Dueling crossovers: subwoofer and receiver xo overlap?

Discussion in 'Archived Threads 2001-2004' started by Jeff Meininger, Aug 19, 2002.

  1. Jeff Meininger

    Jeff Meininger Second Unit

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    On my receiver, I set my subwoofer crossover at 120 Hz due to my very small main speakers. My current subwoofer has no internal crossover, so the setting is easy.

    In the next month or two, I'm going to be upgrading to a passive subwoofer for which I'll need an external subwoofer amp. I've narrowed it down to 2 amps, and they both have built-in crossover settings. One of the amplifiers allows you to disable the built-in crossover as to not interfere with the receiver's crossover. The other one ($50 cheaper) has a crossover setting, but you can't disable the crossover. You can just turn it up to 160 Hz.

    I was looking at the frequency response graphs of crossovers, and the receiver's 120Hz crossover would overlap the sub amp's 160 Hz crossover in a major way.

    My thinking is that the 160 Hz sub crossover will reduce output in the higher frequencies (100-120 Hz) that I DO want to hear. Is this the case? Should I pay the $50 extra for the ability to disable the extra crossover setting?

    There are many posts on the topic, and most people just turn their sub XO to max (150 or 160). For a typical 80 Hz receiver crossover, the resulting overlap is much less than it will be in my case, though.

    Thanks for the help!
     
  2. Dan Hine

    Dan Hine Screenwriter

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  3. Dustin B

    Dustin B Producer

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    No he has a valid point. Crossovers are not brick walls and the receiver crossover slopes are more than likely 12dB/octave. 160hz is well within the first octave above 120hz (240hz being an octave above). So the low pass filter will be rolling off the above 120hz frequencies to the sub at 12dB/octave, and then when the signal goes into the sub, the subs crossover will increase this rate of decline starting at 160hz. The high pass filter that feeds the speakers won't get this second increase in decline of the slop around 90hz to match up with the low pass filter. Which could cause a dip in the system response around 200hz. When the two signals sum in the room.

    How big this dip will be, or whether it makes a big difference in comparison to other room interactions I can't say for sure, but I doubt it will. If the thought of it really bugs you spend the extra $50 for peace of mind. If it doesn't really bug you, then save the $50.
     
  4. Jeff Meininger

    Jeff Meininger Second Unit

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    As I understand it, a crossover is a curve, not a fixed point. A typical receiver crossover set at 80 Hz will actually reduce the output of the sound at 80 Hz by 3 dB, getting progressively closer to "full strength" as it goes lower in frequency. So a crossover DOES reduce output below the crossover point. The crossover point is the "corner frequency" of the curve.
    Two crossover curves, one at 80 Hz and one at 160 Hz barely overlap. This is why I think it's "okay" for most people to just turn their sub's XO to max, and everything works out okay for them.
    Two crossover curves at 120 and 160, however, overlap in a more significant way. Not too badly, but they do indeed overlap.
    I'll be building a DIY sonotube shiva sub, upgrading from a tiny 8" sub in a room way too big for it. It's going to be a massive improvement no matter what amp I choose... but at the same time I don't want to make any mistakes. I assume that cranking the XO to max will be just fine even with my high 120 receiver crossover, but I'd rather run the idea past folks more knowledgable than myself. [​IMG]
    (BTW: I posted this thread here rather than the DIY section because I think speakers & subwoofers is the official place to talk about sub crossovers. Plus I'm sure they're getting tired of my questions over there in DIY. Heh)
     
  5. Dan Hine

    Dan Hine Screenwriter

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    OK, bear with me here. Here's how I thought it worked. Let's use a speaker level input instead as it will show where I'm coming from a bit better. And also assume that the subwoofer has speaker level inputs and outputs. You connect your receiver to your sub using speaker wire then from your sub to your speakers.

    Now, with the sub crossover set at 160hz the "rolloff" for the sub would be the octave going UP to 320hz thus blending with the speakers. While the speakers go to 160hz and then rolloff to the octave down of 80hz.

    I know that crossovers aren't brick walls but I thought that they worked in this manner. I guess I just don't know what a sub would be set up to have a crossover of 160hz and then slope down as the frequency dropped. Are you guys saying it slopes both ways to accomodate the overlap between sub and speakers? Could someone throw me a line then with a more in depth reading on this topic? Could I ask anymore questions? :b
     
  6. Jeff Meininger

    Jeff Meininger Second Unit

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    You could say it slopes both ways, but I think a better way of understanding it is that it only slopes one way, but that slope doesn't begin at the crossover point... it begins at a frequency BELOW the crossover point. A crossover setting of 80 Hz begins rolling off below 80 Hz, and is down 3 dB at 80 Hz.
    Download this: http://www.adireaudio.com/Files/Seal...plications.PDF
    Page 6 has a graph of a typical receiver crossover at 80 Hz.
     
  7. Dustin B

    Dustin B Producer

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    Maybe this will help a bit:
    http://www.hometheaterforum.com/htfo...602#post523602
    Edit:
    It goes from the perspective of a 2000hz crossover between a tweeter and mid/woof in a 2way speaker. But the exact same principles apply to an 80 or 120 hertz crossover with a sub.
     
  8. Dan Hine

    Dan Hine Screenwriter

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  9. Jonathan M

    Jonathan M Second Unit

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    Hi there.

    This is a much more complex issue than it first sounds. First of all, there is more than "just" the receiver crossover and the sub crossover to consider. The speakers you have are small, and thus there own rolloff is acting as an acoustic crossover. Where this happens depends very much on the size of the speaker. How this influences things also depends on the type of alignment - sealed rolls off at 12dB/octave, and vented at 24dB/octave (near enough).

    Also, depending on the alignment of the receivers' xover it will either be 3dB down at 120Hz or 6dB down at 120Hz (Butterworth vs. Bessel), and this will affect the summing of the final output. Add to this room effects and the like, and it's a real dogs breakfast!

    Are you handy with a soldering iron? If so, you may be able to relatively easily remove or make the xover quite a bit higher with some simple modifications if it is necessary.

    Unfortunately it is almost impossible to predict the response until you measure it in your room.

    If it was me, I'd save the $50 and use it for software!

    Cheers
    Jonathan
     

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