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
Thanks, Mike
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...
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...
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.
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.
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.
I think I am sure on the resistor values... I used the active filter designer in WinISD Pro Alpha (a speaker design software) and typed in 80 for the cutoff freq. and 0.1 for the capacitor value. I then used circuitmaker to make a schematic for 2 channels. Why do you ask? Do they not seem right?
> I would prefer the 324
No, it is an awful audio chip. I nearly lost a job over it. I saved my butt by adding the class A drain resistor: that was good enough for live hillbilly music in the park but next day I threw away all my LM324.
741 is an old classic, better than 324, but you can do a lot better.
The best easy-to-use active filter chip is probably the TL071, TL072 and TL074 series. Low power drain: they last a long time on a couple 9V batteries. Zero input current: you can use high-value filter resistors which means small cheap caps (or affordable exotic caps).
I also love the LM833 and LM837. Though you can't take resistor values really high, they can be high enough. These also have much beefier output stages than a TL07x (or semi-linearized 324), so you can drive low-Z filter or load values.
NE5532 is an audio favorite, out of fashion because it is older than many of the people building audio toys. When used with understanding, it is still fault-free and will drive heavy loads to high voltage and current. It isn't a great choice in general filter design because it has large input current, so even 47K filter resistors cause significant offset.
There are a heap of newer sexier op-amps. Some are better, though you may not notice, especially in subwoofer duty. Some are hard to use, they like to become mini radio transmitters.
Get yourself a pile of LM072 and some sockets. Nearly any audio chore "can" be done with TL07x, and not stink. (The LM324 will stink.) With the socket, you can prove basic function on cheap easy TL07x chips, then try sexier chips and see if it makes a difference.
The filter calculator Dave points to is "correct", but was made by a mathematician. You really should turn up a copy of the Active Filter Cookbook(*). Even this excellent book can be criticized for giving "right" answers instead of "best" answers: simplest, cheapest, most likely to work nicely for simple audio tasks. (You do need "right" answers for complicated chores with stiff specs.)
Here is an off-the-cuff unity gain 2-pole (12dB/8ve) 150Hz low-pass:
The cap values were picked to be "available", not "mathematically perfect". It has a 0.3dB peak re DC, though when you look at 40Hz and up the peak is only 0.2dB. Use 5% caps if possible, though it will still be fine in subwoofer work with 10% caps.
To change frequency, scale both resistors higher for lower freq.
72K = 150Hz
91K = 120Hz
110K = 100Hz
Use 5% resistors, 2% if they are handy. Actually, two 10% resistors hand-picked to 5% matching will work, though the frequency will be "off" by the amount of resistance "error". Take two "100K 10%" resistors, find two that are really 104K and 106K, the -3dB point will really be 95Hz and the response-shape "error" will be inaudible. But the resistor and capacitor ratios should be very close to the design: a 20% ratio error (say 0.01µFd and 0.022µFd) will change the neat corner to a slump or peak. You may not be at all unhappy with 0.01 and 0.022 or 0.033: the corner error is about one dB.
Get rid of those LM324. While they "work" on the bench, they are unlistenable and not worth gimmicking-up to make them less unlistenable. (If you track my other opinions, you will know I am generally indifferent to devices, but the 324 does offend even my dull ears.) The TL07x is hardly the ultimate audio chip, but is a hell of a lot better than 324 for general messing-around and even some "serious" work. (semi-Pro and even some Pro audio was full of TL07x for decades; you have heard or not-heard these chips a thousand times.)
(*) I link to Amazon only for reference, to identify the book. I am not endorsing Amazon. They have been OK to me over the years, but are currently on my poop-list for a series of stupid service screw-ups. Buy from the vendor of your choice; the Amazon listing has all the details you need to find it order it at B+N, ABE.com, or Freddy's Books And Bait Shop.)
No, it is an awful audio chip. I nearly lost a job over it. I saved my butt by adding the class A drain resistor: that was good enough for live hillbilly music in the park but next day I threw away all my LM324.
741 is an old classic, better than 324, but you can do a lot better.
The best easy-to-use active filter chip is probably the TL071, TL072 and TL074 series. Low power drain: they last a long time on a couple 9V batteries. Zero input current: you can use high-value filter resistors which means small cheap caps (or affordable exotic caps).
