Dismiss Notice

Psst... Ready to join TalkBass and start posting, make new friends, sell your gear, and more?  Register your free account in 30 seconds.

QSC USA 850 bridging at 4Ohms?

Discussion in 'Amps and Cabs [BG]' started by Pre EB, Sep 10, 2002.


  1. Pre EB

    Pre EB

    Mar 15, 2002
    Denton, Tx
    Does anyone know if you can run this amp bridged with a load under 8ohms? The manual gives kind of conflicting information.
     
  2. 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
    You can, but don't push it so hard that the amp clips, or else you might make the thermal protection circuitry shut it down. Make sure you have a free flow of air available into the back and out the front.
     
  3. Pre EB

    Pre EB

    Mar 15, 2002
    Denton, Tx
    Thanks, Bob. I'll try it and see what happens. BTW, it seems to clip fairly easily when I run bridged into my 8ohm Acme. Any tips?
     
  4. 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
    Do the clip LEDs light up when that happens? If so, there are three things that can make the amp clip prematurely:

    1. Low AC voltage.

    2. Shorted or abnormally low load impedance. (Speaker impedances such as 8 or 4 ohms are just nominal values; they actually vary widely with frequency, and might in some cases dip well below the stated impedance.)

    3. A defect within the amp. (Other symptoms will often show up if this is the case.)

    Or the amp might also be putting out its full power when it clips.
     
  5. Pre EB

    Pre EB

    Mar 15, 2002
    Denton, Tx
    Yeah, the lights go on. It sounds pretty bad when it clips and I really don't want to damage my speakers.
    Also, I have asked this question before and never really gotten an answer; I have compared my rack set-up (425w @ 4ohms) with my Hartke 2000 (200w @ 4ohms) and it seems that there is a large difference in volume. The Hartke seems louder- it doesn't sound nearly as good, but definately louder. This comparison was done with my Mesa cabs (115+210), not the Acme. Is this due to some frequencies being more apparent or what? This is with both rigs set flat.
    Thanks for any help you can give. BTW, I drive the QSC with an Alembic F-1X.
     
  6. submelodic

    submelodic

    Feb 7, 2002
    Seattle, WA
    Are you using a Speakon to 1/4 inch adapter? The Neutrik Speakon NA4LJ adapter is wired +1 / -1, which is inappropriate for the bridged output of the amp.
     
  7. Pre EB

    Pre EB

    Mar 15, 2002
    Denton, Tx
    No, it only takes banana plugs. When I run the Mesas, both 8ohms, I piggy-back the cables into channel one. That DOES make it 4ohms, right???:confused:
    I haven't yet tried to bridge it at 4ohms. I really wanted to try adding my 115 cab to the Acme and see if I got more volume. Not sure if that would leave enough juice for the 410, though.
    Still, it seems weird that my little 200w Hartke would have more voume than my rack set-up.
    Thanks,
    Joe
     
  8. 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
    So the Hartke is louder at the point where it clips? If so, that would indicate that something's wrong with the amp.
     
  9. 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
    Yes, that makes it 4 ohms. What do you connect to Channel 2?
     
  10. Pre EB

    Pre EB

    Mar 15, 2002
    Denton, Tx
    Bob,
    My Hartke doesn't have a clip indicator light, so I'm not sure when it clips. Maybe when it starts to sound distorted?
    On the QSC, the only time I use channel 2 is when I run the 115 out of the low pass of my pre and my 210 full range.
    I do have to say that I get more and better lows when I use the QSC. Perhaps they're eating up all the juice? Would a Hartke be voiced differently, and therefore push more mids, just giving the impression of greater volume?
    I've set the EQ on my Alembic preamp as flat as I can; 2,10,2 and I don't use the deep or bright switches. It sounds great but I'm not getting the bone-crushing volume I love so much.:(
    I also run my Stingray (2 band EQ) with the bass wide open and the treble backed off about a third. I love a huge, deep tone- almost a Dub kind of thing.
    Thanks for your time.
    joe
     
  11. 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
    Why don't you put a speaker cab on each channel instead of two on one? That'll give you some more headroom.

    Yes, different EQs and voicings can give different perceived volumes. Our ears are most sensitive to midrange frequencies, so of two signals of the same SPL, one with midrange boosted and the other with it attenuated, the one with the midrange boost will tend to sound louder. To compare them accurately, you'd really have to get them voiced the same, if that's possible.
     
  12. Pre EB

    Pre EB

    Mar 15, 2002
    Denton, Tx
    I don't have a "Y" cable to run full range off both channels:( :D
    seems to me, though, flat means flat- right?
    I just wish that this was my life's biggest worry right now! Still a mystery, though.
     
  13. First of all: Sorry for my poor english, I'm from Germany.

    I've bought a used QSC USA 850 last week (at a store) and today was my first chance to test it with the band. I'm using a Sansamp RBI as preamp and a cabinet equipped with two EVM 15L im parallel mode, so the cabinet has 4 Ohm. I use the QSC in bridge-mode (putting only an unbalanced 1/4 cable from Preamp-Out into Channel 1 and using both red loudspeaker-terminals for connection to the cabinet)

    We're playing metal, the guitarist uses two 100 W tubeamps with a 4x12" each, but with my very efficient Electro Voice speakers I had never problems to get heard.

    After very nice playing for like 15 minutes the poweramp stopped working and muted. 30 seconds later everything was fine again. Another 5 minutes later, the same happened.

    I know that there's a thermal protection which mutes the power amp before overheating. But there were no signs of clipping (the leds didn't flash and the sound was absolutely fine), the fan works and there were no barriers for the air to flow. The housing of the QSC got warm, but I know higher temperature from other amps.

    My question: Is it possible that the thermal protection is switching too fast? Maybe because it's an older unit?

    Thanks a lot.
     
  14. 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
    From your description, it appears that the amp is overheating. Are the heatsink fingers inside the amp clean and not covered in dust?

    The USA850's fan had two speeds: slow for cool, and fast when it heated up. Does the fan switch to high speed? If not, then the thermal switch may need replacement.

    Actually, your English is sehr gut! ;)