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Old 19th February 2003, 02:35 AM   #1
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Default Variable frequency active crossover

Hello, is there any kit or design for a variable frequency active crossover?
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Old 19th February 2003, 02:45 AM   #2
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Marchand makes some excellent units, fairly priced.
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Old 19th February 2003, 02:50 AM   #3
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Default Crossover design

Hi Francois,
What frequency range are you looking for? What order of filter ?

If it is a second order filter ( 12db/octave) and the variable range about 1:10 or less , you can do it very easily. Quality will depend on the opamps you use. If you use the OPA2134 or similar, you can't go wrong.

If you give some details of what you want , maybe we could give you the solution.
Cheers.
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Old 19th February 2003, 01:26 PM   #4
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Marchand have great stuff!. I have a friend that has this crossover.

Maybe I can build for less??

I was thinking about a
second order linkwitz-riley filter (I find it more "progressive" then the 24dB of Marchand).

Variable frequency 0-10000Hz

3 ways

with an attenuator

The marchand modules seem simple so I guess this is not so hard to build a good and simple crossover.

thanks
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Old 19th February 2003, 01:51 PM   #5
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Default couple ways to do this

(this is a DIY forum, correct ?)<p>

for an analog filter to work correctly you have to closely match the values of R and C -- as the order of the filter increases the need for precision increases too. Thus, a 4th order filter is going to require 0.1% precision resistors. While a 2nd order filter you can use 1.0% -- the way to achieve a high degree of precision is to use the digital resistors from Dallas, Analog, Microchip etc. (It helps if you have a capacitance bridge to match the capacitors since these are seldomly sold with the same precision as resistors.)

The dallas, now www.maxim-ic.com, units can be controlled with a potentiometer.

Since the resistors are all cut on the same chip the acuracy is pretty darned good.

In the PDF for the Analog Devices AD8400 digital resistors (www.analog.com) there's a file for a state-variable active filter -- this is a good jumping off point.

It's is always a good idea, even though it tries our patience, to investigate the physics of what's going on in a filter. I would recommend two books which have cookbook approaches combined with the theory and practice "The Active Filter Handbook" by Tedeschi and "The Active Filter Cookbook" by Don Lancaster. I have the formulas in an Excel Spreadsheet. In regards to education, Texas Instruments has a really excellent tutorial on their site. Of course, TI has interactive filter software which you can download from their website.

There are also some older DAC's which allow access to the R2R ladder.
These might be cheaper than the AD8400, AD8403 etc.
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Old 20th February 2003, 11:36 AM   #6
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I understand but I am searching for a "no-brainer" for now. It is why I was asking for a kit.
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Old 20th February 2003, 11:43 AM   #7
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Default for a no-brainer

take a look at the application notes for the analog devices AD8400 digital pots -- a 3 way state variable filter -- it's on page 17 of this linked PDF: <p>http://www.analog.com/UploadedFiles/...8400_2_3_c.pdf
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Old 20th February 2003, 04:23 PM   #8
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How can I find that TI filter calculator? I cant seem to find it on their site.

Thanks,
Paul
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Old 20th February 2003, 06:00 PM   #9
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Default look on this page

http://focus.ti.com/analog/docs/util...42&familyId=57
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Old 22nd February 2003, 11:45 AM   #10
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thanks for the info, I will look at this.
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