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Question re: -20db light on Stewart amps

Discussion in 'Amps and Cabs [BG]' started by DLM, Mar 22, 2005.


  1. DLM

    DLM

    May 25, 2004
    California
    When playing at low volume, the light is not activated. When I increase the volume to about 1/3, the -20db light starts to light up.

    What is the purpose of this light? I can understand the red clip light which is alerting me to any danger but don't know what the -20db's purpose is.

    Please enlighten? Thanks!
     
  2. DLM

    DLM

    May 25, 2004
    California
    anyone? anyone?
     
  3. Passinwind

    Passinwind I Know Nothing Supporting Member

    That's called a signal present indicator on a lot of amps, and is particularly useful in PA applications when you are getting no sound, since you can narrow things down rapidly by the indicated absence or presence of signal at the power amp. It tells you a bit more than the on-off power indicator, basically. It can be useful for setting gain structure sometimes as well.

    I was going to tell you to download the manual on the website, but it wasn't very helpful in this case. :rolleyes:
     
  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
    My best guess is that it would indicate that the signal level has reached 20 dB below the clipping point.
     
  5. Passinwind

    Passinwind I Know Nothing Supporting Member

    You cheated and read the manual, didn't you? :eyebrow:
     
  6. DLM

    DLM

    May 25, 2004
    California
    Passinwind, I don't think it's merely a signal present indicator since at low volumes it doesn't light up; only at higher volumes.

    But assuming Bob is right, what does it tell me? I guess what I'm asking is why would I be concerned if it's -20db below clipping? I would definitely be concerned when it's, indeed, clipping.
     
  7. Passinwind

    Passinwind I Know Nothing Supporting Member

    The reason a signal present indicator usually works at that sort of threshold is that if it was much more sensitive, system noise alone would trigger it at least some of the time, is my guess. You also wouldn't have a groovy flashing LED to inpress the babes then, no joy in that. -20dB is 1/100th of power at clipping, so it shouldn't be very loud unless you have really sensitive speakers.

    Bob is right, of course; but again, in my experience, it's potentially useful in troubleshooting gain staging or problem conditions in your gear. You might never see the clipping light come on in normal operation, but if the -20 light on your board comes on at the same time as the -20 light on the amp, you can have at least some confidence that clipping should occur aproximately when you see 0dB on your board, assuming there's nothing patched in between that changes that relationship. This is pretty handy to know sometimes. :cool:

    For me, in a bass rig, those lights tell me that both channels of my stereo rig are firing up. That's it, really.
     
  8. DLM

    DLM

    May 25, 2004
    California
    That makes sense: having a higher threshold so as not to indicate a signal when it's only "system noise." So, in the end, maybe those lights do function in the same way as those on, say, a QSC amp.
     
  9. Passinwind

    Passinwind I Know Nothing Supporting Member

    I can't remember what the threshold is on my QSC, but the signal light hardly ever goes off when I'm playing bass with a compressor at standard settings. In some venues, a few unmuted mikes on stage in a noisy club will keep it lit, IIRC. The QSC lights that go signal-->-20-->-10-->clip are more useful in any case, I'd say.