Looking for an active crossover design (low pass only)?

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Ok i know this isnt really an amp as the forum intention but it is the front end to one. I'm building a small active sub to go with some speakers I've built, but I was wondering if anybody had a design for a filter that would be low-pass only with vaiable crossover point and subsonic filter around 10Hz, Idealy with a small semi parametric eq as well? any help would be great other wise i'll be starting compleetly from scratch.
 
This is an active low pass filter that has an adjustable Fc and has a 40dB per decade rejection. Simple op-amp circuit found in most electronics text books.
 

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I just designed and built an active crossover with 2 way stereo and subwoofer with subsonic filter and boost. I used the following pages to design all aspects of my crossover, including the power supply:

http://sound.westhost.com/project09.htm

http://www.snippets.org/filters/crossover.htm

I purchased parts from:

www.digikey.com, polypropylene capacitors, box, power switch

www.allelectronics.com, electrolytic caps, RCA input/output units (good deal), 16VAC transformer ($3.50!)

www.futurlec.com, metal film resistors ($0.20 to $0.35 for 10), misc capacitors, PCB board (I used stripboard), heatsinks

I used the power supply design from Snippets, 2-way from Sound.Westhost, and sub crossover boost from Snippets. In the end I combined parts of each website. I used the buffers from the first site for the inputs, used the Snippet's "Shiva" design to produce the active boost and lowpass for the sub. I did the mono summing after the buffers, then the lowpass, then the highpass subsonic boost.

I used the excel sheet on the Snippets website to calculate the component values I needed for the crossover, except for the buffer stage and output resistors which I used the info from the Westhost page.

I'm no expert, but it works for me. So, good luck!

Edit:

By the way, I started out with the same intentions as you, but eventually decided to just make a biamped system with sub for my high-powered PA/DJ style system and then found a Carvin 1000w amp for my sub and a Audio Centron 125w x 2 for the high on Ebay, all together costing me less than $170 for the extra amps and then $60 for the crossover. Overall, it's not that much more expensive to just make a biamped system, and many will argue it's much better. I haven't yet been able to fully experience my efforts (college is interfering), but I'm sure I'll be very happy!
 
You know :D if you just expand this circuit out, make 11 or more bandpass filters, use a slide pot as a voltage devider for each bandpass to adjust that specific frequency, you would have a pretty good EQ. Just adjust the RC componants for the proper Fc low and Fc high for each bandpass filter. Lots of surface quad op-amp chips, small capacitors, resistors, and soldering skills needed for this though. If you got the skills and the time, you can make an EQ as accurate as you want.
 
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