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Discussion in 'General Electronics Discussion' started by DLJ, Apr 27, 2012.

  1. DLJ

    DLJ

    3
    0
    Apr 27, 2012
    Hey all, first post.

    I'm working on a sound system project, and the audio forums are less than helpful. So, I figure I'll bring my questions to those who know electrical systems, rather than audio systems.

    I have two 4ohm subwoofers, and four 6.5ohm midrange speakers. Two channels, rated at 4ohm minimum impedance. On one channel, I'm going to have just one 4ohm subwoofer. On the other channel, I want the other subwoofer, and at least two of the 6.5ohm speakers.

    My question pertains to the secondary channel, the one with 3+ speakers. What would be an ideal way to wire the 3+ speakers to ultimately achieve an impedance close to four ohms, but not below?

    So, I've got one 4ohm, and four 6.5ohm, one or two of the 6.5ohm can be ditched.
     
  2. BobK

    BobK

    7,607
    1,648
    Jan 5, 2010
    The problem is not just the impedance, but the balance of sound. If you just wire them in a combination of series and parallel, you may achieve close the right impedance, but different wirings will give you different frequency respose. The only way to do it right is with a custom designed crossover network.

    Bob
     
  3. DLJ

    DLJ

    3
    0
    Apr 27, 2012
    Okay, so if I remember right, a passive crossover technically takes a signal from the amplifier, and splits it into separate channels with difference frequencies?

    With that in mind, I still have the problem of impedance. I've spent a couple days rolling around wiring ideas, but for some reason I just can't come close to the proper total impedance.
     
  4. BobK

    BobK

    7,607
    1,648
    Jan 5, 2010
    The impedance is not all that important as long as it is not too low. A higher impedance will just mean you cannot get quite as much power out of the amp.

    If you want to try it, I would put the 3 midranges in parallel, then put this in series with the subwoofer. This would give you 6.2 Ohms.

    Bob
     
  5. john monks

    john monks

    693
    1
    Mar 9, 2012
    Not enough information. The problem is complex because you may not want to drive the subwoofers with the same range of frequencies as the midrange speakers. The best thing to do is open up a professional speaker system and see how they did it. Just to start out you may try connecting a subwoofer across the speaker jacks and placing two 10uF 25volt capacitors back to back in series with two midrange speakers in series or parallel across the same speaker jack. Then play around with the capacitor values until you get the best results. Your impedance will not come out exactly right but it never will anyway.
     
  6. poor mystic

    poor mystic

    1,059
    28
    Apr 8, 2011
    :)
    I expect that the subwoofers and mid-range speakers may filter each others' signals.If that is right, reduced performance can be expected in both ranges because the sub-woofer does not pass mid-range frequencies, while the mid-range will not pass the subs. Though I am not an audio specialist, I suspect that is why cross-over systems are used, as John Monks has just recommended.
     
    Last edited: Apr 28, 2012
  7. john monks

    john monks

    693
    1
    Mar 9, 2012
    Poor Mystic is quite correct. But the problem is even far more complex. The impedance of a typical speaker is far from linear. For example an 8 ohm speaker may be 2 ohm at DC and 18 ohms at 120 hertz and make a dip to about 8 ohms at around 400 hertz. Then go back up to about 20 ohms at about 20khertz. Every speaker is different. To do this right you need the impedance graph on every speaker and treat each speaker as a complex RCL circuit and the math can get quite nasty. Another thing to consider is that some speakers can be damaged if you drive them with two low of frequency at the rated power. Ultimately DLJ is going to have to do some trial and error.
     
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