwitz can address all of these issues. http://www.linkwitzlab.com/pcb.htm#MT1 Davey.

28th April 2005, 03:45 PM	#5
tiroth diyAudio Member Join Date: Dec 2001 Location: Pittsburgh, PA, USA	Baffle step is normally in the 3-4dB range due to room effects. Although you are correct that a 4th-order transition won't be correct, as long as there is a filter of about the right order in the right spot the ripple that results will probably be no worse than the ripples due to diffraction, driver FR variation, etc. I don't think that many people are using 4th order LR crossovers -- most people that are going for 4th order slopes use 2nd or 3rd order crossovers. You are correct about the limitations of state variable crossovers, but that is really irrelevant -- there is no need for a asymmetric crossover to correct for baffle step. Nothing you say is wrong, but when designing real world speakers you have to keep things in perspective. Nothing is flat, and the transition points and magnitude are going to have to be determined experimentally if you want them to be really accurate. If at the end of the day you can get a response that is +-3dB, you are doing great. There is no need to sweat an error of a dB or two, unless of course it is compounding another error. By the same token, an "error" in your baffle step filter may even serendipitously correct for another anomaly! This is why designing loudspeakers is an iterative process no matter how much modeling you do.

28th April 2005, 06:14 PM	#6
RHosch diyAudio Member Join Date: Sep 2003 Location: SouthEast	State variable designs do allow for separation of crossover points in a two way design if you plan ahead. The full SV circuit (as opposed to the abridged version) gives low pass, band pass, and high pass. The transitions from low to band and from band to high pass are independently variable. In a two way, if you ignore the band pass tap and use only the low and high pass taps then you can independently set the crossover point of each. It is true however that many SV designs use a somewhat consolidated topology that effectively has no band pass (the output tap is there in spirit, but its bandwidth is zero I believe) and the high/band and low/band transition points are locked in step. I use this simplified version myself. There is also an even simpler SV version that only provides either high or low pass output (can't remember which), which is nice when you need only one side of an XO to be continuously variable, but otherwise isn't of that much usefulness.

28th April 2005, 09:54 PM	#8
Mikett diyAudio Member Join Date: Aug 2003 Location: Toronto	What feature is on the Marchand? There is a the variable resistor to vary how the two bands sum, but I don't see anything that can shelve a bit. If you implement the correction via staggering the crossover point, in practice that is difficult because in a two way, it would require that the tweeter cross at a very low point wouldn't it?

28th April 2005, 10:36 PM	#10
catapult diyAudio Member Join Date: Dec 2002 Location: Planet Earth	The Bassis is a similar but somewhat different critter. It's a pole shifter or "Linkwitz Transform" circuit to extend and shape the bass response of sealed woofers. It's a 12dB/octave shelving filter plus Q adjustment at each end of the slope. Baffle step is usually only about 3dB/octave. http://www.linkwitzlab.com/filters.htm#9

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