Phase Issue???

Qu Qu General Discussions
1

Last weekend I was running sound for a benefit outdoors. Usually my band plays in small bars. We were set up on the back of a flatbed trailer with the mains (passive sv115 club series Yamaha’s) stacked on the trailer 2 on each side 25ft apart. The powered subs (JBL 18 inch) were on the ground 2 foot infront of the mains. The Trailer sat up on a hill. When standing about 30 to 40ft away right in the middle the sound would nearly cut out when just running mp3’s through Channel 16. Lean left and you could hear the left stack, lean right and you could hear the right stack. I used the same length cables coming from the power amps to the lower mains, then daisy chain to the top mains. The power amps are fed by a DBX crossover which sends a mono signal out to one powered sub, then daisy chained to the other powered sub. The crossover is fed by the LR of my QU16. It was really noticeable with change over music, but could hear it when the other bands were playing, I could hear the acoustic guitars coming in and out which would have been coming out the mains, so it doesn’t seem to be a sub frequency problem. I tried turning the mains in and out. Running just one main on each side. But that was about as far as I got because the bands needed to start playing. I’ve never noticed it before but we usually play indoors with a lot of reflective sound off of walls.

What do you guys think is going on?

I’m sure it’s a phase issue, but how do I fix it? In the QU is there away to reverse polarity on just the left or right channel? Should I run the stage slighly stereo, right now everything was panned to the center. Is it just the limitations of my system, and it’s not made for outdoor venues. Do I need a center speaker to combat the issue? Should I have stacked the mains right on top of the Subs to line them all up (they would have rolled down the hill)?

2

Red…

If you want this troubleshot you’re going to have to give complete information.

  1. How were you connecting the mp3 player to channel 16? Cable type?

  2. Which DBX crossover are you using? Specific model # please.

  3. What kind of cables/connectors are you running from your amps to your speakers?

  4. It’s polarity, not phase…

3

No problem.

1- its a 1/8 inch to rca cable to a rca to 1/4 plug, coming out of an old android phone, into channel 16. I could hear it when other bands were playing also. Not just MP3.

2- It’s a DBX 234xl. Xlr’s from LR of the qu16 to inputs on the DBX running stereo out for mains but mono for subs. XLR’s to crown power amps for mains, then one xlr to left sub input, one xlr from left sub output to right sub input. All cable lengths are the same for mains, 25 ft to left sub, 25ft to right sub.

3- Speakon Cables out the back of the crown power amp XLS2500 to right and left mains. 50 ft. then daisy chained to top Main with 3 ft speakon.

4- Polarity it is then

Thanks
Red

4

OK, so check the wiring in your Neutrik cables to verify there’s no polarity flip from mis-wiring. You can also try running mono out of your crossover to the tops or feeding the crossover from the Alt Outs on the Qu and assign the feed to the output as L+R instead of L/R.

To check the polarity of your main tops you can use the old 9v battery trick and visually observe the direction of the cone excursion when voltage is applied.

Happy hunting and good luck.

5

PS

Don’t forget to check for proper wiring on your XLR’s as well.

6

My guess problem is on the LF speakers. Having passive tops and powered bottoms there is a high likelihood there is a polarity difference between the 2. Just old pin +2 or +3 problem. Could be Speakons on the tops though. So it’s just a matter of going through entire system. The symptom sounds like one side of LF flipped.

7

I forgot to mention that the 234 has polarity flip buttons on each channel. They’re set into the face plate and you have to use a paper clip or the like to operate them. I don’t remember if there are any lights to indicate if the butoons are pushed.

8
9

Thanks guys, I’ll look into it next time I get a chance. Hopefully it’s something simple enough…

10

“When standing about 30 to 40ft away right in the middle the sound would nearly cut out”
“Lean left and you could hear the left stack, lean right and you could hear the right stack”
There has to be a reversed polarity (phase) cable for this to happen in the center of the system,…
if it is happening between the two top speakers on each side, then it is most likely a comb filtering issue.

Provided all the gear is indeed “in-phase” electrically, everything in the center of the system will sound nice and punchy.
I am impressed that strokerRED recognized this issue with the system. So many sound guys would be asleep at the wheel and not even notice.

StrokerRED, the problem is most likely not what I am going to discuss here but something to experiiment with the next time you set up your system. If you have two top cabinets on each side and they are not pointing away from each other enough this causes a serious bad sounding issue called comb filtering. Even if the cabinets are trapezoid design, the angle is usually not enough. You can test this by setting up just two top cabinets, sitting side by side as they would for lets say,… the left side. Angle them away from each other instead of allowing the insides of the cabinets to be flush with each other. Those Yamaha SV115 cabinets claim to have a 90 degree horizontal dispersion so we can not allow them to interfere with each other. Yes, they can sit side by side, but there needs to be a pretty big gap between them. I am only going to guess that they probably need somewhere between 10 and 16 inches between them, I am talking about this gap being in the front of the cabinets with the backs touching each other… spreading the front outward.

When speakers overlap, (conflict with each other) certain frequencies double up and become “HONKY” and other frequencies get cancelled out.
The advice that Gcumbee and Dick Rees have given is good, and what they have offered is a good starting place to make sure that everything has the same polarity.

Here is a picture of some of my Altec and JBL gear set up outside for a concert.

You can see that even though the top cabinets are not identical, they are spread out, the amount of spread or angle between the two cabinets on each side will always be determined by your keen ears and a pretty short listening test after setup.

Experiment with this and the lights will come on and you will say WOW! I had no idea something so simple could make so much of a difference.

Now on a different topic… sometime we can discuss the fact that for most of JBL’s existance, their components were “out of phase” from virtually all other brands of speaker components. It was always stated very clear though…
Positive voltage applied to the BLACK terminal creates a forward cone movement. JBL recently changed this otherwise (stupid)
design and now their components are in phase with most all other brands. Even if we discovered that the subs were out of phase from the top Yamaha cabinets, it would not cause the sound to practically disappear in the center.

Somewhere, ONE of the cables feeding the DBX inputs is reversed polarity, or ONE cable feeding the Crown amp for mains is reversed polarity, OR…
the Crown amp is set in Bridge mode. That will cause this symptom too.

Alright, enough of me rambling :slight_smile: