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Phasing Between Different Amp Makes

Discussion in 'Amps and Cabs [BG]' started by bgavin, Aug 15, 2002.


  1. How can one determine the phasing between power amps made by different manufacturers? Is there a standard?

    For example, I own a QSC PLX 3002 and I bi-amp. Much as I would like to own another QSC for my Highs, I can't find one at a decent price. So... I'm looking into a Carvin DCM-600 for the highs and using the PLX entirely for my subwoofers.

    I want my Highs and Lows correctly phased, but I don't know how to accurately check phasing between amps. Amps are more difficult that putting a DC source on the cabs and watching the cones move.
     
  2. geshel

    geshel

    Oct 2, 2001
    Seattle
    I would hazard to say that all (decent) PA amps should be polarity-preserving. Anybody making an inverting power amp for professional use would be summarily dismissed, I believe.

    Otherwise, time to break out the 'scope! :)
     
  3. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Gold Supporting Member Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    Ah, the polarity issue! About 12 years ago the AES finally settled on XLR pin 2 hot. I think they may have flipped a coin to decide it. ;)

    Most power amps (QSC included) are pin 2 hot—IOW, an input voltage that is positive on pin 2 compared to pin 3 will make the amp put out a positive voltage on the + or red terminal.

    There are some exceptions: older Crest models are pin 3 hot. I don't know if they've changed their whole line over to pin 2 hot yet. But since you're interested in Carvin, I'd ask them directly if their amps are pin 2 or pin 3 hot.
     
  4. Bob,

    Much obliged. The Carvin runs Pin-2 as Hot.

    I'd really prefer a PLX 1202, but even the used prices on eBay are very high. So... I have to look elsewhere. I only need a maximum of 150w @ 4 ohms per channel.

    I contacted a seller who had the Carvin DCM-600 up on eBay and it didn't sell. The reason is, he wants far more than market value for used equipment. Carvin specifically does not transfer their 3-year warranty to the 2nd owner, so it makes more sense to pay a few dollars more and buy a new one.

    Frankly, I'd rather buy a used PLX 1202 for $360 than a new DCM-600 for the same price.
     
  5. OK so that means phasing will not be a problem? I have another question if anyone can answer this...if you run the Carvin bridged, is the speakon connector wired the same as when you run the PLX bridged?

    bgavin, did you end up getting the Carvin? If so, how does it sound compared to the PLX?
     
  6. ganttbos

    ganttbos The Professor Supporting Member

    Dec 18, 2001
    New Orleans area
    Bruce,
    I own one of those $400 phase testers - it produces a signal you can feed into the chain that emits a pulse through the speakers, and then has a detector that you hold in front of the speaker that lights up green or red in response to the initial cone movement (in or out). It's small and light. If you want to pick up two-way shipping I'll send it out to you to play with for a while. We messed around with it a bunch when we got it. This was all precipitated by my putting an EV proline 10" and a JBL E140 15" into a sonic cab, and finding out that JBL speakers are polarity reversed relative to EV's and most others. This was a very simple (speaker) phasing example that could have been diagnosed and fixed (the 9v battery thing) without a phase tester. The pin 2 vs. pin 3 hot issue is actually quite complicated and suffice it to say that several times we found that a phase reversing glitch in the signal chain doesn't always emerge at the speaker end as a phase problem. It was very enlightening to play with and would probably be right up your alley.
    Gantt
     
  7. Gantt,

    Many thanks for the kind offer. I'll put it in the bank for future reference. If I add a Carvin to my PLX rack, that would come in very handy. A second PLX would be the ideal solution, except for the $$$.

    Yes, on the JBL thing, for sure. Some model JBL are Black/Forward, others are Red/Forward. Crap, and you'd think JBL of all people would skip that inconsistent stuff.

    I just received a JBL E155-8 18" Big Mutha, and will be wiring it (with everything else) into Red/Forward polarity. I'm looking forward to getting the cab built for the E155 to see if it is an acceptable driver for down sizing my stage rig. I don't expect it to have any bottom, but it will be loud as hell, and run properly in 2.54 cubic feet.

    If the E155 thing works out, and I go back to all 4-string basses, I will be able to downsize considerably. I won't need a 2nd power amp, and may not even need to bi-amp. The Eminence Magnum 15LF is another one that looks very promising, but it is not as loud, and requires a larger cabinet.

    BTW, thanks much for that D410XLT. Chelsea is driving it with an SWR 2002 and she just loves it.

    Link to My Daughter's Band