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Old 16th April 2012, 01:03 PM   #21
Calvin is offline Calvin  Germany
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Hi,

well, one may call it a subtractive filter, since the HP ist generated by taking the difference of Input signal and Lowpass (TFhp=1-TFlo).
On the other hand is the TF of the Lowpass generated by ´analog computing´ or state variable function.
Quote:
Essentially the output of a 2'nd order high pass filter is...
Just to fan confusion
If only there were a 2´nd order HP!?!
The somehow weird thing with this kind of filter is, that all depends on each other. Without the integrators there wouldn´t be any HP function, which -integrated a couple of times- results in the corresponding LP.
So, no HP wo LP and no LP without the HP.....sounds a bit like a dog chasing its tail, eh?
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Old 16th April 2012, 07:31 PM   #22
NebuK is offline NebuK  Germany
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UUUh... UUuuuuuuuhm. Now i'm a "little" overwhelmed. I must really say that i just can't quite follow what you're going on about here. But please DO go on, maybe something cool comes out? :P

Anyways, in the meantime i've been reading a few OpAmp datasheets trying to find a cost-effective yet well-suited choice for this schematic.
@Andrew T.: I do understand that i have to do the matching, still, my preferred german parts supplier carries 0.1% resistors for a good price, so why not just use them? Capacitors i would've planned on hand-matching.

Also, about OpAmps, is there something you could recommend for this usecase?

@Calvin: At what frequencies did you start getting those oscillation problems? I would like to use this circuit as a base for subwoofer crossovers, so crossover frequencies at 50-100Hz should be possible.

Anyways, i've updated the schematic and pcb, which both can be found on:
ActiveCrossOver - Adjustable active 24db LR crossover
in the eagle 6 format (ltspice files as well!) For convenience, here's the images of both:
Click the image to open in full size.
Click the image to open in full size.

What do you think of this pcb design? Any good for this? Comments and Critique *warmly* welcome!

As a sidenote, the R11-R14 are drawn as verticalized resistors because i wanted to use those 2-pin-ic-socket-like-thingies so i could just plug 4 new resistors in to change the crossover frequency ...?

Thank you so much for your help up to here . Regards
- NebuK

Last edited by NebuK; 16th April 2012 at 07:34 PM.
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Old 17th April 2012, 07:42 AM   #23
Calvin is offline Calvin  Germany
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Hi,

I wouldn´t recommend the filter below ~200Hz. But I didn´t try to compensate the circuit since I changed to a different topology that I found alot more practical. As said before, a filter without equalizer is hardly good for anything.
So its all up to a test, if You experience oscillation probs or not.
As Opamps You might use the all present NE5532, or more costly the LME-Seeries or AD797, all of bipolar type.
Alternatively You may consider types with FET- or JFET-Input. Due to the highly varying impedance values in filter circuits FET (or JFET) OPamps hold a slight advantage over bipolars.
The OPA134/2134/4134 or their cheaper brethren 132/2232/4232, the OPA604/2604, or AD711/712 are relatively cheap and well sourceable JFET-OPs.
If it were for high quality but costly the AD8610/8620 or OPA627 come to mind.

jauu
Calvin

Last edited by Calvin; 17th April 2012 at 07:49 AM.
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Old 17th April 2012, 09:01 AM   #24
godfrey is online now godfrey  South Africa
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Quote:
Originally Posted by Calvin View Post
Mathematically they exploit the fact with filters of even order (2nd, 4th, etc.) the denominator of the transfer functions of Highpass and Lowpass are the same. The TFs only differ in the numerator.
I don't follow the math, but the idea seems to work fine for odd-order filters as well. Here's a nice simple 3'rd order Butterworth.
Attached Images
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File Type: gif but resp.GIF (12.8 KB, 214 views)
File Type: gif but phase.GIF (12.8 KB, 208 views)
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Old 17th April 2012, 09:19 AM   #25
godfrey is online now godfrey  South Africa
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Quote:
Originally Posted by Calvin View Post
While the filter worked well at frequencies of a couple of hundreds Hz on, I sometimes got oscillation effects with low crossover-frequnecies.
On second thoughts, I don't see a reason for low frequency oscillation. Was it high frequency instability or low frequency motorboating? Roughly what was the frequency of oscillation?

Quote:
Originally Posted by NebuK View Post
I would like to use this circuit as a base for subwoofer crossovers, so crossover frequencies at 50-100Hz should be possible.
It's probably a good idea to increase the capacitors then, perhaps to 100nF. IMHO, a good rule of thumb is to keep the resistors between 10K and 100K.
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Old 17th April 2012, 02:20 PM   #26
Calvin is offline Calvin  Germany
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Hi,

one channel worked flawless and one eventually didn´t.
Didn´t investigate into that matter any further, since I gave up on these kind of ´pure´ filters, because of their impracticality without additional equing circuitry.

jauu
Calvin
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Old 24th April 2012, 08:16 AM   #27
NebuK is offline NebuK  Germany
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Heyyas,

i've once again found a little time to tune the capacitor values and play around a little. I've also refined the PCB a bit. So once again, here's the board layout and schematic:
http://pics.kanojo.de/acrossover-schematic.png
http://pics.kanojo.de/acrossover-pcb.png
(the opamp name is just for reference, eagle just happened to have it, i think i'll go with the opa2134 as you advised. I already quite like it in my CMoy :P).

So, the 68nF gives me the following:
15K 155Hz
22K 106Hz
33K 70Hz
47K 49Hz
68K 34Hz
82K 28Hz
100K 23Hz
Which is just E10 + 82K + 100K. A set of four of each for tuning my woofer crossover (roughly) shouldn't be that bad, right? So that later when more refinement is needed one knows where to look for intermediate values...

Sure, for a general 2-way crossover where the crossoverpoint is somewhere from 1500Hz to 4000Hz, i'd need different capacitor selection to have a nicely adjustable range somewhere up there.

But for now ... do PCB and schematic look ok? Do you have any other advise on how to improve it? Do you think i could maybe get the PCB simpler and more home-etchable by using 4x or 8x opamps (i guess today the crosstalk between the channels is negligible for my application?)?

Anyways, thanks alot once again :P! Regards
- NebuK

PS: Eagle and LTSpice files are again on ActiveCrossOver Git Source Tree - Adjustable active 24db LR crossover ...
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Old 29th April 2012, 09:40 AM   #28
NebuK is offline NebuK  Germany
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Really no final comments? :/
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Old 29th April 2012, 11:09 AM   #29
AndrewT is online now AndrewT  Scotland
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I fear noise performance may be poor due to the adoption of relatively high resistor values. Was that to keep capacitor cost/size as low as possible?

Maybe for bass this is not a problem.
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regards Andrew T.
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Old 30th April 2012, 08:39 AM   #30
NebuK is offline NebuK  Germany
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Hm, i certainly won't fear a bit higher building cost if it improves performance ... for this i've tried to adopt what godfrey posted
Quote:
a good rule of thumb is to keep the resistors between 10K and 100K.
... doesn't that apply here for some reason? Also, could you give a little beginner like me a little hint why larger resistors in this schematic will promote noise? By intuition i would've thought otherwise ;/

thanks
- NebuK
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