Need help with my new QSC amp, very frustrated!!!

Discussion in 'Home Theater Projects' started by BryanDO, Apr 5, 2007.

  1. BryanDO

    BryanDO Stunt Coordinator

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    Hey, I just got my QSC MX-700 amp and excitedly hooked up. I grabbed the remote, turned her on and played some music. To my disappointment there was barely any output! I had both chan 1 and 2 gains full on the amp and there was barely any bass going to my sub. I checked all my connections and everything looks correct, I can't figure out whats going! Here's my set-up

    Yamaha HTR-5640 subout to:
    Behringer FD pro 1124 to:
    QSC Mx-700 amp.

    I properly bridged the amp by bridging both channels together (also tried both chan 1 and 2 seperately) in the back of the amp and checked the gain level on my receiver and BFD. Whats going on here? Isn't 700 watts supposed to be able to push my Sealed shiva hard!?!? I'm running out of patience and I'm starting to think I bought a crappy amp off of ebay.

    PLEASE someone help me out!
     
  2. Wayne A. Pflughaupt

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    Pro amps use a hotter signal level than home gear does. Turn your sub output from the receiver up all the way. Also, if you’re using a lot of cutting filters in the BFD, that will give a net loss in signal output. You can check by comparing your in-use signal to the base (incoming) signal (I think you push and hold the "In/Out" button to do that).

    Regards,
    Wayne A. Pflughaupt
     
  3. bobbyg2

    bobbyg2 Supporting Actor

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    I had the same problem with my dads receiver, he had the subwoofer out put on 8 ohms, my subwoofer was 4 ohms.
     
  4. BryanDO

    BryanDO Stunt Coordinator

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    Well, my BFD is definitely cutting alot of the output, but it' much needed to flatten the frequency. I believe your Correct Wayne, my receiver isn't putting out enough voltage through the sub out. I tried turning the volume up on the receiver to unheard of levels and the sub did get louder, unfortunately I also hit clipping and blew a coil in my Shiva. Seeing as the signal clipped long before the amp was outputting it's full 700 watts, proves it's the receiver level output. Is there anyway to fix this without buying a new receiver?
     
  5. Wayne A. Pflughaupt

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    Something sounds funky there. I don’t see how you could damage the sub if the receiver’s sub output was clipping, especially if the amp isn’t getting to full power...

    Well, you could lower the level of all your main speakers. That would require turning up the receiver higher to compensate, which would increase the signal to the sub. But I think the problem is really the way you set the equalizer.

    When you EQ’d the sub, where there any low places where you could have boosted, but you cut everything down to the lows instead? I’m betting that’s what you did. If so, that’s the source of your problem. I’ve seen this problem many times before with people cutting everything in site. With a regular home sub, it’s not too big of a deal, you can just turn up the volume to compensate. But if you’re using a pro-audio amp, it can be disastrous – as you’re seeing. They’re barely getting enough signal from our home gear as it is – substantially reducing that marginal-to-begin-with signal ensures the amp will never reach full power. Look into re-doing the equalization with a combination of boost and cut filters, and you should be in business.

    Regards,
    Wayne A. Pflughaupt
     
  6. Jon Carlson

    Jon Carlson Extra

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    Could you have bumped the Operating Level button on the BFD while hooking up your new amp? It needs to be in the -10dBV (home) setting, not the +4dBu setting. A long shot, I know. I'll go back in my shell now.
     
  7. BryanDO

    BryanDO Stunt Coordinator

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    Wayne, I never use boost when equalizing a sub. I gives an inconsistent response when in different seating positions around the room. I basically equalize any large peakes. If I have dips, I find another place in the room for the sub or found out whats causing it. I'm only using 4 filters from 16-100hz with the BFD. Lowering all the speaker inputs is an idea, i'll have to try it out. One thing I don't understand is it seems as though it only sounds the sub is blown (plastic hitting plastic sound) when the EQ is on? Very weird. When the sub got clipped power it was clipping at both the amp and BFD. The output for my sub on the receiver is already pegged. I'll keep experimenting. Is there any way to boost the line level voltage of the receiver?

    Jon, good thinking, it was actually the first thing I checked [​IMG]
     
  8. Robert_J

    Robert_J Lead Actor

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  9. Mattak

    Mattak Stunt Coordinator

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    Then you were already sending too much power to the shiva. Clipping isn't a magical speaker killer...it's too much power. You could send a clipped signal all day long to the speaker as long as the signal is within the speaker's power handling ability.
     
  10. Wayne A. Pflughaupt

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    Most of us don’t try to get ideal response at every seat in the house, but for a primary listening position. But typically EQing for a primary position gives acceptable response for other seats as well, as long as they’re away from boundaries. If you have REW, it can average several readings and you can EQ from that curve. Makes things much easier, as long as you don’t mind not having “perfect” response anywhere in the room.

    Regards,
    Wayne A. Pflughaupt
     
  11. BryanDO

    BryanDO Stunt Coordinator

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    The Shiva would have easily taken the power it was being given and then some. It was not getting the amplifiers maximum output.

    I've had the EQ playing along with the 400 watt plate amplifier and it sounded great, flat response and big spl. I was using the same EQ setting with the pro amp.

    All in all, I just think I need more line input voltage and all will be good.

    And I thought going from a plate amplifier to a pro amp would be easy, go figure!
     
  12. Leo Kerr

    Leo Kerr Screenwriter

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    Level mismatch can come around and bite you when you least expect it.

    I've worked with a number of different pro sound and video pieces, and even there, there are differences.

    Leo
     
  13. BryanDO

    BryanDO Stunt Coordinator

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    Finally got all the kinks out of the set up. The Behringer feedback destroyer has been re calibrated and now the subwoofer is rocking. I found the best way to do it was turn the amps gain to full and then control the output by the receiver sub out level. Everything sounds great.

    There is some ground loop noise though. Looking for a hum eliminator to get rid of that.

    Is it OK if I disconnect the fan in the amp? It's only on for an hour or 2 when watching a movie and that's at respectable volumes. If not, are there replacement fans that are quieter? The thing sounds like a harrier jet taking off.
     
  14. Wayne A. Pflughaupt

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    When you say “recalibrated the BFD” does that mean that you re-equalized so that its output level is not so low?

    Hopefully that will get it, but it may simply be noise from the electronics. Poor S/N is a common consequence of a low signal/high gain scenario like you’re dealing with.

    Regards,
    Wayne A. Pflughaupt
     
  15. BryanDO

    BryanDO Stunt Coordinator

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    re-calibrated = re-equalized​

    I actually just moved my sub around and I'm not using any equalization. Not the smoothest respose but not bad for it not being corrected any.

    [​IMG]
     
  16. Mattak

    Mattak Stunt Coordinator

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    Is the QSC using a 24v 80mm fan? If so, Digikey part number P9739-ND NMB/Panaflo 24v fan. This is pretty much the best fan you can find assuming it's 24v 80mm (there's a 12v version as well). I replaced the one in my Behringer EP2500 with this and have been using it for a couple weeks now. Before I couldn't stand using the amp for my theater application because of the fan noise. Now it's good.
     
  17. BryanDO

    BryanDO Stunt Coordinator

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    The QSC does use an 80mm 24v as well. After opening the amp up to clean it out of dust etc. I noticed the aluminum heat sink is fairly large. I decided to try the amp without a fan and put it through it's paces with a couple bass heavy films.The heat sink never reached above luke warm! For home use it seems perfectly fine to not use a fan at all. But I can't speak for everybody. Either way I'm not too worried, if for some reason the amp does get too hot, thermal protection will shut it down and at that point I'll look into the panaflo fans.
    Thanks Mattak.
     
  18. Mattak

    Mattak Stunt Coordinator

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    I agree, it probably doesn't need one for home use. I didn't stress mine without a fan, but did have it disconnected for a little while and it was barely warm. But, since my amps are right next to my computer/HTPC...which has fans and hard drives...and my projector has a fan...well, it didn't add any extra loudness to put the NMB in (22dba if I remember correctly).
     

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