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Simple audio band-pass filters with sharp cut-offs

Discussion in 'Electronic Basics' started by [email protected], Nov 27, 2005.

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  1. Guest

    I want to split an audio signal into 5 bands by frequency range (top
    and bottom can be low-pass and high-pass) and send these into speakers
    with moderate power, say 10 watts. I can use amplified speakers if a
    5-way audio amp gets complicated.

    This is not for quality audio, but for educational demo purposes, so I
    don't need an even response and distortion is okay. I'm really looking
    for the sharpest cut-offs I can get, in as simple a box as I can build.
    Can anyone suggest how to accomplish this?

    I haven't made many coils before (rather embarassing for a ham op), but
    I'm fine with simpler circuits. I found a post about an IC that might
    do the trick, but I'm doubting it'll handle audio frequencies.
  2. Randy Day

    Randy Day Guest

  3. Bill Bowden

    Bill Bowden Guest

    I'm really looking for the sharpest cut-offs I can get, in
    Maybe try active bandpass filters using op-amps. There is a calculator
    to help out at this address:

  4. Bob Masta

    Bob Masta Guest

    One good resource is Don Lancaster's "Active Filter Cookbook".
    You will not need any inductors (one of the virtues of active
    filters), but you will need some matched components. Look
    at "equal component Sallen-Key" designs. These avoid the
    need to have 2 different specific capacitors in each stage to
    control Q and frequency. Instead, you use 2 equal-value
    caps, and equal-value Rs, and you adjust Q with a separate
    gain factor. Stability is alleged to be poorer than the unequal
    Sallen-Key, but IMHO that's more than compensated by the
    ease of correct tuning in the first place.

    You will have to make some tradeoffs between complexity,
    sharpness of slopes, and passband ripple.

    Also, with only 5 bands to cover the audio range,
    the bands will be too wide to use classical
    bandpass fitler stages. You will need to create each band
    from a separate high- and low-pass stage, or a chain of
    simple bandpass filters summed together.

    In fact, you may find it simpler to change your design to use more
    than 5 bands, just to allow simpler bandpass stages. (They should
    be well under an octave to use bandpass stages. Narrower
    allows sharper tuning.)

    You will come to appreciate why designers prefer to make
    filters like this digitally. One A/D and DSP chip, and you can
    generate any combination of filters and slopes with perfect
    stability and no parts tolerances. You can use FFTs (a
    whole bunch of simple bandpass filters, equally spaced)
    or Finite Impulse Response (delay line with summed weighted
    taps) or even the trickier Infinite Impulse Reponse types
    (like analog filters) to create fitlers for this job.

    Best regards,

    Bob Masta

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
    Home of DaqGen, the FREEWARE signal generator
  5. john jardine

    john jardine Guest

    Amplified speakers would be the way to go. Implementation can then be by 5
    off, passive or active filters.
    For passive component simplicity, would suggest for the HP, LP and 3
    bandpasses, '3nd order' Butterworth responses.
    Simplest is a passive, standard LC design (HP,LP,BP's using say, 1000ohm
    source and termination resistances). To avoid the ballache of hand winding,
    then use of Toko style inductors would be preferable. Needs in total, 11
    inductors and 13 capacitors.
    For demo purposes the roll-offs are quite fast with a nice flat response
    over each pass band.
    T'other method is to use 5 active filters. As a minimum, would need a total
    of 5 opamps. 27 resistors and 18 capacitors. The HP and LP are standard 3rd
    order Butterworth design but the 3 bandpasses (2nd order this time) use a
    jiggery pokery component arrangement to give a wide flat top response using
    a single opamp.
    (My "service" supplier of 6 years has ditched A.B.S.E. so I can't post an
    example circuit).
    'Wannadoo' is a just a cabal of f***** useless management wankers.
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