Why was my power amp freaking out?

Discussion in 'Amps and Cabs [BG]' started by fttfbass, Dec 10, 2005.

  1. First off here is my rig:
    Warwick Corvette Standard
    Tech 21 Sansamp RBI
    QSC PLX1602
    Tech 21 B410 & B115 cabinets
    Furman power strip

    I've never had a problem with this rig before either at home or in the practice room. Tonight was our first show since reforming our band back together. I had everything setup and turned everything on. I turned everything to a low volume just to make sure that I had sound. At the low volume that I had it at my LED light on the poweramp would be real low, and then all of a sudden shoot up to the red clip light. This would make my speakers pop like crazy. I ended up borrowing someone elses amp to finish the show. The amp was a real old Peavey of some sort, which sounded like crap and hardly had any volume to it at all.

    When I got home to figure out the problem.......there was no more problem. Everything works fine. Any suggestions what the problem was? Could it have been a problem from the outlet that I plugged into? Should I invest in a power conditioner instead of my power strip?

    Any help from all you experienced TBers would be great. Thanks!!!
  2. Id like to hear the answer to this. That is the worst part of the show, the initial plug in and power up. There are so many other things running on these electric circuits in these bars, including the big one....lights. One night, I had helicopter noises coming from the cabs, made a great intro. I feel for ya bro.
  3. fdeck

    fdeck Supporting Member Commercial User

    Mar 20, 2004
    Madison WI
    HPF Technology LLC
    Possible short circuit in your speaker wiring, which somehow corrected itself when you plugged into the spare amp. Take a long hard look at your speaker cables.

    Or, possibly low power line voltage. If you don't get a power conditioner, it's at least worth getting a voltmeter. Of course a power bar with voltmeter built-in gives you the best of all worlds and avoids having to dangerously stick voltmeter probes into a power socket.
  4. C-5KO


    Mar 9, 2005
    Toronto, Canada
    Check inside your RBI.

    There's a cable that runs from the front Input jacks, and connects to the back Input. It's about 1/3 the width of a floppy drive cable. On my RBI, this was loose. It gave me an intermitting signal.
  5. Bump......anything else to look for?
  6. 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
    No, a power "conditioner" won't actually help anything.

    It's not clear from your description--did this happen on one amp channel or both? Did all of the signal level LEDs light up when their clip LEDs lit?
  7. Yes, it happened on both channels. And yes all of the signal level LEDs did light up when the clip LEDs lit.

    In the practice room my levels are usually fairly high, because it's hard to cut through in such a small room. My level on the preamp is about 3/4 the way up, and the power amp is usually around halfway (22-24dB). I've never had a problem at all with the setup like that ever.

    At the show when I turned everything on my preamp was set only at about 1/2 the way up, and the power amp was just turned loud enough to get sound (about 8-10dB). At these levels it was just enough volume so I could make sure everything was working. I played a few notes and all of a sudden the LEDs would randomly shoot up to clipping. When the amp would clip it would make a crazy popping sound through my speaker cabinets.

    In reguards to a power conditioner. I know that they are basically just glorified power strips. But I thought maybe it would be a good idea to get one with the meters on them, so I know if I need to find a different outlet to plug into. And it would also be a little more convenient with my rack. Right now I have to mount my power strip in the back of the rack case, so it's a wire mess in the back of the case. If I got a rackmounted power conditioner all of my cables would be further in the rack case and out of the way.
  8. I remember playing in this place before. I had an Ampeg SVT-CL played through an Ampeg 810e cabinet. My settings on the CL were master turned all the way up, and I used my gain as my volume. I remember that I had to turn my amp up quite a bit louder than usual to be heard. This place is quite small, too. I found it weird that I had to turn my amp so loud, to get the volume out of it that I did. Could I have been experiencing a similar problem before, but didn't notice it since my amp was all tube?
  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
    Okay, that's good to know. Was the amp in bridged mono? Were the inputs in parallel (switches 4–6)?
  10. Any chance of a dodgy solder joint on your cables?

    That's what it sounds like to me... Hapens to me all the time when a joint breaks/jack needs changing; clip lights come on and feedback-like noises come.
  11. Yeah, the amp was in bridged mono with switches 4-6 set in parallel mode.
  12. I don't think it's a cable issue, but I will check them to make sure. I have another 4 weeks until our next show to solve this problem. I'll probably have 4-6 practices in the meantime for anything to go wrong.

    And yes my speakon cable is wired correctly for bridged mono mode. I figured I'd answer that question before it pops up.
  13. irjason

    irjason Supporting Member

    Nov 17, 2001
    Louisville, KY
    My guess would be bad power at the club since you say it was fine when you got home. Low voltage can make electronics do all sorts of crazy stuff. I've had to replace all the outlets in my basement because of the poor job the builders did wiring the outlets. one was only putting out around 80 volts. Anything I plugged in there didn't work right.
  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
    I doubt that it was bad power, unless something other than the power amp was being affected. Really low AC voltage (<90 volts) would make the amp shut off, not go crazy. It's not out of the question, though; I played an outdoor gig a couple years ago with a generator providing juice for the PA, backline, lights, and some fair booths, etc. The AC dipped periodically as we got rocking, and my preamp twice went into a weird unstable mode; I had to shut it off and back on again to clear it. Of course, my bass rig was more powerful than all the backline put together and possibly more powerful than the PA, too, so I was probably responsible for most of the current draw. ;)

    A power conditioner will not cure low voltage, but an AC voltage regulator could, if it's designed for the high current of a power amp.

    First, I think we can rule out a problem downstream from the amp outputs, like in the speakers or speaker cabling. A short anywhere in that section would cause the clip LEDs to light before all the signal level LEDs did. I think it would have to be something either upstream from the amp or in channel 1 of the amp. IIRC, the SansAmp RBI is powered by a singled-ended DC supply; intermittents in audio circuitry with single-ended supplies often don't interrupt only the signal, but also cause large pops because of sudden DC shifts.

    If you can try to duplicate the problem with the amp's inputs in parallel (BTW, you don't need to parallel the inputs in bridged mono, and it's better if you don't) but the amp not in bridged mono, then it should be easy to tell where it's coming from. That's because in straight parallel-input mode, the only thing common between the two channels is from the input connectors to the parallel switches; after that, they're on separate paths. Then if the pop shows up on both channels, it's most likely coming in from upstream. If it shows only on channel 1, then the problem would probably be in channel 1 of the amp.
  15. I had an issue once where some condensation shorted the input pad switch and/or the gain knob on my pre effectively turning my whole rig to 11, clipping big time. After it dried out some everything went back to normal. I don't know if you encountered any drastic temp/humidity shifts, rain or run-off from a beverage but it may explain it fixing itself.
  16. I have a feeling that I won't be able to duplicate this problem until an important show comes up. That's usually the way it goes.
  17. Well it must have been bad power at the place we were playing at. I haven't had a problem with my amp since. Last night I was talking to the one band that we played with that same night. They said that all of there amp's were acting funny, too. So I believe the situation has been figured out. Thanks!