QSC RMX-850 versus A750 Hypex?

Discussion in 'Archived Threads 2001-2004' started by steve nn, Feb 26, 2003.

  1. steve nn

    steve nn Cinematographer

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    I am in the process of having a sub built for me and I am a little miffed on what direction to go. As things stand now any sub I hook up to my system will have a hum if I turn gain up on the back of the sub to 1/2. This does not present a problem for me now because I only need to turn the gain to 20% on the bottom sub and to 30% on the top sub. I run duel Vel ct-150's presently. I plane on selling one of these and have Brian build me a AV 15. My question is if I go with the QSC RMX-850 amp will this present a problem since gain needs to be at full? I can see pro's and con's with each decision. Any knowledge or past experience my way sure would be appreciated. I gave up trying to fix the hum to. I have tried 432 different easy fix solutions.__
    If I went with the SVS Black Box would it take care of any problems I might run into if I went with the QSC?
     
  2. Pete Mazz

    Pete Mazz Supporting Actor

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    Can't imagine a pro amp solving your hum problem, and possibly making it worse.

    You may want to get an amp with a lower input sensitivity than the QSC. Some amps feature a switchable input to mate better with consumer gear.

    May want to get something locally, if you can, to try first.

    Pete
     
  3. steve nn

    steve nn Cinematographer

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    This was my thought to Pete. From what I have herd this could make things worse. It is not that I have a big hum problem but if I turn any sub up to 70% I will here it. I would like the extra 100 watts but maybe the HYPEX 500 would be better for my situation? No options in plate amps at 800 watts and the HYPEX comes in at 720 so would loose a db or so. "All that for a extra $250"[​IMG]
     
  4. John A. Gordon

    John A. Gordon Stunt Coordinator

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    steve nn,
    To solve your hum problem, you need to find out where it is coming from. You've tried 432 fixes, but did you find the source? Yes, finding hum can be a daunting task. I use 3 QSC amps in my system without issue. I have had hum, but after tracing it down, my DVD and cable orientation was the culprit, as well as cable TV (cable is a major culprit).

    As for gain, why does the gain have to be at full with the RMX 850? My gains for sub are set to about have, and for mains they are about 3/4 full. Remember, gain on an amp does not mean you are getting less power. The amp can still put out full power regardless of gain setting.
     
  5. Brian Bunge

    Brian Bunge Producer

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    John,

    That was my recommendation as I've always heard to run the gain wide open using pro audio amps. If this is not the case then I stand corrected.
     
  6. Martice

    Martice Screenwriter

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    I've also heard the same thing. [​IMG] I was told if I didn't have the gain turned all the way up, I stand to have harmonics come into play or something to that effect.

    Please advise.
     
  7. John A. Gordon

    John A. Gordon Stunt Coordinator

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    Gain is one of the most misunderstood controls in audio, for both pro and consumer. I am by far no expert, but I do know that gain needs to be integrated both with processor and amp for best results. For some good info regarding gain, check out: http://www.rane.com/library.html, scroll down to the “setting sound system level controls.” You will also find other good info on this site as well. The dictionary is quite nice too.

    A good way to set proper gain would be to turn off all power amps and turn gains all the way down. What needs to be done is to set the levels on your processor to where they just start to clip. Without gain tools or instruments, this is difficult. On a mixing console this is easier to accomplish because of the meters on the board. Once levels have been set on the processor for maximum gain without clipping, you then turn on the amps, with gains all the way down and adjust them. You would slowly turn the gains up on the amp(s) until they just start to clip, and then bring it back just a tad. If you are able to do this, you have maximum gain structure. This of course is not a complete guide, more a guideline. And of course, how do you do this in the home without instrumentation. Some people have been successful by using test CDs with sin wave and pink noise on them. Hope this helps.

    And, I will say it again, gain does not imply power output. And amp can still put out full power even if gains are at half, or even less. If you are getting too much system noise, then your gain structure is probably not set correctly.
     
  8. Brian Bunge

    Brian Bunge Producer

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    John,

    Your explanation of setting gain is the way I'd heard and suggested in the past. But once I came over to this forum and heard people saying that you should turn your sub out's level down and run the sub amp's gain control wide open I really started to get confused. I know the theory was that many receivers' sub outs distort very quickly and this is the way to maximize a clean signal. But if you just took it as an absolute statement on how to setup a sub you really weren't maximizing your S/N ratio.
     
