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  1. #1
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    Setting Up a Crossover in a PA System...

    Hey all

    I have some questions about setting up a PA rig, and I have some specific questions that I'm not able to find answers to online -- so I thought I'd bounce them off you all to see if you know the details I'm trying to figure out. Okay, here's what I'm thinking (and this will probably be a LONG post)...

    The setup would be:

    1. 1 mains amp (Crown XLS 802)
    2. 1 monitor amp (Crown XLS 202)
    3. 1 sub amp (Crown XLS 802)
    4. 1 crossover (dbx 223XL)
    5. An EQ (but that doesn't factor into my questions)
    6. 4 8-ohm speakers
    7. 2 8-ohm sub speakers

    From the desk, I'll have channels 1 & 2 which will go into the crossover channels 1&2. Here I'll split to my subs from the crossover LOW output, which will be 1 XLR to the sub amp -- and here's where I start getting confused. Which sub amp INPUT do I go into? Is this where I use the "bridge mono" mode of the amp? Or, do I just pick a channel and go with it, knowing I'll be totally wasting a channel on the amp?

    The same thing for the mains. Do I go from the crossover's MID to say, channel 1 of the mains amp -- and go from the crossover's HIGH to channel 2 of the mains amp? THEN I'll know that channel 1 is for my mids, and channel 2 for my highs? That sounds great, and I guess I'd just take the channel 1&2 speakon outputs over to the speakon inputs of each speaker pair master, and hookup the secondary speakers in parallel for 4-ohms...?

    If the above is correct...

    I'll have 2 subs from 1 channel, in parallel for 4-ohms, from the Sub amp. I'll also have 2 HIGH speakers in parallel at 4-ohms (from channel 2 of the mains amp), and 2 MID speakers in parallel at 4-ohms (from channel 1 of the mains amp).

    Does that sound right?

    TIA,
    Maury

  2. #2
    Let's see if I have this right - your mains have separate inputs for mid and high frequencies, allowing you to feed those drivers separately? Important to know.

    For the sub, it's easier on the amp if you drive both channels in parallel, and connect one sub to each channel. The "correct" way to do this would be to set the switch in "dual" mode and wire up a siamese-twin XLR Y-cord to feed both channels. (Some amps have a switch position allowing you to drive both channels from one input, but yours doesn't.)

    The "sneaky" way to do this is to set the switch to "bridge" mode, put the input signal into channel 1, and then hook up one sub to each output - but with the polarity flipped on the channel 2 sub. When an amp is set to bridge mode, channel 2 puts out the same signal as channel 1 but with the polarity flipped. (180 degrees out of phase, if you insist.) So if you flip the wires going to sub 2, the polarity is flipped twice, which is to say it's back "in phase" like it should be.

  3. #3
    First things first. The amp is only designed to run at 8 ohms when run in Bridged mono. Unlike the very easy answer given above, Bridged Mono mode is not to be taken lightly. With a 4 ohm load (two 8 Ohm Boxes) you would destroy the amp, and/or possibly the speakers, or both.

    Your rig should go like this:
    Left and Right Outputs from mixer/EQ/whatever your final piece of processing gear is before the Xover to the Channel 1 and Channel 2 inputs of the Crossover.

    In this case, set the xover unit up to run in Stereo 2-way operation, and pay attention only to the markings above the connections. (Mode switch should be in the 'UP' position. the LF summed switch should be at 'NORMAL'.

    Make sure the 'XOVER FREQ' buttons are in the 'UP' or off position.

    Plug the outputs with the marking 'LOW' above them to your sub amps two inputs.

    Plug the outputs with the marking 'High' above them to your mains amps two inputs.

    Run the amp input switches on 'DUAL' mode.

    Plug one speaker into each channels output (preferably with professionally made speakon cables).Obviously, the sub amp outputs go to your sub boxes and the mains amp outputs go to your mains. If you plan on panning anything to a stereo 'image', make sure your Left sub is under/near your Left Main box. typically, Left is channel 1 and Right is channel 2...

    on the Xover:
    Set the input gains at -0-

    Set the Xover Freq at around -120- so, ,one notch below 140, or right between 70 and 140. This can be played with to find out what works best with the subs you have.

    Set your Low and High outputs at -0-

    On the Amps, go ahead and set the input attenuators all the way down.

    Once everything is connected, Play a CD that you are very familiar with through the mixer. Make sure the channel input on the mixer is not being clipped. (the level of the signal should read -0- on the meters when you solo or PFL the channel). Set the main L&R outputs so that the main meter reads -0- at the loudest parts on the CD.

    slowly bring up the level on the AMP level controls.

    Once you reach the desired level (volume) or the AMP level controls are all the way up, that is a rough start. You may just want to turn it all the way up. as long as you are not clipping anything in the signal path, you should be fine to do this. Also depends on how you matched your amps power rating to the speakers they will drive.

    From here, adjust your Low and High outputs on the Xover accordingly, to make the CD sound how it 'Should' sound in your head. Taking the crossover levels down is usually a better step than up, as you may just add distortion/clipping by going up.

    Of course, you need to watch your amps lights, too. The 'Signal' (Green Lights) should be on. The 'Clip' (Red Lights) should only flicker on the hardest hits, and you may want to turn down the crossover levels if you see the red lights.

    The 'Fault' (red) should not come on. if they do, something is wrong.

    This should get your rig running and sound good (and keep you from blowing up in a mushroom cloud) !!

  4. #4
    as for the speakers offering mid and high inputs. if you can run them without the two inputs (usually an option on mains boxes), it will greatly reduce the risk of headaches/high frequency horn failures.

    Multi-way rigs are best left for those who have a little more knowledge of how to protect each component that is being driven/used.

  5. #5
    I wasn't sufficiently specific.

    I meant to connect Sub 1 between the output and ground of Chan1 and Sub2 between output and ground of Chan2, with polarity flipped. That way each channel drives an 8-ohm load. As Bassred says, if you connect both subs in parallel across the outputs, as you normally would in bridge mode, that amounts to a bridged 4-ohm load which the amp can't drive.

    I was assuming a mono program and bi-amped mains, which is what the original post seemed to suggest.

  6. #6
    Senior Member SC358's Avatar
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  7. #7
    Quote
    Quote: karl eilers
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    I wasn't sufficiently specific.

    I meant to connect Sub 1 between the output and ground of Chan1 and Sub2 between output and ground of Chan2, with polarity flipped. That way each channel drives an 8-ohm load. As Bassred says, if you connect both subs in parallel across the outputs, as you normally would in bridge mode, that amounts to a bridged 4-ohm load which the amp can't drive.

    I was assuming a mono program and bi-amped mains, which is what the original post seemed to suggest.
    Did you even look up the gear that was mentioned? This is the bottom-of-the-ladder consumer amp here. It is certainly not going to do well in bridged mode with a rookie at the helm.

    Are you trying to explain how to wire them in series, rather than parallel? That would give a final resistance of 16 ohms.

    When bridging a power amp, you should not try to wire anything to it in any other manner than designed for. You are certainly a dangerous person to listen to, and I truly hope you have not caused too many people to lose or damage their gear.

    I only go this far because I've seen post after post of half-truths and bad info, and it would really do you well to do some more discussion with actual sound pros/equipment designers to get your information straight before loading a bunch of junk into amateurs heads..

    To all who are inquiring about audio help. READ YOUR GEARS MANUALS, everything you need to know is in there, it might take a few times through it to understand, but it's all in there.

  8. #8
    Bass: do I have to draw pictures?

    An 8-ohm speaker connected between the hot and common of one channel of an amp is an 8-ohm load on that channel. Period.

    An 8-ohm speaker connected between hot and common of the other channel of the amp is an 8-ohm load on that channel. Period.

    When connected this way, each channel drives its own load and does not know or care what the other channel is doing.

    The only difference when you switch the amp into bridge mode is that the output of the second channel is flipped in phase. The load is not changed.

    Granted, under normal conditions this would cause the subwoofers to operate out of phase, so their sound would largely cancel and you'd get very little acoustic output. That's why I said to switch the wires going to sub2 so they would operate in phase.

    Granted, this is an unusual way of using bridge mode. But it will work.

    Granted, if two 8-ohm speakers were connected in series across the two hot terminals that would be a 16-ohm load. That isn't what I said to do, if you reread my post, but it would work just fine. The amp will see a bridged 16-ohm speaker as, essentially, an 8-ohm load across each channel, just as it sees a bridged 8-ohm speaker as a 4-ohm load across each channel. Each channel will put out 500 watts into its half of the load, for a total of 1,000 watts.

    You can safely wire things in odd ways if you know what's inside the amp. Classic impedance matching went out with tubes, which is nearly 40 years ago now, but the memo does not seem to have circulated. A modern power amp is essentially a stiff voltage source with essentially zero actual output impedance, and it doesn't care how you load it as long as: 1) the impedance is not lower than the specified minimum (higher than specified minimum will not damage the amp); and 2) said impedance does not have too much inductive or capacitive reactance in it.

    I should add that there's no such thing as a "bottom-of-the-ladder consumer amp" where Crown is concerned. Also, it was Crown who invented the high-power, weird-load-protected amp, about 40 years ago, and they know how to do it if anybody does. I was the Minneapolis/St. Paul Crown service tech for many years, and went to the Crown service school in Elkhart. That was a while back, but I have a hard time believing Crown amps today are less competent than those of 40 years ago.

    As for "not do(ing) well in bridge mode," the amp is designed to be used in bridge mode. The only thing Rail could do to cause a problem would be to connect both subwoofers in parallel, across the two hot terminals. This would be a 4-ohm load, which I do recall saying the amp can't drive.

    As far as doing discussion with actual pros and equipment designers goes, I am an actual pro and equipment designer. Please cite other half-truths and bad info. If I've made mistakes - I do make them, everybody does - I should correct. If not, I should clarify.
    Last edited by karl eilers; 31st May 2009 at 20:00.

  9. #9
    I do agree with one thing Bass said. It is worth taking the time to read user manuals, carefully and repeatedly. When I can, I even get schematic diagrams and study those. One of the keys to getting superior results is to know your equipment as thoroughly as possible.

  10. #10
    Just went thru earlier posts, and there's another thing I completely agree with Bassred on. Running separate amps to midranges and tweeters is begging for trouble. Aside from the high probability of blowing the tweeters, it's almost impossible for someone who doesn't have a golden ear and/or sophisticated test equipment to match levels between them. Then you wind up trying to fix the problem with EQ, and pretty soon everything is screwed up.

    There's also a beam-tilt problem. In the crossover range, where both midrange and tweeters are working, if the sound wavefronts of midrange and tweeters are not perfectly in phase, the speaker's coverage angle will tilt up or down. In extreme cases (which, on probability alone, will happen 50% of the time) you can even get cancellation. In many speakers, the crossover is around 2kHz, which puts the problem right in the middle of the frequency range most critical for vocals.

    Without careful study, you have no way of even knowing what the proper phase angles at crossover are supposed to be. This is a complicated issue and most people are better off using the mid/high crossover built into the speaker.

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