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output setting on power amp

Discussion in 'Live Sound [BG]' started by TheVoiceless, Sep 29, 2009.

  1. TheVoiceless


    Jun 11, 2008
    New Jersey
    Ok so I was taught to run the power amp to FOH at 75% or 3/4 the way up. But my singer is saying that he was taught to run the amp at 100% all the way up.

    I am wondering how the TB'ers run your power amps. And if there is any benefit/negative effect by running the amp at either of these settings.

    Thanks, TheVoiceless
  2. My stuff is always wide open. Most people I know do it this way.
  3. jaywa


    May 5, 2008
    Iowa City, IA
    Power amps go wide-open. That's how I roll, anyway.

    In fact I've always wondered why they even bother to put knobs on power amps at all.
  4. TheVoiceless


    Jun 11, 2008
    New Jersey
    Yeah I wondered why would you run something at 100% and have knobs for adjustment? Also thinking back I was told about the 75% thing from a studio engineer. So maybe that does not apply to live sound. Thanks for the feedback!!
  5. Hi.

    There's no volume control on any pro- or semi-pro PA amps that I know of.

    The knob is an input attenuator.

    The use of such a device is necessary as the input sensitivities of the amps vary quite a bit, as do the output capabilities of the devices that feed the amp inputs.

    Ideally You want to match the signal chain for the best S/N ratio, i.e., if the input sensitivity of the amp is for example 2 V RMS, You run the preceeding stages at 0-level and adjust the amp attenuators to the onset of clipping (0 as well). Assuming of course that the preceeding stages are capable of the 2 V RMS ;).

    Unless You know what the signal chain puts out, the position of the knobs don't mean a thing.

  6. TheVoiceless


    Jun 11, 2008
    New Jersey

    OK so it seems that if I want to run at the proper settings I should set my master faders at 0 then adjust the amplifier to a level that gives me a 0 gain reading.

    Other wise I should just run the amp wide open.

    Thanks Sam!!!:D
  7. Revvv


    Oct 31, 2007
    I leave a standard PA power amp in the 100% mode. I use the sliders and faders on the board to give me the volume adjustments I need.
  8. If you run your amp WFO, you'll have to lower the level, of preceding gear, which will adversely affect, the signal to noise ratio. For best S/N you want the hottest signal possible, while staying safely away from clipping. That level should be set at the beginning, and remain the same "unity gain," through the rest of the signal chain.

    Long version:
  9. TheVoiceless


    Jun 11, 2008
    New Jersey
    Thanks Ulrich, So from what you say and what I read, that the input sensitivity needs to be set for the output of the board. So for a quick fix what I can do is run the mixer at unity, then adjust the input on the amp so that its set right before clipping.

    Thanks for the link. Eventually we will use the pink noise test.
  10. The mixer, compressor, EQ, crossover, get set to about the safest level before clipping. Use the amp input attenuators, to set volume level.

    Pink noise generator, oscilloscope. LOL Well it's nice to know the right way to do things. I end up punching the bypass switch (where available) in, and out while adjusting level. When the level sounds the same with gear bypassed, or not, it's good enough for rock music. Sometimes you just have to guess.

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