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crossover component formulae

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Category: General
Forum Name: Advanced Discussion
Forum Description: Advanced discussion area for higher lifeforms
URL: http://forum.speakerplans.com/forum_posts.asp?TID=98017
Printed Date: 24 March 2017 at 12:00pm
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Topic: crossover component formulae
Posted By: snowflake
Subject: crossover component formulae
Date Posted: 08 March 2017 at 7:33pm
Hi

the formulas for the component values in a second order butterworth filter are:

L=Z/(root2.Pi.f) and C=1/(2.root2.Pi.Z.f)

are there similar formulae for Linkwitz Riley and Bessel alignments?

cheers
Phil



Replies:
Posted By: Conanski
Date Posted: 08 March 2017 at 10:37pm
Originally posted by snowflake snowflake wrote:

are there similar formulae for Linkwitz Riley and Bessel alignments?


Yes.. there are.Big smile


Posted By: snowflake
Date Posted: 08 March 2017 at 11:04pm
Originally posted by Conanski Conanski wrote:

Originally posted by snowflake snowflake wrote:

are there similar formulae for Linkwitz Riley and Bessel alignments?


Yes.. there are.Big smile

you tease Wink


Posted By: _djk_
Date Posted: 08 March 2017 at 11:20pm
The formula only apply to resistive loads, and drivers that are ruler flat for a large distance beyond the crossover point.

In other words they are of little use in designing a real crossover.


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djk


Posted By: madboffin
Date Posted: 08 March 2017 at 11:33pm
I'm sure I have seen some online calculators that will give the theoretical filter component values. But the only use for a calculated crossover is as a starting point to get you in the ballpark before doing a proper design exercise - for the reason DJK gives in the previous message.

The component values will need a lot of tweaking before they work properly with real drive units.




Posted By: MarjanM
Date Posted: 08 March 2017 at 11:43pm
No they wont get you even close to the  ball park.
Crossovers are made with acoustical measurements, not by calculators.


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Marjan Milosevic
MM-Acoustics
www.mm-acoustics.com
https://www.facebook.com/pages/MM-Acoustics/608901282527713


Posted By: madboffin
Date Posted: 09 March 2017 at 12:12am
Well they will get you into the back row of seats if not the actual ball park...

When I worked for a major speaker manufacturer (long time ago) we would design crossovers using an iterative process. After examining the frequency response and impedance curves of the drivers (and maybe building zobel networks), the starting point would be a "calculated" set of filters built with component substitution boxes.

Then of course, lots of measurements - using either B&K chart recorders or the original TEF analyser (I said it was a long time ago...)

The substitution boxes made it easy to change components just by setting switches, so although it involved a lot of adjustments it was not difficult, just very time-consuming. But I used to get through an awful lot of chart paper...




Posted By: snowflake
Date Posted: 09 March 2017 at 1:15am
guys, I don't want to start a debate about crossover design. I'm actually quite into it and am trying to set up some calculations for filter components loaded with different zobel networks. I need the formulas not online calculators. does anyone know them or should I start trying to work them out from the transfer functions?

btw all the crossover i've designed in akabak have come out so near to what I expected that it wasn't worth tweaking them.


Posted By: snowflake
Date Posted: 09 March 2017 at 1:25am
okay the 2nd order LR components are the same value as the first order butterworth components. obvious when you think about it as LR is two cascaded Butterworth filters. Still stumped on the Bessel values.


Posted By: snowflake
Date Posted: 09 March 2017 at 1:52am
won't pretend I understand why but the Bessel values are a factor of root3 different to the LR values - just verified this by tapping numbers into the online calculator. so for normalised 2nd order bessel:

L=root3.Z/2.Pi.f
C=1/2.root3.Pi.Z.f

http://www.rane.com/note147.html" rel="nofollow - http://www.rane.com/note147.html


Posted By: audiomik
Date Posted: 09 March 2017 at 12:02pm
Phil
bench with many components here still if you want to try things empirically!
Mik

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Warning! May contain Nuts
plus springs, washers, screws, etc, etc.


Posted By: snowflake
Date Posted: 09 March 2017 at 12:15pm
thanks Mik, will show you what I come up with. Hopefully model once, build once ;)


Posted By: odc04r
Date Posted: 13 March 2017 at 9:00am
Best way is to look them up in a reference design table as a modification to the standard formulae you mentioned in the first post.

If you want to know where they come from then draw out the transfer function of a series LC lowpass filter, you will get a result with a polynomial on the denominator. This structure is what all more complicated filters are broken down into, because it is the basic filter building block. Ok a 1st order is really the simplest, but with one pole only you can't manipulate it as much.

Once you have the polynomial (coefficient of the s^2 term should be one to be in standard form). The coefficient of the linear term can then give you the Q factor of the filter. So if you want a certain Q and Wo, you reverse engineer from this point to get your L and C. Or you can just look them up as you did!

More reading https://en.wikibooks.org/wiki/Signal_Processing/Filter_Design" rel="nofollow - here , Matlab (or Scilab=free) is very handy for evaluating filter designs and generating nice visual plots. Python is probably quite good too, not used it myself for this purpose.

Edit - Forgot link



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