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  1. #1
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    Aug 2009
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    Michigan
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    PA Hookup, Balanced XLR to Unbalaced 1/4

    Hello,
    I just purchased a fairly cheap PA setup and bought an Alesis 3630 compressor/gate/limiter to run after the mixer. The mixer's main out's are only XLR Balanced. The compressor has only 1/4 unbalanced ins and outs. I know you can buy patch cables that change XLR to 1/4. Would it be best just to buy these cables or to exchange the compressor for one with XLR ins and outs? I am worried these cables may degrade the sound before entering the compressor and then out to the power amp.
    Thanks for any suggestions.

  2. #2
    What mixer do you have that has only balanced main outs? If this is the case, is there a mains insert point?

    But yea, don't put an unbalanced unit in the middle of a balanced signal chain. It ruins the attributes of a balanced system. There is no cabling/balun settup that can set it right.

    This Alesis isn't really designed for a main output of a board, but as an insert on a single channel, or an insert on a aux bus.

    But I'm curious; what amps are you using that have balanced ins?
    'I think my intimate relationship with electronics started as a child when I was playing with a screwdriver and a wall plug, Doc, and...'

  3. #3
    Actually, you can get this to work fine as long as mixer, compressor and amp are fairly close to each other and plugged into the same outlet. If they're spread a hundred feet apart, the loss of balancing is going to bite you.

    The following advice is based on the most common balanced mixer output circuit. Let us know make and model so we can be sure.

    You will need to make your own cable from mixer to compressor. Single-wire shielded. XLR pin 1 to shell of 1/4" connector. XLR pin 2 to tip of 1/4" connector. XLR pin 3, no connection. If you have 2-wire shielded cable, just ignore one of the wires.

    If you buy a cable, there's a good chance you'll get one that shorts XLR pin 3 to ground. Not good.

    From mixer to power amp, depends on the amp. M/M#?

  4. #4
    The inputs to the Elesis are hi-z though, and a balun would have to be in there to make the impedence match. You can force anything to work if you know how. But the point is to make things work properly. Keeping a balanced system balanced is just plain proper.
    'I think my intimate relationship with electronics started as a child when I was playing with a screwdriver and a wall plug, Doc, and...'

  5. #5
    New Member
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    Aug 2009
    Location
    Michigan
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    thanks for the replies. Here's the gear:
    Mixer: Behringer RX1202FX Eurorack Pro
    Compressor: Alesis 3630
    Power Amp: QSC GX5
    Speakers: B52 MX-1515

    The main outs on the mixer are only XLR balanced. There is a Control Out but that's for phones/control room. The Mixer does have aux sends and returns to the main bus. Would this be the best way to rought the compressor? I want it to tame the overall signal so as to not damage the speakers at all from pops and cracks from drums or vocals.

    The manual suggests putting the compressor in chain after the mixer but before the power amp. Can I get the compressor to do the same thing through the aux main send and returns? I would then just be careful not to clip the mixer and the output signal should be good for the speakers to handle?

    All the units will be very close together.

    The power amp does have balanced XLR and unbalanced 1/4" ins.
    Thanks Again

  6. #6
    Gotta say something about "impedance matching."

    Classic impedance matching is thus: the actual source impedance of the sending unit is the same as the actual loading impedance of the receiving unit. The receiving unit "terminates" the line.

    This hasn't been the norm in audio equipment since transistors took over from tubes, nearly 40 years ago, but it seems old concepts refuse to die.

    A typical "600 ohm" mixer output has an actual output impedance of, typically, 50 ohms. It is not intended to be connected to a 50-ohm load, and won't work properly if you do. It is intended to be connected to a load of 600 ohms or higher. Anything up to and including an open circuit is kosher. Power amplifiers are the same way: the actual output impedance of an amplifier designed for an 8-ohm load is usually a few hundredths or thousandths of an ohm.

    Audio outputs nowadays are essentially stiff voltage sources, like a battery. They have a specified minimum load impedance, which you cannot go below without overloading them. But they do not require matching.

    A 600-ohm mixer output can be fed directly into a high-impedance compressor input with no problems.

    I'm speaking only of analog audio. When audio is digitized, the resulting bit stream has frequency components into the megahertz, and then impedances have to be matched to avoid reflections in the line which screw up the data.

  7. #7
    Are you looking to use the compressor as a limiter for your amp? Why not just watch your output meters on the mixer (make sure you aren't constantly RED) and the lights on the amp. This has worked for countless techs for years and years. Make sure you are not sending too hot a signal, and that the amp is not receiving a signal that will cause it to act badly (distortion, blowing up your speakers, etc...)

    I have used a compressor on the main outs of a mixer before, but it wasn't really doing anything, because I know what I'm doing. I had it in place for the rest of the crew who didn't really know what they were doing. To properly set the thing up, you are going to want to know what you are doing. Learning to run the system properly without it would be better in the long-run.

    You would be better off using the compressor in the more traditional/conventional way of inserting it on the channels that are the most transient (usually vocals)... The mic channels have insert jacks, so you could use each side of the compressor on a different channel.

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