Help with math regarding a 3-Way Speaker Crossover circuit

Discussion in 'General Electronics Chat' started by mjsummers, Jan 2, 2013.

  1. mjsummers

    Thread Starter New Member

    Jan 2, 2013
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    I was asked to design a circuit for a 3-Way Speaker for some home made speakers as a way to keep my skills from developing cobwebs since I graduated college. The attached pdf file is my circuit, it is a single channel circuit. The circuit has 4 bandpass filters, three L-Pad Drivers, and a notch filter in addition to a tweeter, mid-range, and woofer speakers. My Problem is one of calculating the impedance that will be seen at the amplifier. I am looking for an impedance between 4 and 8 ohms. I have included the dc resistance for the inductors. I have text notations of speaker power by the resistors that make the l-pad for each speaker. The signal input is 100W (RMS) from the amplifier.


    Any help would be greatly appreciated, if you need any other information please let me know.
     
  2. Brownout

    Well-Known Member

    Jan 10, 2012
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    Start with the impeadance of the speaker(s) and work back towards the source, using parallel and series formulae for each node/loop. Use the formulae:

    XL=ωL for inductors, and
    Xc=1/jωc for capacitors.
     
  3. mjsummers

    Thread Starter New Member

    Jan 2, 2013
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    thanks for the advice, how do i account for a signal that covers from 20 Hz to 20kHz in the formula?
     
    Last edited: Jan 2, 2013
  4. Brownout

    Well-Known Member

    Jan 10, 2012
    2,375
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    BTW, there are design methods for standard filters, for example, the Butteroworth filter, which take into account injection point and load impeadances. Use those formulas and you won't need to be concerned with calculating those impeadances, they will be automatic. I suggest goolging "butterworth filter" etc.
     
  5. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    I recall there are phase issues depending on what order the filter is. It's been a while since I did crossovers. You definitely want a butterworth so it will have a flat bandpass.

    If I recall, a first order LPF is OK for the woofer, second order is also OK. The midrange needs a second order bandpass, and the tweeter gets a second order HPF.
     
  6. mjsummers

    Thread Starter New Member

    Jan 2, 2013
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    I have included butterworth filters in to the design already. Im using a 3rd order butterworth filter that is designed to handle the tweeter/mid and mid/woofer crossovers so there isnt a noticable difference in the switching of the speakers.
     
  7. mjsummers

    Thread Starter New Member

    Jan 2, 2013
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    Everything for the design is included in the PDF. I would like to make sure that I will not blow the amp or the speakers when I connect them, thus the reason for the calculation. The amp in question is discontinued and the speakers are going to be made out of a wine barrel
     
  8. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    It looks wrong to have low value resistors (R7, R5) parallel across the speakers as it will attenuate the speakers severely.
     
  9. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    There's a design tool here. You need to know the crossover frequencies, filter slope, etc.

    http://www.diyaudioandvideo.com/Calculator/APCXOver/
     
  10. mjsummers

    Thread Starter New Member

    Jan 2, 2013
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    I am using that one already. I am trying to find the right crossover frequency(ies) that I can buy standard parts for.
     
  11. mjsummers

    Thread Starter New Member

    Jan 2, 2013
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    Those resistors are apart of the l-pad circuit for those speakers. I am trying to maintain an 18dB attenuation for the speakers
     
  12. Audioguru

    New Member

    Dec 20, 2007
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    Why are you throwing away more than half the power of the amplifier?

    Get rid of the resistor in series with the woofer since it destroys the excellent damping of the woofer resonances provided by the extremely low output impedance of a modern amplifier. Also get rid of the resistor parallel with the woofer.
    Oh, the amplifier is discontinued? Is it an old vacuum tube amplifier with a high output impedance that needs an old woofer that damps its own resonances??

    With the resistors at the woofer removed then the resistors attenuating the midrange and tweeter speakers might be able to also be removed.
     
  13. mjsummers

    Thread Starter New Member

    Jan 2, 2013
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    I was trying to determine how much power the speakers will actually see. This is a first draft of the circuit before I buy the parts to make the card for the speaker box. The amp is a 7.1ch yamaha rx-v659 that is no longer made
     
  14. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    You will probably need to adjust the amount of attenuation to the mid and tweeter on a home made speaker because of the wide disparity in speaker efficiency'

    Guess how I learned that.....:p
     
  15. Audioguru

    New Member

    Dec 20, 2007
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    Will you plug your ears then run the amplifier at full blast all the time?
    Most people don't. They simply turn down the volume control.
    The volume control is at THE INPUT of an amplifier, not at its output.
     
  16. Audioguru

    New Member

    Dec 20, 2007
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    The amplifier produces 100 real Watts per channel at a low distortion.
    Its Damping Factor when it drives an 8 ohm speaker is at least 120 so its output impedance is only 0.067 ohms or less for excellent damping of the resonances of a speaker.
     
  17. mjsummers

    Thread Starter New Member

    Jan 2, 2013
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    For the record, I'm an RF Engineer not an Audio Engineer. I am treating the circuit as part of an RF package
     
  18. mjsummers

    Thread Starter New Member

    Jan 2, 2013
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    Dont know if you saw the resistance for the woofer (4OHMS not 8) besides the tweeter is rated for 45W RMS and the mid is 60W RMS and the woofer is 140W RMS, so aside from the input the speakers fall within +/- 60% of the rated output of the amp, hence my concern
     
  19. Audioguru

    New Member

    Dec 20, 2007
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    The amplifier is spec'd to produce 140W into 4 ohms at fairly high distortion.
    Simply keep the volume control turned down a little (half of full power is only a little less louder) so you do not break your midranges, tweeters and your hearing.

    I used a pair of little speakers for many years on my Yamaha 70W per channel amplifier. The speakers were rated at 40W each. The woofer on one speaker stopped playing and when I opened the speaker I saw that the woofer was stamped "5W Korea".
     
  20. mjsummers

    Thread Starter New Member

    Jan 2, 2013
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    did you account for the sensitivity of the tweeter, mid, and woofer when building a speaker box?
     
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