Band Pass Filter

Discussion in 'The Projects Forum' started by noingwhat, Aug 29, 2010.

  1. noingwhat

    Thread Starter Member

    Jul 23, 2010
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    Hi! I want to create some sort of band pass filter for my stereo setup to filter the sound to my sub woofer. I want either 100hz and below, or 80hz and below, it doesn't make much of a difference. I am wiring the sub woofer in parallel with my left and right speakers.

    So my questions start here. First off I want to know if I should use a passive or active filter because I am still a beginner so I am not sure if I want to go into something such as an active filter. Also, will I need to build some sort of circuit to redirect the untouched sound to the smaller speakers because it will be wired in parallel? So what kind of circuit should I use?

    Also I need some sort of mid-range and high pass filter for my stereo speakers, all I want to do is filter out the 100hz that will go to my subwoofer, because I have such a huge bass boost on that my stereo speakers sound distorted with their tiny 5" woofers.

    So what kind of circuit do I need to seperate the audio at the 100hz line and send the low end one way, and the high end the other?

    Thanks to all that help!!!
     
  2. windoze killa

    AAC Fanatic!

    Feb 23, 2006
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    Firstly I would go with active filters as they can be better controlled and adjusted. You do realise that what you are considering can be bought off the shelf at nearly all electronics stores. I don't know where you are but here in Australia they are freely available. Depending on quality they range from about $10 each upto $100s. Of course buying one takes all the fun out of it.

    Try doing a search for audio crossover circuits on google. Comes up with heaps.
     
  3. windoze killa

    AAC Fanatic!

    Feb 23, 2006
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  4. noingwhat

    Thread Starter Member

    Jul 23, 2010
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    I have thought about purchasing a pre-built filter, but for one I am not really sure where to get one, and two, I think that is a bit more expensive, especially if I were to go with a passive filter, as I can build one for about a dollar, maybe less, and I am sure purchasing one will be more. but where could I buy one? Radioshack? Bestbuy? Or something else, I live in LA, California, in the US.
     
  5. windoze killa

    AAC Fanatic!

    Feb 23, 2006
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    I have no idea of the stores you have over there, I know the Radio Shack equivelant here used to stock them. AND they were a rip off. Not sure what they are like over there.

    Also you maybe able to build one for $1 BUT you are trying to drive a subwoofer. I don't think you will be able to do so with small cheap components unless you plan on running it through an amp after the filter.
     
  6. marshallf3

    Well-Known Member

    Jul 26, 2010
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    http://www.partsexpress.com
     
  7. noingwhat

    Thread Starter Member

    Jul 23, 2010
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    Hmmm... One thing I forgot to mention is that this setup is mostly going to be used for music, and after a bit of testing, it sounds like the frequency I want is 200hz and below, and (because I know all sound filters do this) to be completely, or almost completely cutoff by 450, 500 area. So, What kind of thing should I look for, what is the name of the circuit called need to Google where it will cut the audio in two, and send a single input to two separate outputs, one for high frequency (450+) and one for low (450-)?

    Thanks to all who have helped!
     
  8. Papabravo

    Expert

    Feb 24, 2006
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    At 200 Hz. you are not really in the sub-wofer category anymore. Middle C is at 256 Hz. A low-pass filter passes low frequencies and blocks high frequencies. A high-pass filter passes high-frequencies and blocks low frequencies. A crossover network does a bit of both:

    http://sound.westhost.com/lr-passive.htm

    The other thing you asked about is a power divider. They are used in RF circuits, not sure about using one in audio. It's possible but I'm not familiar with it.

    BTW I think connecting the speakers in parallel is a bad idea.

    Good luck on your cost targets which are an absolute pipe dream
     
    Last edited: Aug 30, 2010
  9. noingwhat

    Thread Starter Member

    Jul 23, 2010
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    Yea, I learned i wasn't really in the sub category anymore. At first I wanted the speaker to be like, 100hz and below, but when I tested that, I realized music doesnt get that low too often, so I changed my idea.

