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Old 6th October 2002, 06:56 AM   #1
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Default Crossover order and phase question

All right I did some searching here and couldn't find a definent answer, but let me tell you what I know and you guys correct me as needed.

I was leaning towards a 1st order crossover for its simplicity,but because of the target tweeters (Audax TM025F1) fs (1200hz) and freq resp (2500-20K) I was thinking it would be better to use a 2nd order for the steeper slope in the 2500-2800hz area (mating with a peerless 6 1/2 850467). However I keep hearing that the phase can change between the tweeter and the woofer. But the problem is sometimes poeple say it happens on the odd orders like 1st and 3rd, and some poeple say its the even orders. Now I've heard you can turn the polarity on the tweeter around and it will fix this, so is that true? I could just use the 12db second order and reverse the polarity?

Also is there a good website that shows the different wiring schemes of different crossovers and possible explain what a zobel (sp?) crossoever is and all that good stuff.

Man I need to dig out that loudspeaker cookbook!
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Old 6th October 2002, 07:51 AM   #2
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Default Re: Crossover order and phase question

For every pole (ie order) there is a 90 degrees phase shift. A 1st order XO is inherently phase correct (assuming everything is lined up & you have perfect drivers).

If you go to the FRD Group and download the passive crossover design calculator, it has many of the text book filters. There is a set of accompanying pictures to illustrate the topologies being calculated.

dave
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Old 6th October 2002, 11:11 AM   #3
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So then a 4th order would have a 24db curve and be completey in phase @ 360*? Would a 3rd order be like a 1st order just opposite? Also that page mentioned above looked very promosing, I'll download some of those when I get home.

Thanks much.
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Old 6th October 2002, 12:09 PM   #4
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Cool Phase and Delay in Crossovers

Hi Westrock2000,
One of the great misunderstandings in audio is that the phase of the crossover is not constant.
To give some examples:
A simple firstorder lowpass filter for the bass [a inductor] changes the phase from 0 degree at 20Hz to -90 degree at 20kHz. -45 degree at the crossover; 400Hz.
A fourth order Bessel highpass filter for the tweeter at 6000Hz has 0 degree phase at 20 Hz and -360 degree phase at 100kHz.
-180 degree at the crossover(6000Hz)
The delay is flat just up to the crossover point , then falls in a straight line on paper with double logaritmic scales (dB/frequency)
When combining all kinds of filters in a 3-way system the situation gets more complex. Also the above was for zero resistance of the coils. I real life the resistance of the coils is of course not zero and this complicates the situation even further. Also the driver does not have a constant impedance across its used frequency band. Overviewing all this the construction of a loudspeaker crossover is a work of Art and a lot of trial and error <B>and</B> listening!
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Old 7th October 2002, 01:05 AM   #5
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Ok heres another quick question. If you crossover a woofer with a 2nd order at 2500 and the same for the tweeter, than at 2500 each driver will be down 12db right? So is it better to say cross a woofer at 2600 and the tweeter at 2400, so that you get a little better overlap?
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Old 7th October 2002, 03:09 AM   #6
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Quote:
Originally posted by Westrock2000
If you crossover a woofer with a 2nd order at 2500 and the same for the tweeter, than at 2500 each driver
will be down 3 bB for a Butterworth XO, LR 6 dB?

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Old 7th October 2002, 06:22 PM   #7
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I have found this a very good website for designing cross-overs http://www.lalena.com/audio/ Also this link http://www.speakerbuilding.com/content/1044/ outlines the many problems to avoid with parrellel, those wired in parellel, cross-overs (otherwise known by me as the cross-overs most diy dudes use). Although he avoids phase shift problems by building a series cross-over, you probably won't want to do that. The link also has many other good projects.

To help you understand why people say it happens on 1st and 3rd cross-overs is because and correct if I'm wrong any one, is because inductors shilf the phase lets say, back 90 degrees for each inductor and capacitors shift it forward a little bit. So 1st and third ordet x-overs maybe maybe about 85 degrees and 255 degrees out of phase respectively but at you don't have almost total cancelation of frequencies like you do when they're 180 out-phase. That is to say, the wave peaks meet but one is high and one is low if you look at a graph or the ocean. Read the second link above it explains this part much better than I can.
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Old 7th October 2002, 06:34 PM   #8
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Nevermind the explaination it seems to be all wrong, now that I've read Elso Kwak's post again, but the links are still pretty chill.
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Old 9th October 2002, 05:36 AM   #9
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Most of what we know about crossovers is wrong, not in
the theory sense, but in making real speakers.

It's a fact, and any manufacturer is welcome to argue with
me, that real crossovers are the result of a tremendous
amount of screwing around and listening and measuring.

Days. Weeks. Months.

The Audax tweeters are known for very forgiving characteristics
if you don't drive them too hard, and I have had a lot of
success with about 3 uF in series with them and that's all.

But this depends a lot on what your are mating them with.
If it's the Audax carbon fiber 8", you're in luck. It will work
with either no crossover or maybe .25 mH. The result can
be extremely good.

Some other driver? Back to paragraph 2.

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Old 9th October 2002, 06:24 AM   #10
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Dear Mr Pass,

I think what westrock want is a starting point. should he use 12db/octave, 6db, 12db LR or someting in between or other.

As a rule to make my life as simple as possible I prefer to use drivers that do not require steep XOs as that complicates things for a DIYer who does not have access to measuring instruments.

I have seen speaker manufactuers use driver that have all sorts of peaks and valleys in their freq. curves and than use passive XOs (notch filters, etc...) to compensate for all of this in the end having XOs with as many as 50 components. I find it hard to believe that this would result in a good sound but I am not one to judge. The market does that better than me.

given this and your vast expereince what would you suggest Westrock do? 6db high pass with 6db low pass or 12 db low pass or even a Low pass that is 9db or so? Where does he start?

I ask this as a DIYer and one in India where I do not have access to LDC or other books and drivers are hard to find leave alone measuring instruments I have to look to the DIYAUDIO forum for help, pointers, and guidance. I would like to know how to start too? My drivers are not the same as his but are similar (Vifa TC series 6" and 1").
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