Can someone explain how sub crossovers work

Discussion in 'Speakers' started by SteveW, Jan 30, 2004.

  1. SteveW

    SteveW Agent

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    I currently have my speakers running full range set to small. With my new set-up I want the front l/r to be have a crossover point above where the sub goes below. Do I accomplish this by using the speaker level inputs on the sub. If so I am a little confused. The sub I am getting is the TN-1220HO with 250 watt amp. On the specifications it says that this amp has line level low pass but not line level high pass. I guess I'm a little confused on the line level low pass and high pass. Can someone clear this up for me.

    Also are there different settings on new receivers that allow crossover points to be set. My older Onkyo does not. But I am currently looking at either the Onkyo TX-SR601 or the Denon AVR-1804. I don't know if they do
     
  2. Nathan Stohler

    Nathan Stohler Second Unit

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    Yes, your sub would only have a low pass filter, which will allow frequencies below the crossover frequency and filter out those above.

    The most common way to set up your crossover to your sub is:
    1) set your speakers to small (which you've done)
    2) hook up a mono RCA cable (or specialty sub cable) to your receiver's sub pre-out output
    3) if your sub has a "bypass filter" input, connect the other end of the cable here. Otherwise, connect it to the normal sub input and turn up the crossover setting on your sub as high as it will go.
    4) If your receiver allows you to change the crossover frequency, adjust it to where you want. 80 Hz is a good starting place, but if your mains have good bass extension, you might set this lower.

    You might use a calibration disk like Avia to see if there is a "hole" in the frequencies near the crossover. If there is, you will need to adjust your crossover frequency on your receiver.

    Some receivers fix the crossover at 80 Hz. Some have bass management features to allow you to adjust the center, mains and surrounds separately.
     
  3. SteveW

    SteveW Agent

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    Ok this still confuses me a little. Why would the amp have speaker level in and out if it is not going to filter out the lows and pass the highs to the front speakers. What is the point of using speaker level inputs then.

    The speakers I will be using are the Axiom M22Ti. Should those still be set to small?? The ones I currently use are more of a satellite size. That is why I have them set to small.

    And is anyone familiar with either of those two receivers and how they handle crossover and bass management.

    Thanks
     
  4. Nathan Stohler

    Nathan Stohler Second Unit

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    I was suggesting using the line level inputs, which makes things much easier in my opinion.

    If you are using the speaker level inputs, you will need to connect the sub in parallel with your main speakers. Then you can use the crossover knob on your sub. I believe the frequencies that your sub filters out will then be passed to your main speakers.

    However, if you want to use the bass management features of your receiver, you'll want to use the line level inputs, not the speaker level inputs.

    Try to find the product manuals for the particular receivers you're interested in to find out how they handle bass management.

    --Nathan
     
  5. Lew Crippen

    Lew Crippen Executive Producer

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    I don’t know anything about the Onkyo, but the Denon has crossover points of 80, 100, 120 and 150 Hz.

    Something that you probably already know, but I’ll just mention that setting a crossover point does not result in all the higher frequencies going to the main speakers and all the lower frequencies to the sub. It is a gradual process. You can sort of think that your main speakers will still be producing some of the lower (bass) sound at half the crossover point, and the sub possibly at twice the crossover point (this all depends on exactly how any particular crossover is designed)—so if you select 100 Hz as the crossover point your main speakers will be reproducing more of the sound than the sub at 90 Hz, while the sub will be reproducing more of the sound at 60 Hz than the mains (but they will both be getting some). Usually the slop of the curve for the higher frequencies is a bit higher than the lower ones, so the sub is not asked to contribute as much as the main speakers.

    With this in mind, the crossover for many satellite speakers should be set higher (perhaps 120 Hz), while full range (or at least nearly full range speakers) can utilize the lower crossover points.

    Of course by setting all your speakers to large, your subwoofer will only be called on to reproduce the Low Effects Channel (LFE), or the .1 channel sound the other speakers getting the full 20-20,000 Hz range.

    You will need to experiment for yourself and decide what sounds best, but unless your new Axioms are flat to below 39 or 40 Hz, I’d recommend beginning with a small setting for all speakers and a crossover of 80 Hz. Next I’d try large for the R/L mains, small for the rest and a crossover of 100 Hz (maybe even 120 Hz). But in the end, use the settings that sound best to you.
     
  6. SteveW

    SteveW Agent

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    Thanks for the input guys. Much appreciated. That clears up alot for me.
     
  7. Edward J M

    Edward J M Cinematographer

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    To specifically answer your question, most subwoofer amps have a fixed high passed filter (set at around 100 or 120 Hz), and a variable low pass filter.

    Why the control doesn't vary both is beyond me, except as a cost issue, as Dustin B recently pointed out.

    Any high or low pass filter will have an associated slope (the rate of decrease). The slope of a filter can be 1st, 2nd, 3rd, (etc.) order. 1st order is 6 dB/octave, 2nd order is 12 dB/octave, and so on.

    A typical BM circuit in an AVR will have a 2nd order high pass filter, and a 4th order low pass filter.

    If you set all your speakers to small, each channel will be high passed at whatever frequency you select in the AVR, again typically at a 12 dB/octave rate.

    At the same time, the AVR will sum every surround channel, and the LFE channel together, and low pass them at the selected frequency and send the signal on to the subwoofer.

    At the selected filter frequency, the amplitude of the signal to both the surrounds and the sub will decreased (typically by 3-6 dB). The sum of the sub output and the surrounds output at the selected xo frequency "should" result in a flat response at that point, but in reality that is not usually the case.

    Irregularities in the FR at the xo are all too common, partly due to room acoustics, and also partly due to the fact that the slopes of the high/low pass filters are different, and the natural roll-off of the surrounds is not always 2nd order. A vented surround for example has a 4th order roll-off below the tune point.

    In the day, THX recommended an 80 Hz xo and the 2nd order high pass and 4th order low pass filter rates because THX certified surround speakers were sealed units with a natural 2nd order roll-off and a tune point of 80 Hz.

    The combined effect of a 2nd order high pass filter at 80 Hz, and a 2nd order natural speaker roll-off at 80 Hz, resulted in a 4th order roll-off - not coincidentally the same as the low pass filter rate imposed on the subwoofer. The final result: a 4th order high pass and 4th order low pass filter rate at the selected xo of 80 Hz and a nice neat crossover.


    Unfortunately, nearly all surround speakers these days are vented, and imposing a 2nd order filter on a vented speaker results in a combined 6th order roll-off - not the same as the subwoofer.

    In addition, surround speakers can have different tune points and F3 points, making it hard to determine what the combined filter/roll-off slope will look like. Using vented surround speakers with an F3 of say 50 Hz might result in a slight emphasis in the 80-60 Hz region because the vented speaker has not yet begun to naturally roll-off, and is only being artificially filtered at a 2nd order rate.

    People desiring better bass management have turned to external BM units with adjustable filter points and filter rates for each channel, including the LFE channel.

    With all that said, if I was going to recommend the best BM for "most" systems and enthusiasts, it would be all speakers to small, 80 Hz xo, subwoofer to on/yes, a sub cable from the AVR to the sub, and bypass the filter at the sub (use an unfiltered or LFE input).

    Regards,

    Ed
     

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