  9. Martice

    Martice Screenwriter

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    This is from Crown's web site regarding setting the levels on a power amp.

     
  10. steve nn

    steve nn Cinematographer

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    Very informative. Thanks for the help and it is starting to make some sense. I can use the gain on my receiver to adjust-regulate gain to the QSC and I do not necessarily need to have gain set to full on the QSC. So I do have options and some latitude. Just like calibrating a regular plate amp some-what? More gain on receiver then less gain on the plate or vise versa? I will check that link out John._______

    Sorry Brian we ain't done yet.[​IMG] I might need the next QSC up the ladder if you have a very sharp pencil?
     
  11. Brian Bunge

    Brian Bunge Producer

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    Steve,

    You know where to find me! [​IMG]
     
  12. Rob Formica

    Rob Formica Stunt Coordinator

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    Is it possible that the other threads (posts) that mentioned turning the gain all the way up for the power amp were more in reference to the lower output level on most consumer preamps versus the professional ones? I don't have a pro-amp to really verify this, but I was under the impression that they required a much higher level from the preamp so that the sub level output adjustment may not go high enough to compensate for this?

    It's one of the reasons I've never experimented combining pro equipment with consumer ones... :b but I have been reconsidering this for a projected DIY sub.

    Thanks...
    Rob
     
  13. Martice

    Martice Screenwriter

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    I had to pull the trigger on a QSC RMX 850 today. It was going for $200 even. I have a Stryke AV15 coming in a 25" PowerCube cabinet. The 850 has 830 watts Bridged into 4 ohms and although it is more than enough for the AV15, when and if, I decide to drop the HE15 in the cabinet later on, I should have enough power to push the HE15 convincingly enough although not to it's max.

    I also saw a QSC 2450 for $425 at the same place. Knowing the possibility of me moving on to the HE15 later, do you think I should invest in the QSC4250 or is my original amp at 830 watts powerful enough?

    By the way, the QSC RMX 2450 is class H amp as opposed to the Class AB 850 and is rated at 1500watts into 4 ohms bridged.

    Thanks
     
  14. Robert_Gaither

    Robert_Gaither Screenwriter

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    Martice, where the heck did you find them that cheap? A big thanks if I can get one as well (just got my AV15 and still haven't decided on alignment).
     
  15. Martice

    Martice Screenwriter

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    Hi Rob. YGM!!!
     
  16. David Lorenzo

    David Lorenzo Stunt Coordinator

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    Hey Martice,

    Any chance of me getting info on where you got it? I'm also getting a AV15 and haven't decided on an amp yet. Thanks.
     
  17. John A. Gordon

    John A. Gordon Stunt Coordinator

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    For most QSC amplifiers, to drive them to full power you only need between about 1 to 3 volts depending model. Most processors can easily do 1 to about 7 volts, and some even more. So there should not be an issue with driving the amplifiers. I’ve used a couple of processors in my system with my 3 QSC DCA amplifiers and believe me, I can drive the amps to full power if I wanted to (of which a benefit of QSC amps, and most pro amps, is the clip indicator. You know for sure when you are clipping the amp, and of course if you do not want to do that). Or as my wife says on occasion, “it’s too loud, turn it down.”

    Calibrating your system to its best performance is a wonderful thing. It takes a little more work, but if you enjoy tweaking, as many of us do, the end results can be pure joy. And, once you have the system calibrated, the great thing is you are set and do not have to re-tweak it anymore—unless of course you add a new component to your system of which you would be rechecking anyway. Of course, it’s not a bad idea to periodically recheck the system, just as you would recheck channel levels with your SPL meter.
     
  18. Martice

    Martice Screenwriter

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    I apolojize to the thread for slightly misleading you on my post about the QSC amps. I should have said that it was a shelf demo and the QSC 2450 for sale at $425.00 is a second hand but flawless unit. That should clear up the stampede of e-mails I've received about the price of my RMX 850.

    Oncea again, I apolojize for the the screw up.
     
  19. Robert_Gaither

    Robert_Gaither Screenwriter

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    No problem Martice, still a big thanks as now I have a selection of amps to choose from.
     

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