***Official Linkwitz Transform Thread*** (1 Viewer)

[Anthony_Gomez]

Dan *L* ok thanks=)

a parametric EQ can function as a psuedo LT for sufficiently low Fb/Q systems. I find that q>0.6 are don't yield a very nice responce. Also, a low pass filter does not do what an LT does. It may act on the gain side as a parametric EQ, but on the low frequency side, it divebombs.

 

[Dan Pawlowski]

Anthony
"Also, a low pass filter does not do what an LT does."
I disagree, check with http://www.linkwitzlab.com/filters.htm#9
That is a low pass filter designed to equilize the hi pass function that the speaker + box creates.
The filter design would be a lot simpler if the speaker + box response were .707 (ie add a cap to existing active filter or gain stage with in the low frequency path)
Dan

 

[Dan Pawlowski]

Anthony

Also, a low pass filter does not do what an LT does."

ok ok its a bass boost or treble cut depending on whether you think the glass of beer is half empty or half full.


sorry for the confusion on my part

but an bass boost can be created by adding 1 cap to an existing filter or gain stage.

dan

 

[Anthony_Gomez]

ok dan...I see what you mean. it would work, but it would mess up your XO between the sub and mains. but if you are careful, you would be fine=)

 

[Dan Pawlowski]

Anthony

yes i agree it affects xo
my xo is at 55hz and i did a simulation on LSPCAD then after moditying swept the input and measured the response of the ouputs to conferm ythe filter was what i wanted...LSP is pretty close


dan

 

[Anthony_Gomez]

good to hear you got something that worked!

 

[Dennis XYZ]

I think there's quite a bit of confusion out there about what a Linkwitz transform is and how it works. Basically it's three parametric filters rolled into one. Engineers, please excuse the non-technical jargon; this is an explanation for the new guys. Poles and zeros tend to draw blank stares.
This is how you would go about building one with one of the popular DSP equalizer/crossovers.
The main part is a shelf filter (not a lowpass) that boosts the bass. The curve of that one is flat at high frequencies, then it slopes up at lower frequencies, and then it goes flat again lower still. It starts sloping up at the frequency where the speaker starts rolling off.
The second filter is a regular bell-shaped filter like the BFD does. That one has a center frequency the same as where the speaker starts rolling off and the shelf filter starts boosting. It adjusts the natural rolloff of the speaker so its Q matches the Q of the shelf filter.
The third filter is another bell shape. It's down at the lower frequency where the shelf filter flattens out. This one adjusts the final Q of the system.
So, let's say your shelf filter has a Q of .7 and your sub is a small box that rolls off at 40Hz with a Q of 1. You want a Q=.5, 20Hz final response. First step, apply a bell filter at 40Hz so the rolloff looks like Q=.7 and is 3dB down at 40Hz. Then apply your shelf filter and boost the bass so it's more or less flat and rolls off Q=.7, -3dB at 20 Hz. Finally apply a bell down at 20Hz and adjust the final rolloff so it's Q=.5.
With all that in mind, you can look at the graphs on SL's page linked above and maybe make some sense of them. Or maybe not.

 

[Anthony_Gomez]

Nicely put Dennis=)

Maybe I should just upload an image=)

 

[Dan Pawlowski]

Great post Dennis....Thanx for the perspective

 

[Anthony_Gomez]

Basically it's three parametric filters rolled into one. Engineers, please excuse the non-technical jargon;

As I said before, your post was a pretty good explaination, but I would say that an LT is more like a pair of parametric filters and one shelving filter
Hope that isn't too technical ..hehehe