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Old 6th November 2003, 10:05 PM   #11
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Would this work? I got the values for 80hz and 100hz and the schematic from WinISD Alpha. I will also have the 150hz resistors and caps, but circuitmaker doesn't have a 3-position switch.
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Old 6th November 2003, 11:24 PM   #12
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why not move c3 and c4 to the other side of the second switch so that you reduce the number of excess parts. you probably could do the same with c1 and c2.
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Old 6th November 2003, 11:28 PM   #13
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here is the revised pic. are you sure on the resistor values?
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Old 7th November 2003, 02:19 AM   #14
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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?
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Old 7th November 2003, 02:21 AM   #15
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now that i look at the picture again, I noticed that you are right. Only the resistors for the top circuit are right... I forgot to change the bottom ones from 1K to the correct value.
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Old 7th November 2003, 03:24 AM   #16
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i just usually see the circuit with equal resistors and different caps. haven't looked into that cicuit's analysis yet.
what 100nF caps are you going to use?
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Old 7th November 2003, 05:18 PM   #17
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> 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:
Click the image to open in full size.

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.)
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Old 7th November 2003, 05:57 PM   #18
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PRR...your gonna get slammed for that post!! LOL...dont forget to include the fet input op amps (353,356)...I thought I was the only old bastard who still actually screws around with those components..haha

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Old 8th November 2003, 01:02 AM   #19
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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
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Old 8th November 2003, 08:03 PM   #20
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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.
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