# KEF - style Acoustic Butterworth 3rd Order w LRC?



## Synthsayer (Dec 19, 2007)

Jeff Bagby's Passive Crossover Designer has a calculator for an Acoustic Butterworth 3rd Order passive high pass filter. What makes it unique is an RC circuit that goes from the positive input of C1 to the output of C2. The RC circuit is calculated for a crossover Fr using driver Re, Fs, and L1 resistance.
From my understanding this is a KEF designed circuit intended to dampen the tweeter's impedance rise at it's Fs? in conjunction with the 3rd Order Filter?? 
Question: Should an LRC circuit for eliminating a tweeter's impedance rise at Fs be used with the Acoustic Butterworth 3rd Order passive filter?
It seems to me like it might be redundant from my novice understanding of the crossover topology and the purpose of the RC circuit described above.
Thanks for the help in advance
Have a nice weekend


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## robbo266317 (Sep 22, 2008)

What is the nominal impedance of the tweeter you are using compared to it's impedance at Fs.

I have not seen any passive crossovers that use this topology. Most simply compensate for the drivers rise in impedance with a simple RC circuit across the speaker.

Cheers,
Bill.


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## Synthsayer (Dec 19, 2007)

Here is the schematic from Jeff Bagby PCD 2.03 - it's in later versions, too .









Thanks again.


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## Synthsayer (Dec 19, 2007)

Nominal impedance of tweeter is 6 ohms. L1 resistance is .35 ohms. Fs of Tweeter is 1.5kHz. X-Over Fr = 3kHz.


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## robbo266317 (Sep 22, 2008)

Synthsayer said:


> Nominal impedance of tweeter is 6 ohms. L1 resistance is .35 ohms. Fs of Tweeter is 1.5kHz. X-Over Fr = 3kHz.


If the crossover frequency is twice Fs then I wouldn't worry too much about it. I would be more concerned if it fell within the crossover range. 

I must have a look into Jeff Bagby PCD 2.03 to see why it is recommended.

Do you have values for L,C and R in the crossover? I am intrigued.

Cheers,
Bill.


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## Synthsayer (Dec 19, 2007)

From my understanding this is supposed have a similar effect to conjugate impedance network to smooth impedance hump at resonant frequency much the same as series LRC.
To me, it looks like a series RC contour filter but the topology has me stumped. 
The tweeters I am using are a bit 'edgy'. Titanium dome and high Fs = 1.5kHz
Used LR 2nd order w series LRC conjugate impedance at resonant frewuency.
Have heard about the 3rd Order ACB nut never tried it. Have along weekend from work.
Seems like it is mentioned in Loudspeaker sookbook I'll douible check.
Thanks again


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## Synthsayer (Dec 19, 2007)

Screenshot of PCD Values for:
View attachment VIFA - d25ag-05-06e.pdf

Screenshot of entered values for VIFA Tweeter PDF:
View attachment Jeff Bagby - PCD 2_03 - 6ohm - 3kHz.pdf


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## robbo266317 (Sep 22, 2008)

The tweeter is fairly well behaved. The Fs resonance is ~ 8 Ω dropping to 5 Ω at mid range and then climbing to 8 Ω at 20 kHz.
I doubt that you need to worry about it. 

If you don't like the sound of the driver you could try something else.

Cheers,
Bill.


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## JimH (Nov 11, 2011)

Interestingly, the full-blown simulation section of PCD doesn't include this circuit option. You might want to post your question over on the Parts Express forum as Jeff B is a frequent poster there.


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## fusseli (May 1, 2007)

There's more than one way to skin a cat. I haven't seen this sort of topology before either. It may be usefull but I'm sure you can accomlish the same thing other ways. As far as I know, if you are crossing high enough above Fs then you may not even need to notch out the resonance at Fs. Naturally, other things help dampen the rise at Fs also. Such as the parallel leg of a tweeter l-pad.


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