Using midrange roll-off in crossover - diyAudio
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Old 27th July 2009, 01:40 PM   #1
Stu is offline Stu  Australia
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Default Using midrange roll-off in crossover

I'm thinking of building a tri-amped 3-way with active crossovers... and in theory I don't see a reason that the following shouldn't work:

4th order Linkwitz-Riley crossover between the midrange and tweeter

2nd order Butterworth high-pass on the midrange at the -3 dB frequency of the midrange (sealed box, Q = 0.7)

4th order LR low-pass on the woofer at the -3 dB frequency of the midrange

This should give an effective 4th order crossover between midrange and woofer, right? So the two should be in phase at the crossover frequency without reversing either polarity?

Sorry if this has been asked before, tried a quick search and couldn't find anything.
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Old 27th July 2009, 06:05 PM   #2
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Sure, that will work fine. That's how the THX spec for the subwoofer crossover in AV receivers works. It's assuming a sealed main speaker with Q=.7 at 80 Hz. The electrical filters are a BW2-80 highpass and an LR4-80 lowpass.
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Old 27th July 2009, 10:48 PM   #3
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Without measurements we can only do guess work. But of course, sometimes guess work can turn out fine, but it needs a lot of maths and simulations.

In my active OB 4way sepakers under the final stage of development, an electrical 2nd order HP (close to BW2) on the tweeter at 2kHz combining with the tweeter's natural rolloff yields an almost perfect LR4 HP. An electrical 3rd order BW LP at 2kHz yields an accoustical response between LR4 and BW3 on the midrange. Note that a perfect accoustical LR4 is not targeted if there is (usually) an offset between the tweeter and midwoofers, in which case the phase would no longer be perfectly aligned. That is why I have an accoustical response between BW3 and LR4 on the midranges so that the phase at the XO point is aligned with the tweeter. They call this asymmetric XO.

Going down in frequency from there, an electrical 2nd order high Q HP yields a perfect BW3 accoustical response for the midrange at 180Hz. I also shaped the response of the woofer to be BW3 LP at 180Hz, using an electrical filter that looks like 4th order. I didn't bother asymmetric XO this time (too complicated and time-consuming). Instead, I introduced an allpass filter to the woofer to align the phase, so that the XO response looks like a perfect BW3. The allpass filter changes the phase at the XO frequency so that the woofers and midranges sum flat. This serves the purposes of both aligning the phase due to driver physical offset as well as swapping the polarity in a BW3 to obtain better group delay, sort of one stone killing two birds, in the simplest possible circuit.

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Bill
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Old 28th July 2009, 01:34 AM   #4
Francec is offline Francec  Australia
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Stu,
Why not use all 4th order and save yourself any potential grief?
Rod Elliott's 24dB L/R crossover will explain all you need.

Frank
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Old 28th July 2009, 02:39 AM   #5
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Quote:
Originally posted by Francec
Stu,
Why not use all 4th order and save yourself any potential grief?
Rod Elliott's 24dB L/R crossover will explain all you need.

Frank
Because what counts is the acoustical response. If the driver is rolling off 2nd order and you add a 4th order electrical filter, the acoustical response will be 6th order.
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Old 28th July 2009, 04:58 AM   #6
Stu is offline Stu  Australia
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Quote:
Originally posted by Francec
Stu,
Why not use all 4th order and save yourself any potential grief?
Rod Elliott's 24dB L/R crossover will explain all you need.

Frank
I'm thinking of using that, actually, but I was playing around with a few box designs in winISD and trying to figure out how to avoid the acoustical phase issues around the F3 frequency... the obvious choice being to turn the bug into a feature and let it form half the roll-off.

Thanks for the responses, I thought it would have been done before.
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Old 28th July 2009, 05:22 AM   #7
Francec is offline Francec  Australia
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Stu,
I chose drivers that had an extended response well past the cross-over points so that any 'additional' roll-off was irrelevant.

(I've had comments that I didn't do it the "right way" but skinning a cat can be done different ways too. )
Frank
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