    The speakers being in parallel a bad idea, yea, I agree, that is why I want to have one input then it will output to two separate parts, the woofer, and my high-range speakers.

    And I am just saying I don't want to spend an arm and a leg on it, the dollar was just if I had built a very VERY simple low-pass filter, which I think i would like more than that, but not if it is gonna cost me like $50. I don't need anything that spectacular, just a little improvement over what I already have.
     
  10. marshallf3

    Well-Known Member

    Jul 26, 2010
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    Some have made decent LPF inductors by buying an entire 1/4 lb roll of #18 AWG magnet wire and optionally adding a cap across the output.

    It's become far more commonplace now to use an active LPF and a self-powered sub.

    Many 2 and 3 way crossovers have 400 - 500 Hz as the low end, and don't understimate just how much music actually exists around 40 Hz.

    Crossovers can be simple and useable or elaborate and acurate keeping the phase relationships close to equal as well as overall sound level balance. Plenty of books and info out there on the subject, even most of the older stuff from the monophonic Hi-Fi days still applies.
     
  11. noingwhat

    Thread Starter Member

    Jul 23, 2010
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    I want the 40hz, I want the woofer to be about 300hz and below, the 400hz cutoff I was talking about is the highest point. Most crossovers have a sort of fade off freqency(s) they don't just stop at exactly 300hz, so I said I want somewhere around 300hz and below to be perfectly preserved, and 500hz and above to be completely shutoff, so they will not come out of the woofer. Sorry if I didn't explain that well before.
     
  12. marshallf3

    Well-Known Member

    Jul 26, 2010
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    With passive components you'll never be able to build a circuit that will have a perfect cutoff point, you can only make the slope of the cutoff curve steeper and steeper by adding more poles to the filter but remember that every pole is going to consume a bit of your power.

    Crossovers work by slowly shifting the speaker certain frequencies are directed to; at some points you'll have both the woofer and the midrange (or tweeter) both reproducing some of the signal.

    The speakers themselves do a pretty good job of assiting the design, a large woofer becomes more and more inefficient reproducing higher frequencies just as a midrange or tweter don't reproduce bass worth a darn.
     
  13. Audioguru

    New Member

    Dec 20, 2007
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    First, you need a sub-woofer speaker in an enclosure designed for its specs. It should have its own power amplifier that is driven from a lowpass filter.

    Second, you need stereo speakers that go as low as your crossover frequency to the subwoofer. They need a stereo amplifier that is driven from highpass filters.

    The lowpass and highpass filters have the same cutoff frequency where the subwoofer and stereo speakers are all working together at half-power.

    As the frequency gets lower then the subwoofer produces more sound and the stereo speakers produce less sound.
    As the frequency gets higher then the stereo speakers produce more sound and the subwoofer produces less sound.

    Forget about trying to make passive filters because the inductors will be huge, expensive and will steal most of the amplifier power. A passive lowpass filter ruins the damping of the subwoofer causing it to sound boomy.
     
  14. marshallf3

    Well-Known Member

    Jul 26, 2010
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    There's no doubt that active crossovers are the way to go with the sub and I just thought of another drawback. Subs need a lot of power to drive, if only one amp were used youd stand the chance of frying your regular speakers.

    I measured a 100' roll of #18 wire and it only came out as 1.55 mH which equates to about 0.4 ohms of reactance at 40 Hz. 100' of #18 would also introduce 0.65 ohms of DC resistance. The DCR isn't so bad but to get the inductance up high enough to be effective you'd need one heck of a core which would probably end up saturating long before you reached peak output.

    That link I posted, http://www.partsexpress.com , used to sell subwoofer amp plates that had the crossovers built in with the amplfier. You supplied the speaker and enclosure.

    Couple of alternates would involve finding one or more of those old 36" EV woofers they used to make or building some Voice of the Theater cabinets without the horn. Either of those would be pretty darn efficient.
     
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