question about passive filters

Discussion in 'General Electronics Chat' started by lokeycmos, Jun 6, 2013.

  1. lokeycmos

    Thread Starter Active Member

    Apr 3, 2009
    432
    7
    i have a question about RC highpass filter and LR lowpass filter. I attached a diagram. the top one is high pass RC. the bottom one is low pass LR. this is in reference to basic speaker crossover. My question is, can the 8 ohms of resistance of the speaker itself take the place of R1? or does the speaker go in parallel with the resistance of R1? thanks
     
  2. LDC3

    Active Member

    Apr 27, 2013
    920
    160
    In order for the filters to work, they need the resistor. Also the filters are usually placed in front of the amplifier (which has a high impedance input).
    I believe that if you placed the filters just before the 8Ω speaker, the frequency of the filter would change. You would have to model it in SPICE.
     
  3. #12

    Expert

    Nov 30, 2010
    16,313
    6,817
    Ignore post #2.

    This can be done both ways. Regard: Air core inductors for audio speakers in retail version, http://www.parts-express.com/cat/air-core-inductors/297
    If you couldn't do it that way, they wouldn't be available in retail form.

    There are advantages to doing the crossover before the amplifier, but the disadvantage is that you must use 2 amplifiers if you're going to do the frequency division before the amplifiers. This is called, "Bi-amping". One advantage is that the inductor is a lot smaller because of the high input impedance of the amplifier. In fact, the impedance is so high that you can do it with resistors. Another advantage is the loss of intermodulation distortion. Another advantage is that the exact impedance of the speakers becomes irrelevant unless you use a certain form of negative feedback.

    In the drawing you made, the resistor symbols are just symbols used to represent the speakers. Speakers do not have the DC resistance the label says. Their AC reactance changes with frequency. Consider the fact that the speakers "voice coil" is literally, a coil. Google, "Zobel". If the high frequency driver is a piezo type, the resistor would seem to be necessary because of the high impedance of the transducer (speaker).

    This is enough information to get you started. Please return to this site after you have developed your ideas to a higher level and we can use some real numbers to find some real answers.

    Edit: The graph provided with the speaker is labeled, "frequency response" but the impedance of the voice coil can be imagined by looking at the fall off in response above 4 KHz.
     
    Last edited: Jun 7, 2013
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