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Author Topic: NEXO phase checker problem  (Read 6408 times)

Tim McCulloch

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Re: NEXO phase checker problem
« Reply #10 on: May 19, 2014, 10:14:25 AM »

Hi Dion-

A couple of other things about "phase poppers"....

They will react differently when one pass band in a speaker system is of reversed polarity (let's use mids as an example).  If the detector is closer to the mids, it will show the polarity as negative, but if you move it a few cm closer to the lows or HF it will show positive polarity.  You can see how trying to determine polarity while running full range could be a problem.

Also these checkers do not deal well with full arrays, they work much better if you are checking one loudspeaker/transducer at a time.  They do not deal well with reflected sound, either.  You need to be close (around 10cm or so) to each individual transducer and check each pass band individually (for the reasons shown above).

Our shop owns Galaxy Cricket® polarity checkers and we were a beta tester for Galaxy Audio (they were about 20 blocks from our previous shop).  The Cricket also functions as an XLR cable tester (will show pin 2/pin 3 mis-wiring).  I suggest you use a cable tester capable of showing mis-wires as well as opens or shorts, and verify that all cables and snake return lines are wired consistently.  Check every cable your own, not just the cables in your amp racks, and check cables owned by your crew members, too.  Cables have a way of creeping into rigs...

When Galaxy finalized the design we went through every XLR cable and every snake channel/return line in our business.  We checked other devices (EQs, system dynamics and power amps) to see if they inverted polarity without saying so in their owners manuals.  Note that some power amps are inverting*, and if you mix them with amps that do not invert you'll have polarity issues that can drive you crazy if you're a 'mix & match' shop.  We then verified polarity with a single speaker system (an S4 clone), and checked every speaker cable with that loudspeaker, too.  Once we knew that our cabling was consistent, we went on to test every transducer in every pSeudo4 in our inventory (48 of them at the time) and found a couple internal wiring errors that likely crept in when we serviced them.  My boss did most of this and called me to say "the mids are out of phase!"  Well, I corrected him with "polarity, not phase" and then described my recent processing changes for that system (it made the acoustic crossover sum much better).  He'd never done that and was surprised, but I digress.  You just need to know that whatever you find is consistent for all your inventory of identical loudspeakers.

But if you're finding that one side of the PA (regardless of pass band) is consistently opposite polarity of the other side, it means you have a pin2/pin3 swapped in a drive cable somewhere.

How are you connecting the sending device?  At the console end of the snake?  At the amp rack(s)?  There are lots of places to have this error creep in.... don't forget to test every interface cable, too (it could be in your drive rack, between the EQ and the amp rack, or between the console and the EQ, between processor output and amp input, etc).

Have fun, good luck.

Tim Mc

* - Power amps are inherently "single-ended devices".  Applying a positive voltage to which XLR pin causes a positive voltage swing on the RED (or positive) terminal?  With the Crest Pro 001 series, that was pin 3, but for the Crown MacroTechs it was pin 2.  If one routinely mixed Crown racks and Crest racks within the same system deployment there would be problems.  We dealt with this by swapping pins 2/3 in the XLR that connected to each Crest amp's inputs and marked the XLR as "pin 3 hot".  In the Crown racks we marked the XLR "pin 2 hot".  The inputs to all racks were wired pint 2 hot for consistency with the rest of our inventory.  It didn't matter what rack you put where, it would just work right.
« Last Edit: May 19, 2014, 10:33:46 AM by Tim McCulloch »
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Re: NEXO phase checker problem
« Reply #10 on: May 19, 2014, 10:14:25 AM »


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