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phmosele 5th December 2012 07:55 PM

Phase switch on crossover
 
Hi
I have a phase switch on my electronic crossover. I am using a two-way system crossing from 2x15s at about 250Hz into passively crossed over EV 3-ways for tops; do I need to worry about phase switches? All my drivers are wired to move in or out in unison, which is what I always thought should happen.

"Phase" in my mind is synonymous with something canceling something out. I can't see that being a good thing.
Thanks
Phil

wg_ski 5th December 2012 09:36 PM

If your bass bins and your tops are both relatively flat at the 250 Hz xover, AND there is no significant physical offset between the two, the drivers should be reasonably in phase with each other. If that's true, leave the switch "in phase". If either the sub or top is rolling off near the 250 Hz xover they will become progressively out of phase with each other. This "excess phase" causes cancellations (peaks and dips). Reversing the phase of either may (or may not) help in this case.

The crossover itself has a phase characteristic of its own - if it's a 4th order it has the proper phase for high and low pass to sum in phase if the drivers phase is perfect. In many applications that's good enough. In more complex rigs it's not. One reason for the popularity of these DSP based digital crossovers is that the high and low pass filters don't have to be symmetric - or at the same frequency. This can (almost) completely compensate for excess phase in the drivers/cabinets - assuming that you can measure your rig to determine what to tweak.

phmosele 5th December 2012 10:24 PM

Hi
That's a very good answer, thanks. Because the guy I am helping wants maximum flexibility (don't know why...his idea is to do live drumming to a Karaoke machine!!) I was planning on having the capacity to lie the 2x15 down and use a long, adjustable pole up to the EVs when circumstances dictated. But that doesn't sound like a good idea at all if I understand you right.

It would seem I should keep the 2x15 upright and either mount the EVs directly on top, or just use enough pole to lift them slightly above ear height if there is no stage.

If the stage is high enough (and there are a few old places around here) I was thinking leave the 2x15s on the floor and just put the EVs on the stage. In those circumstances, I was thinking lie the 2x15 down to get maximum bass "bounce" from the floor proximity effect. That sounds like a useful principle. But because of the corner protectors, the box is never completely coupled to the floor except by the corners (raised maybe 5mm or so).

I am not entirely sure if this crossover (a Behringer Super X (not the Pro)) will allow me to overlap the crossover frequencies a little or if it's a hard cutoff at 24dB at the frequency mark. If I can, I will overlap em a bit, but i don't think I can.

Thanks
Phil

Enzo 5th December 2012 11:35 PM

5mm off the floor? That is close enough for rock and roll. This is a PA system for a band, not a NASA space probe.

Bodies - like the dance floor is full of - absorbs highs. You want the speakers to be high enough that the horns are not aiming right into the people. MY rule of thumb is that if I cannot SEE the horns from the floor, I cannot HEAR them either. Bass notes kind if go everywhere. SUbwoofers are not line-of-sight things.

When WG said no significant offset, I think he means front to back, not vertical. If your woofers are substantially farther forward or back, then the phase relationship can change. If all the speakers are more or less the same distance from the listener, they should be in phase if wired properly. SUbs ought to be on the floor, and you can lift the tops as high as you need to for coverage. Then again, I may be wrong about his intent.

Phase means a relationship, it doesn;t specify what that relationship is. In phase means working together, out of phase means cancelling.

If you have never heard phase problems, it is simple to demonstrate. Listen to your home stereo. I assume it is properly connected. Now listen to the same music, but reverse the wires on ONE of the speakers. Now the speakers are out of phase, and the sonic result should be obvious.

Now your crossover. Set your system up and play music through it. SOund OK? Now play the same music again, and while it plays, push a phase switch. Try each phase switch you have, one at a time, and listen for any changes. Usually you can hear that "this way" doesn;t sound as good as "that way." Let your ears experience it, then you will be better equiped to recognize phasing problems in the future.

To my ear, out of phase drivers sound "not right." A certain spaciness, or a "can't quite put my finger on it" bad quality.

The bottom line for the sound system is: does it sound good to you?

phmosele 6th December 2012 02:02 AM

Quote:

Originally Posted by Enzo (Post 3272212)
5mm off the floor? That is close enough for rock and roll. This is a PA system for a band, not a NASA space probe.

When WG said no significant offset, I think he means front to back, not vertical. If your woofers are substantially farther forward or back, then the phase relationship can change.

Ah, yes of course. Front to back. Makes a lot of sense now. I can't imagine them ever being offset by more than a few inches whether on poles or not.

I have been questioning myself on the crossover frequency. These are not "subs" in any sense of the word, and they would not see anything below 60Hz at all, so that makes them "bass boxes" only IMHO. The original plan has been to cross these over into a pair of 3-way EVs for the tops at round 200Hz, but I have scads of power for the bass boxes, but the EVs will only handle 200 watts (although I understand that crossing them over changes that playing field somewhat. Plus I could load them with some 300w Celestions; not that that changes the 8" or the horn handling of course).

It seems to me that crossing over these 2x15s at some arbitrary low-mid frequency is just going to cost me power I need not lose. The question of where bass becomes directional isn't going to apply. I can understand that the higher they play, the more frequencies will come out of the ports. As you said though, this is rock 'n roll, and I've used plenty of DJ boxes with 2x15 and a horn and the 15s have played happily through 3+K and I was never unduly distressed by anything that came out of the ports (as long as bass did I suppose).

I certainly wasn't planning on taking these anywhere close to 3K before hitting the EVs, but I always intended the EVs to be the part of this little rig that pops up over the heads just to get that mid/top projection.

I guess I'll just have to set it all up and just fiddle with the crossover point until I get the best I can get. I have a CX 2300 and I don't seem to be able to find a way to get creative with an overlapping crossover point. 2-way, it seems, means 2-way

Thank you
Phil


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