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Old 2nd November 2003, 01:50 AM   #1
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Default Need help designing crossover

I need to build a low pass opamp crossover. Second order should be good. It needs to have a selectable frequencies of 80, 100 and 150hz. I can take care of wiring the 3 frequencies. What resistors and caps do I need for each? Also what opamp should I use? I have a 741 (somewhere) and a ton of LM324. Any chip from national I can get. I would prefer the 324. Could somebody help me by providing a schematic for the crossover? My plan is to run it off of the +- 28V supply powering the sub amp, and take power from the + and gnd through a 5V regulator (7805) and to the opamp. Any help would be appreciated.

Thanks, Mike
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Old 2nd November 2003, 05:09 PM   #2
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use a dip14 socket and 14pin socket header ( both are cheap at www.allelectronics.com )

i will post the schematics later. i like the lm837n, but its not at allelectronics. its at another place, 4 for $1! low noise, ect...

in any case, the freqency and Q of filters can be changed with modules containing 2 resistors and 1 capacitor per lowpass stage (2nd order lowpass). this works out to allow a 4th order lowpass for a single chip. the end filter allows for modules to affect Q, f3, and order with just a socket and a couple jumpers.

i's use a lm317 - lm337 combo, or any other +-12 to +-18v regulator (well, +-18 for my opamps, not all like this, so check to see if it'll work!)

i actuall have a 3rd order lowpass at home that i made last year...
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Old 2nd November 2003, 08:42 PM   #3
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here's the schematic. easy.

the circled components are on a module.
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Old 3rd November 2003, 01:56 AM   #4
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What would the values of the resistors and caps be for each freq? And will the LM324 work?
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Old 3rd November 2003, 02:36 AM   #5
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what order of filter are you looking for? the lm324 will technically work, but isn't an ideal canidate because of the higher than optimal input noise. if the noise floor is high in your listening environment, or you just want to make something, it will pass audio though.
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Old 4th November 2003, 01:29 AM   #6
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What chip would be best for this job? Something from National Semiconductor would be the best. Like I said in the beginning, it needs to be second order.
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Old 4th November 2003, 01:37 AM   #7
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look for a low noise opamp. i've always used lm837n as they are very cheap and low noise. ne5532 (i think) is another classic choice. the opamps you list will work, but are not low noise type opamps.

in any case, if al you need is 80 - 100 - 150 you could use jumpers to simplify the circuit. i like the modules because you could chnage frequency and Q and even order if you make the circuit right.

there are calculators for sallen-key filters. search yahoo for second order lowpass and you'll find some.
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Old 4th November 2003, 01:44 AM   #8
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i can't find the online calc i really like, i'll do some calcs later.
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Old 4th November 2003, 07:44 AM   #9
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On-line Filter Calculator

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Old 4th November 2003, 08:38 AM   #10
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The LM324 has class C output stages with 3VBE dead zones. It generates excessive cross-over distortion unless you take measures to bias it in class A. If you must use it, put a resistor between the output and the negative supply line with a value such that the op-amp output is always sourcing current and never sinking any.
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