I also love the LM833 and LM837. Though you can't take resistor values really high, they can be high enough. These also have much beefier output stages than a TL07x (or semi-linearized 324), so you can drive low-Z filter or load values.
NE5532 is an audio favorite, out of fashion because it is older than many of the people building audio toys. When used with understanding, it is still fault-free and will drive heavy loads to high voltage and current. It isn't a great choice in general filter design because it has large input current, so even 47K filter resistors cause significant offset.
There are a heap of newer sexier op-amps. Some are better, though you may not notice, especially in subwoofer duty. Some are hard to use, they like to become mini radio transmitters.
Get yourself a pile of LM072 and some sockets. Nearly any audio chore "can" be done with TL07x, and not stink. (The LM324 will stink.) With the socket, you can prove basic function on cheap easy TL07x chips, then try sexier chips and see if it makes a difference.
The filter calculator Dave points to is "correct", but was made by a mathematician. You really should turn up a copy of the Active Filter Cookbook(*). Even this excellent book can be criticized for giving "right" answers instead of "best" answers: simplest, cheapest, most likely to work nicely for simple audio tasks. (You do need "right" answers for complicated chores with stiff specs.)
Here is an off-the-cuff unity gain 2-pole (12dB/8ve) 150Hz low-pass:
An externally hosted image should be here but it was not working when we last tested it.
The cap values were picked to be "available", not "mathematically perfect". It has a 0.3dB peak re DC, though when you look at 40Hz and up the peak is only 0.2dB. Use 5% caps if possible, though it will still be fine in subwoofer work with 10% caps.
To change frequency, scale both resistors higher for lower freq.
72K = 150Hz
91K = 120Hz
110K = 100Hz
Use 5% resistors, 2% if they are handy. Actually, two 10% resistors hand-picked to 5% matching will work, though the frequency will be "off" by the amount of resistance "error". Take two "100K 10%" resistors, find two that are really 104K and 106K, the -3dB point will really be 95Hz and the response-shape "error" will be inaudible. But the resistor and capacitor ratios should be very close to the design: a 20% ratio error (say 0.01µFd and 0.022µFd) will change the neat corner to a slump or peak. You may not be at all unhappy with 0.01 and 0.022 or 0.033: the corner error is about one dB.
Get rid of those LM324. While they "work" on the bench, they are unlistenable and not worth gimmicking-up to make them less unlistenable. (If you track my other opinions, you will know I am generally indifferent to devices, but the 324 does offend even my dull ears.) The TL07x is hardly the ultimate audio chip, but is a hell of a lot better than 324 for general messing-around and even some "serious" work. (semi-Pro and even some Pro audio was full of TL07x for decades; you have heard or not-heard these chips a thousand times.)
(*) I link to Amazon only for reference, to identify the book. I am not endorsing Amazon. They have been OK to me over the years, but are currently on my poop-list for a series of stupid service screw-ups. Buy from the vendor of your choice; the Amazon listing has all the details you need to find it order it at B+N, ABE.com, or Freddy's Books And Bait Shop.)
So, I think I will go with the lm837, as long as it will work
theChris-where did you come up with those 100nf caps? I am using 0.1uf.
My schematic will work, right?
Also, I figured that since I have a large bag (over 1000) of the LM324s, I tend to prefer to use what I have before buying aditional parts. But I do want a very clean sounding sub, so I, as I said above, will go with the 837
theChris-where did you come up with those 100nf caps? I am using 0.1uf.
My schematic will work, right?
Also, I figured that since I have a large bag (over 1000) of the LM324s, I tend to prefer to use what I have before buying aditional parts. But I do want a very clean sounding sub, so I, as I said above, will go with the 837
well, really what you should do is get a socket and then try the lm837n vs the lm324. this way you can identify how much difference there is with these opamps.
1000nF = 1uF, 0.1uF = 100nF. for crossovers in audio i prefer to keep the nF unit as most of my filters use values between 5 and 100 nF.
1000nF = 1uF, 0.1uF = 100nF. for crossovers in audio i prefer to keep the nF unit as most of my filters use values between 5 and 100 nF.
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