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Old 28th October 2015, 11:37 PM   #1
surfstu is offline surfstu  United Kingdom
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Default help explaining active filters

I spent last night soldering a simple LPF filter together, attached the schematic and hope that xtronic don't mind.

Could someone explain what kind of filter this is? I understand that there are two simple RC filters here. R 6 and C 12 coupled with a pot seem to sweep the value of R (first filter) 50HZ - 150 HZ cut off points

There then seems to be a second duplicate RC circuit through R 5 and C 11.

Does this mean it becomes a 2nd order filter?

Also, is the first opamp just there as a buffer?

Any light shone on this for a noob would be much appreciated

Stu
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Old 29th October 2015, 12:00 AM   #2
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The first section, around IC3A, is a second-order Sallen and Key highpass. Judging by the RC values used, it is intended as a subsonic filter.

The two RC sections you mention together indeed form a tuneable second-order low pass, but with a very smooth (gradual, weak, not sharp) transition from passband to stopband. Due to loading effects, the poles of the two RC-sections move away from each other, so you get a response that is even more gradual than a second-order Linkwitz-Riley filter.

What's the purpose of this thing?
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Old 29th October 2015, 01:11 AM   #3
surfstu is offline surfstu  United Kingdom
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Thanks Marcel, the purpose is a home subwoofer to compliment some bookshelf speakers that suffer in the low end department.

Are you sure the first section is a high pass? That would not make sense if this cicuit were designed for a sub?

Stuart
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Old 29th October 2015, 01:12 AM   #4
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Thats a very simple crossover.
It just 2nd order.

I used a 4 th order on my system.
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Old 29th October 2015, 01:47 AM   #5
surfstu is offline surfstu  United Kingdom
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Thanks for pointing me in the right direction, I looked up the sallen and key highpass - it seems that the first section is a subsonic filter with a cut off of 22.5Hz. I guess this is to protect the sub.
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Old 29th October 2015, 01:50 AM   #6
surfstu is offline surfstu  United Kingdom
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Thanks for posting your filter design Nigel, it seems a little more complicated than I would like right now. Will I be able to simply add another opamp section if I want something more refined in the future?

S
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Old 30th October 2015, 09:30 AM   #7
AndrewT is online now AndrewT  Scotland
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have you read ESP website?
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Old 31st October 2015, 12:47 AM   #8
surfstu is offline surfstu  United Kingdom
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Thanks Andrew, Yes I've read the Elliot Sound Products website over and over whilst trying to absorb the info at a noob pace.

I've already built my 2nd order RCRC low pass filter so I will try it but i'm already thinking I will be making a new filter very soon!

My bookshelfs are Kef reference 101 which are apparently flat frequency response down to 90Hz, then they drop to -10db at 47Hz.

Not quite sure what I would want my sub filter to do to compliment my speakers, where would the frequencies be best to cross over?

And do you guys make a 'sweepable' cut off point so that the sub can be adjusted to ones taste?

Stu
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Old 31st October 2015, 08:12 AM   #9
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Are those closed boxes or bass reflex speakers (or something else)?

If they are closed boxes with approximately a second-order Butterworth high-pass response, you could:

1. Make a second-order Butterworth low-pass with the same cut-off frequency for the subwoofer. Depending on the placement of the subwoofer and the other speakers, this may give you a 0 to 3 dB bump around the crossover frequency.

2. If the path lengths from the bookshelf speakers and subwoofer to your ears are nearly the same, which would give a 3 dB bump if you take approach 1, you could instead decide to put second-order Butterworth high-pass filters in the path to your bookshelf speakers and make a fourth-order Linkwitz-Riley low-pass for the subwoofer.
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Old 5th November 2015, 12:52 AM   #10
surfstu is offline surfstu  United Kingdom
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Thanks, the kef reference 101 are closed boxes. I have been reading for days about filters and speakers, learning but at a very slow pace. There seems to be so much science. Were you able to guess the second order Butterworth High pass response from the info I gave you (flat to 90Hz, -10dB @ 47Hz)?

Would an ideal sub woofer partner to these speakers then require a bode plot that is symmetrical to this? The bode plot for the frequency response is included in this link:

http://kef.com/uploads/files/en/muse..._Model_101.pdf

I may be wrong, but would a second order filter drop 12dB over an octave? Which means we are almost accurate if we drop 10dB from 90Hz to 47Hz?

Would I then want the opposite effect for my LPF to match the speakers?

BUT, I would get a 3dB bump at approx 90Hz - I kind of understand this I think - would a 3dB bump be extremely undesirable or is it something that some people can live with?

Surely commercial sub woofers that simply have one control for cut off and one for volume must encounter these issues and worse all the time,

Stu
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