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Low Pass Filter for subwoofer

Discussion in 'Electronic Basics' started by Andrew Howard, Mar 3, 2004.

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  1. I'm thinking of making a low pass filter to use before an amp, so it can
    drive a subwoofer. The most economic design seems to be the RC one. The
    input is from a computer soundcard, the output is to a 10W amp driving a 4"
    speaker. The questions are:

    What values are good for R and C to get a good cut-off point for this

    How do you find this? (I think it is 1/2piRC, but I don't understand
    this very well, and anything I put into it comes out completely off.)

    Does it matter what wattage the resistor is rated at? (can I use a
    standard pot?)

    Andrew Howard
  2. Bob Masta

    Bob Masta Guest

    Let's say you want a cutoff of 100 Hz (about right for subwoofers
    in general, but with only a 4" driver this might be lower than it can
    handle.) Start by picking a value for C, since the R is easier to
    adjust. Try 0.1 uFd and rearrange the formula to

    R = 1 / 2*pi*f*C = 15915.5 ohms

    Note that C is in *Farads*, so you have to use 0.1*10^-6
    in the formula. I'm guessing that's where you went wrong.
    You can use whatever C values you have on hand and
    compute the resistor, or use a pot as you suggest.

    This will not be a very sharp filter. The output will only
    fall at -6 dB/octave maximum. You may want to look into
    "active filters" using op-amps for a steeper response
    if this one isn't acceptable.

    The other reason yo might want an active filter is that
    the simple RC type is sensitive to the source and
    load impedances, whereas the op-amp type
    can be made immune. In the above example, the
    output impedance of the source will be in series
    with R, and the input impedance of the load will
    be in parallel. So it will work fine driven by a low
    source impedance and feeding a high amp impedance.
    The sound card output impedance is probably
    low enough to ignore here, but the amp input
    impedance may be in the range of 50 K or so,
    so you'd need to tweak the R value.
    Wattage requirements are very low since this will go before the amp.
    Bob Masta

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
  3. Thanks for the help so far, but I just have one more question.

    The capacitor is bi-polar right?

    Andrew Howard
  4. Bob Masta

    Bob Masta Guest

    Yes, and at the low capacitor values you will be using
    that's probably the only type you will find. Once you
    get to the 1.0 uFd range or above you will find polarized
    types the norm. You don't want or need those here.

    Bob Masta

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
  5. Thanks for your help.

    Andrew Howard
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