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Old 1st April 2003, 07:25 PM   #1
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Smile Activ Filter

Hi

I'm thinking about changing my activ filter, from my current standard 24dB/oktav opamp design, to a design I saw on lc audio's homepage: http://www.lcaudio.dk/filters.htm
I put in a buffer stage to to be sure.
I'm just a little insecure about the values in the filter section.
Is it the same as with an opamp?

Would also like to know if anyone has tried this type of filter.

Hope you can help

Best regards
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Old 1st April 2003, 08:16 PM   #2
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Forgot the picture, sorry
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Old 6th April 2003, 09:11 AM   #3
EC8010 is offline EC8010  United Kingdom
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The filter you have shown is a standard Sallen & Key topology using an emitter follower with ring-of-two constant current sink.

Its disadvantage compared to the same circuit implemented with op-amps is that the gain of the emitter follower is fractionally less than 1, and this affects the filter response. For a 12dB/oct filter, it probably wouldn't matter, but for a 24dB/oct filter, it will cause phase errors around the filter frequency. (I'm assuming you are using it as a loudspeaker crossover.) It is possible to compensate for the non-unity gain using the equations given in the original S&K paper, but the equations are horrible.
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Old 8th April 2003, 05:48 PM   #4
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Hi EC8010,

Thanks for your answer
It's correct that it is to be used with a loudspeaker.

My current filter is 24dB/oct. linkwitz-riley filter, using TL082 opamps. Caps are BC Components MKP460/464 1% I believe.
I'm planning to improve it, even though it sounds ok.

I have read in other threads that 12/18 dB/oct. sounds better than 24. Isn't that a question about what speaker units you've got?
Think I've heard something about the better unit the softer crossover. Is that true?
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Old 9th April 2003, 10:41 AM   #5
EC8010 is offline EC8010  United Kingdom
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TL082 is a pretty crummy op-amp (but useful for checking that the crossover works). I have used OPA2604, or you could use OPA2134. Either will be much lower distortion and noise. Because S&K filters work by positive feedback, it is important to have a low-distortion gain device, otherwise the distortion goes up around the crossover frequency. I think you will hear a significant improvement simply by changing the op-amps!

24dB/oct Linkwitz-Riley is the ideal filter from about 700Hz up when drivers can no longer be considered coincident and phase/amplitude response is important. At lower frequencies, <300Hz, it can be more useful to use an 18dB/oct filter because the power response is flatter.

The most important thing is matching the two channels and getting driver levels right, for which you need some sort of MLS measurement system, such as Liberty Audiosuite (or whatever its current incarnation is called). Having the loudspeaker in the corner of the room is going to add problems...
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Old 9th April 2003, 10:50 AM   #6
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The main advantages of LR4 are:

1.) flat frequency response
2.) less IMD and THD generated by the drivers due to better separation
3.) good vertical radiation

The main disadvantage compared to some lower order crossovers: It has worse temporal behaviour.

Regards

Charles
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Old 10th April 2003, 03:53 PM   #7
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So when my crossover is around 300 a sallen & key is preferred?

About the speaker in the corner. Isn't that maintly with vented speakers there is a problem or is the lower frequencies boostet even the cabinet is closed?
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Old 11th April 2003, 11:23 AM   #8
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Default Boundaries

Any loudspeaker in a corner is in trouble. When you put a loudspeaker near a boundary place (for example standing on the ground in an open field) there is a reflection of the source. In effect, the loudspeaker sees on a 2pi (hemisphere) space, rather than 4pi (sphere). Output rises by 3dB. But when the path length from the source to the mirror, and back again is half a wavelength, there is cancellation. With only one plane, the cancellation is quite mild. With two planes, (floor and one wall) the cancellation becomes deeper, and the rise below cancellation is 6dB. With three planes (two walls and floor) the cancellation becomes noticeably deep, and the rise reaches 9dB. The relevent paper is, "The Influence of Room Boundaries on Loudspeaker Power Output" Roy F Allison JAES June 1974 Vol22 No5 pp314-320.

300Hz is a grey area. Get those speakers out of the corners, and then start worrying about crossover slopes.
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Old 11th April 2003, 12:10 PM   #9
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Default OT...

But they are beautiful speakers and I believe audio is for living...Technically you shouldn't have your electronics between speakers because this introduces vibrations. I have heard as this configuration described as the American setup. Not sure why really...

The best way to listen to music is to have your speakers in an anechoic chamber without you in it, and without your electronics aswell...Not possible...

Just listen and enjoy...My opinion is that some people (although not many - and I'm not getting at ANNYONE here) is that they are so caught up with setting things up right and making the perfect equipment, that they only hear the problems with it...

Again, lovely speakers,
Gaz
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Old 13th April 2003, 11:14 AM   #10
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EC8010,

I can get the speakers about 1m out from the corner, don't know if it's enough? The main problem is that the speakers stand 1.7m and has a weight around 110kg and the livingroom is only 30 m2

Rarkov,

Couldn't agree more. Many people haven't the slightest interest in the music. They are just looking for flaws in their gear.
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