Question regarding sub x-overs

Discussion in 'Archived Threads 2001-2004' started by chris c, Oct 22, 2001.

  1. chris c

    chris c Stunt Coordinator

    Joined:
    Jun 30, 1997
    Messages:
    206
    Likes Received:
    0
    Trophy Points:
    0
    Can someone please explain to me why so few sub manufacturers offer defeatable crossovers? To me it is entirely an unforgivable omission. Who in this day and age doesn't perform bass-management in the digital domain of their pre/pro. The vast majority of sub users are forced to cascade the crossovers in the sub and the processor which is, believe me, not a great idea. I had the ACI Titan sub (no defeatable x-over) for a short while - There was no way to get the thing to integrate properly except to feed it full range signals from the left/right channel outputs. Sent it back. Just don't get it. Perhaps the Dahrman (sp?) guy can help us understand why he didn't include this feature in his sub?
     
  2. Jeremy Anderson

    Jeremy Anderson Screenwriter

    Joined:
    Nov 23, 1999
    Messages:
    1,049
    Likes Received:
    0
    Trophy Points:
    0
    All you have to do is turn the crossover on the sub to its highest setting, and you've essentially defeated it. I mean... if you turn the sub's crossover to 120hz, and have your receiver or pre/pro crossed at 80hz, I don't see how you'd have any gap in the frequency range or problems with cascading. Someone please correct me if I'm wrong on that point.
     
  3. chris c

    chris c Stunt Coordinator

    Joined:
    Jun 30, 1997
    Messages:
    206
    Likes Received:
    0
    Trophy Points:
    0
    The reason you are incorrect in your analysis involves the problems when two x-overs, regardless of their settings, both see the same signal. Not that I am an expert, but I have read much data confirming that problems occur when this happens - for example, phase anomalies, cancellation, etc.
     
  4. John Kotches

    John Kotches Cinematographer

    Joined:
    Mar 14, 2000
    Messages:
    2,635
    Likes Received:
    0
    Trophy Points:
    0
    Chris,
    Yes, but by the time you get to 120hz, the response is down from the processor sufficiently that it is a non-issue from the subwoofer. Well over 90% of the output would be from the mains. And if your subwoofer can set its crossover frequency as high as 2x the processor/receiver it is truly irrelevant.
    Me, and many others, have said the best input is an LFE / unfiltered input from a processor, but lacking that, setting the internal crossover on the subwoofer as high as possible is an acceptable alternative.
    Regards,
    ------------------
    John Kotches
    Contributing Writer
    Secrets of Home Theater and High Fidelity
     
  5. chris c

    chris c Stunt Coordinator

    Joined:
    Jun 30, 1997
    Messages:
    206
    Likes Received:
    0
    Trophy Points:
    0
    John, while I agree that setting the x-over to the highest setting may reduce the problem, my experience has been that it is still there, although perhaps only noticable to an obssessive such as myself (although many of us in this hobby fit that description ha, ha). Nevertheless, I still don't understand why most manufacturers don't offer this option - is it costly enough to make it a nonissue?
     
  6. ling_w

    ling_w Second Unit

    Joined:
    Sep 3, 2001
    Messages:
    426
    Likes Received:
    0
    Trophy Points:
    0
    For a 4th order crossover, which has a 360 deg phase shift at the crossover point. The start of the phase shift starts much before that. It is not as if at 119 hz, you have no phase shift and at 120hz, you have a 360 deg phase shift. So somewhere along the spectrum below 120hz, it will induce a 180 deg shift. Now combine that with your 90hz, which has its own phase shift properties, there might be a good chance where the 2 signals will null each other out.
    Just hook up your sub in that manner and run a freq sweep through your sub, and preferably hook up an DMM to the amp out. Check for points where there is a severe voltage dip, creating something like a notch filter.
     
  7. Wayne A. Pflughaupt

    Joined:
    Aug 5, 1999
    Messages:
    6,123
    Likes Received:
    40
    Trophy Points:
    6,610
    Location:
    Katy, TX
    Real Name:
    Wayne
    Very few subs, receivers or pre-pros have 4th order (24dB/octave) crossovers. Typically they have only 2nd or at best 3rd order crossovers.
    In addition, not all 4th order crossovers are the same. Filters with Linkwitz-Riley and Butterworth alignments have different phase characteristics.
    But really, this is all academic. All that really matters is getting smooth response through the crossover region. If you determine you have a crossover-induced null at a certain place, you can “fix it” with your sub equalizer.
    Regards,
    Wayne A. Pflughaupt
    ------------------
    My Equipment List
     

Share This Page