Crossovers - Do they effect Impedance?

Discussion in 'Home Theater Projects' started by Tab Nichols, Jun 19, 2003.

  1. Tab Nichols

    Tab Nichols Stunt Coordinator

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    I am building some budget speakers for my brother, but these are my first "full range" set that I have ever built. I have never dealt with anything but subs before.

    My question, if I put a crossover into the speaker system, will it, and if so how much, effect the final impedance? For instance, I see 4 ohm and 8 ohm crossovers, do I have to take these numbers into account when calculating the impedance load?

    Your help is always appreciated.
     
  2. Patrick Sun

    Patrick Sun Studio Mogul

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    Yes they do, but you need to either get good simulation software to find out how low/high the impedance load is throughout the frequency response, or actually measure the impedance through its frequency response to find out where the impedance dips to a low value to present problems to an amp.

    You can't think of impedance as a static value when talking about speakers, though speaker makers try to use "nominal" impedance as a way to indicate whether or not their speakers would present a high impedance load through the frequency response (usually in the 8 ohm range), or a low impedance load (usually in the 4 ohm range, especially if the driver configuration has drivers parallelled, like speakers with a MTM configuration, but are basically 2-way speakers with their 2 midrange drivers hooked up in a paralleled fashion.

    Some speaker makes use 16 ohm midranges to get their MTM speakers to measure closer to a nominal 8 ohm load when the midranges are parallelled.

    -----

    Getting back to the problem at hand, I would not just use "generic" crossovers and expect good results with them. You're much better off buying a speaker kit with a crossover that takes into account the drivers' characteristics and does its best to optimize the performance of the drivers in concert with its enclosure, etc.
     
  3. Tab Nichols

    Tab Nichols Stunt Coordinator

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    So to get this straight, If I were to have a crossover tat was rated 4 ohms, I would have to factor it into my calculations as another speaker? (ie. 4 ohm speaker in series with 4 ohm crossover = 8 ohms total load?)

    Id love to just build my own crossovers, but I dont know enough about them to even know where to begin. Id also need a program to calculate the impedence...

    Can you recommend a good site to learn about how they work,and building them?

    I have little electronic, soldering experience....
     
  4. If you are really up to it, then try looking at this:
    http://www.speakerworkshop.com/SW/Project/Main.htm

    This site has free software (you still need to build a jig/pre-amp/mic to take your OWN measurements).

    ...but that link in particular takes you to a walkthrough of designing a speaker.
     
  5. Patrick Sun

    Patrick Sun Studio Mogul

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    You can't simply add the "rated" impedance of a crossover network to the "nominal" impedance of a driver and expect to have it be an additive process. To get it right, it requires a bit of complex math and an understanding of the impedance load that crossover components (resistors, capacitors, inductors) have in various network configurations. The Speakerworkshop page is a good start, but it's a bit tedious, and can be difficult for beginners to not only grasp, but execute (in terms of creating ways to measure what is needed in order to optimize a crossover for the drivers being used in a speaker design).

    To do it right, you have to invest in making your own measurement setup for drivers (mounted in the front baffles of the speaker enclosure), and you'll need to measure its frequence response, its impedance profile at a minimum, and there are tricks to derive the phase response from those measurements. This is not a trivial exercise. But no one should expect to walk off the street and do it right the first time. It's going to require a lot of educational investment in the hobby of speaker building to do it right. Otherwise, if the investment time/cost is too high, build speaker kits from plans you find on the internet which have had good response from others who have built the kits.
     

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