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weird poweramp issues

Discussion in 'Amps and Cabs [BG]' started by Davidoc, May 12, 2002.

  1. I just recieved my QSC USA 850 from QSC. I hooked my Carvin PB-200 into it from the effects loop through channel 2 of the QSC. The sound no louder than with the Carvin! I maxed it out, and still no louder. (maybe preamp gain, because it wasn't clipping)

    Anyways, I bridged it, and it was distorted and teh clip light went on, and it was real quiet... At that point I realized I didn't put on the bridge switch! I immediatly turned off the amp.

    When I plugged it back into channel 2, it didn't work at all. The same thing with channel 1. Won't work bridged anymore either

    Could I have fried the amp by not putting the bridge switch on in bridge mode for 10 seconds?

    There are two weird things.
    When I turn it on, after a few seconds, there it buzzes for about half a second, then stops.
    Also, after I turn the amp off, if i take out and put back the bannana plugs, the speaker makes clicking (connecting) sounds. After about half a minute, it won't do that.

    Also, can anyone tell me why the binding posts screw in and out?

    AMJBASS Supporting Member

    Jan 8, 2002
    Ontario, Canada
    I doubt that the FX loop has enough gain to sufficiently drive the power amp. You would be far better off using the pre-amp out. Bridged mono mode means you need to have the amp in Parallel mode(both switches to parallel), and have both switches to bridge mode on. You can only operate the amp through channel 1.

    If you take out the banana plugs and put them back in right after you turn off the unit, it will make a popping sounding because the voltage hasn't been fully drained.
  3. thanks for the information.

    My amp doesn't have an out labeled preamp out; i was under the impression that's what the effects loop was.

    About the bridging switches, there's a single switch that selects bridge or stereo. The problem started after I played with the speaker plugged into both hot jacks with the switch in stereo mode. Do you think that could have messed it up?
  4. Pre EB

    Pre EB

    Mar 15, 2002
    Denton, Tx
    I have the same amp and although I have never had a problem with mine, I always have it bridged. The manual says to never run a signal into ch.2 when in bridge mode and to keep ch.2's gain all the way down for safety. For a "weak and distorted sound", it says this: "The amplifier clip light comes on during the distortion: There is a shorted speaker cable, the speaker is blown out, or the amplifier channel is defective". I hope this helps you:confused:
  5. Bob Lee (QSC)

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

    Jul 3, 2001
    Santa Ana, Calif.
    Former Technical Communications Developer, QSC Audio
    Davy, you might've blown something in the amp. I'm not clear on what you were doing when you thought the amp was "bridged," but running into Channel 2 and not setting the bridge switch are glaring indicators that you did some things very incorrectly.

    If you don't have a manual let me know (bob_lee@qscaudio.com) and I'll e-mail you one. If you use things the right way, you're less likely to have problems. Anyway, you'll probably need to have the amp serviced.

    The barrel of a binding post screws in and out in order to allow you to insert a wire, probe, banana plug, etc., into the side hole and then clamp down on it to insure a tight and secure electrical connection.
  6. Thanks for the information Bob Lee.

    I think I'll have to get it serviced, just wondering if there was something obvious.

    I did have the signal in channel 1 when I bridged it, sorry if i didn't make that clear. It was the switch that messed it up.
  7. Oh yah, does anyone have any clue what it would cost to have it serviced? I'm low on cash right now. (buying the amp cleaned me out). I didn't think 10 seconds of misuse could have caused that much damage. oh well.:(
  8. Bob Lee (QSC)

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

    Jul 3, 2001
    Santa Ana, Calif.
    Former Technical Communications Developer, QSC Audio
    Davy, there's no way of knowing what's wrong with the amp without having a look at it first.

  9. You had both channels running into ONE cab in stereo mode? That seems not right to me .... The point of stereo or bi-amping for that matter, is to drive two different cabs with two different amps (or channels). So like, I dont' know what cab you have, but it does seem like this could have caused some problem, especially if this is when you experienced the extreme clipping.

  10. I guess that's what did it DiN. Thanks.

    Thanks Bob for your help. Btw, thanks for sending me the manual! (a few weeks ago)

    I just realized how retorical the price question came across as.:cool:. Does anyone know how much servicing would cost? Thanks!
  11. Don't know anything about your particular amp but would like to offer a couple of points regarding bridging in general.

    For a single chan (mono) amp
    With DC coupled amps it's vital that the centre rail - that's the bit that drives the speaker, with the speaker return going to the powers supply 0Volts rail - has also to sit at 0Volts. If it doesn't the amp's not working properly / at all because it's faulty in some way.

    When you drive the speaker the centre rail goes +ve and -ve wrt 0Volts to drive the speaker. It will return to it's centre position when you stop playing.

    For a bridge amp
    The speaker sits between the centre rail of two amplifiers: there is no 0Volts return for the speaker in this mode. The two amplifiers are driven anti-phase; that is, when one amp is pulling the speaker in a +ve direction, the other amp pulls it -ve.

    If one or both of the amp's centre rails is not @ centre rail because of a fault, the system usually shuts down. This is due to sensing circuitry built in to the system to prevent it happening.

    My guess, therefore, is that there's a serious fault with one or both power amps. Serious but not terminal, I suspect:D

    You've doubtless fried one / some output devices in either one or both amps.

    It's definitely a job for the amp tech.

    Best of luck with it.


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