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Old 30th November 2004, 08:48 PM   #1
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Default Line-Level Low-Pass Filter Design

Greetings,
This is my first post so try and take it easy on me. I'm building a bridged/paralleled LM3886 amp to drive a single 12 inch sub. I want to put a line level low pass filter before the input to the amp so I can save $$ with not having to buy expensive high voltage crossover components.

My current setup is for 100% HT with stereo two way monitors powered by a leach-amp. The lower 3 dB on my monitors is 50 Hz. If the world were perfect (a nice big anechoic chamber) I'd design the filter with a center frequency of 50 Hz with a second order rolloff to complement my monitors.

However, the world is far from perfect in my dorm room of 9'W 14'L 6'2"H with concrete walls. Thats right i said 6 foot high ceilings (our beds are lofted to create three rooms within one). Obviously I'm getting substanial room gain, so alignments are practically useless.

Basically my question is what should my center frequency be, and what order slope should I use? Phase is of minimal concern so I'm probably going to use a sallen-key active topology. I have plenty of 741 opamps lyin around so I'd like to make use of those, but I'm definitely open to suggestions of better opamps?

Thanks in advance for suggestions or comments

Justin Crosby
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Old 30th November 2004, 09:06 PM   #2
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What are you connecting it to? If it's driven by Dolby Digital 5.1 or DTS then you don't need a filter as it's part of the decoder/amp unit/soundcard. Look for an output marked LFE.

If you only have stereo outputs and are making do with Prologic then make the filter -3 dB at 50 Hz.

Room gain starts at the frequency where the longest room dimension is equal to a half-wavelength. I'd say that this is a little lower than 50 Hz in your room.

Frg = 344 / (longest_dimension x 2)
using metres
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Old 30th November 2004, 11:35 PM   #3
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Quote:
Originally posted by richie00boy

If you only have stereo outputs and are making do with Prologic then make the filter -3 dB at 50 Hz.
I do not have a full system yet, so I am currently making due with just prologic. I have no LFE output, plus I want the sub more custom to my monitors. My idea is to design a full range amp and put this filter before the input. For me, modularity is key as right now I have little money for components. So later on when i change out my monitors, or get a new sub, I can still make use of the amp. Thanks for your help.

Anyone have any ideas on a good op amp for low frequency filtering?
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Old 1st December 2004, 12:16 AM   #4
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Default Re: Line-Level Low-Pass Filter Design

Quote:
Originally posted by JustinAtGT
Greetings,
I have plenty of 741 opamps lyin around so I'd like to make use of those, but I'm definitely open to suggestions of better opamps?

Thanks in advance for suggestions or comments

Justin Crosby
The '741's are best used as door stops for mouse-holes. For any filter you need an opamp with a bandwidth at least 10X the highest frequency it's going to pass. And good opamps are as cheap as dirt.

Go to the Texas Instruments website and download FilterPro -- you can set terms to your heart's delight -- for a sub you could probably just get away with using the LM3875, LM3886 or LM4780 as the active element in a second order RC filter, just remember that the optimal gain is about 20X for these devices.

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Old 1st December 2004, 02:11 AM   #5
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Default Re: Re: Line-Level Low-Pass Filter Design

Quote:
Originally posted by jackinnj

Go to the Texas Instruments website and download FilterPro -- you can set terms to your heart's delight -- for a sub you could probably just get away with using the LM3875, LM3886 or LM4780 as the active element in a second order RC filter, just remember that the optimal gain is about 20X for these devices.
Awesome program, thanks for the advice, sure beats my mathcad sheets. One of those national devices would be nice but I am trying to make a modular design so I can use the amplifier elsewhere if the need should arise. So my predicament is that I do not want preliminary gain. If I were to design for a gain of 20, wouldn't I be driving my amp into clipping, say I have a gain of 30 for the sub (making this number up since I havent designed the amp yet)?

Would these national devices perform ok at unity gain?

Secondly, is bandwidth really a concern for my design (upper cutoff @ 50 Hz). I'm not designing a high pass for a piezo tweeter here so I don't see where bandwidth would be an issue. If you think it is an issue, any suggestions on high bandwidth op-amps meant for unity gain?

Thanks --Justin
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Old 1st December 2004, 07:46 AM   #6
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I think the National chips must be used with a gain of 10 or more, although I could be wrong. Check the datasheet

I agree that the 741 is comparatively poor, but if you have some that you want to use then go ahead as they will be OK in your subwoofer application. Good alternatives are dirt cheap though, TL071/072 are brilliant for the money and if you want something better go for the OPA134/2134.
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Old 1st December 2004, 06:20 PM   #7
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Quote:
Originally posted by richie00boy
I think the National chips must be used with a gain of 10 or more, although I could be wrong. Check the datasheet

I agree that the 741 is comparatively poor, but if you have some that you want to use then go ahead as they will be OK in your subwoofer application. Good alternatives are dirt cheap though, TL071/072 are brilliant for the money and if you want something better go for the OPA134/2134.
If you bridge the OVerture chips you can use G=10 for each leg of the bridge (thus the total gain will be 20). One Chip the recommended Gain is 20. I made the gain in the circuit above ~ 20. If you don't want it set up as a filter, just remove the extraneous bits.
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Old 2nd December 2004, 07:35 AM   #8
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you CAN take output from the inverting input of the opamp to get a gain of 1 (i think). however the load must be taken into account. if the load is expected to be capacitive, or dynamic, i'd avoid this. however if the load is a 10k resistor or such, you are fine.

a neat thing about the circuit posted is that you can actually make it unstable by chaning the gain.
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Old 30th May 2006, 11:48 PM   #9
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I know this isquite the ancient thread, but I don't want to lie and say I have searched before posting.

I am in a similar situation as the original poster, I wish to build an active filter to cut the frequencies to 2 different amps, one for subs XO'ed at 80 hz, and another for the Mains.

Are there any cheap solutions right off the bat? Maybe a unit I cna purchase for under $50? Otheriwse, I wish to build a filter.

Thanks!
~Jerick
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Old 31st May 2006, 07:41 AM   #10
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Build one as above, or have a look at car audio places as they often do little active crossovers for cheap.
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