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Power amplifier level controls: Do they really mean anything?

Discussion in 'Amps and Cabs [BG]' started by Ostinato, May 21, 2005.


  1. Ostinato

    Ostinato Guest

    Feb 7, 2005
    Toronto ON
    I've heard different opinions on this website about the db indicators on the level controls (ie. -1db, -3db, -6db, -10db) and what they mean. I've always assumed that they mean what they say.
    That is to say, if your amp is 400 watts per side, and you crank the attenuator, your signal lights the indicator up to 0db, and you get 400 watts. If you turn the attenuator to -3db, and your signal lights the aforementioned indicators to 0db, you get 200 watts. Am I right or am I missing something? :help:

    Please feel free to chime to in and verify or destroy my reasoning herein.
     
  2. alexclaber

    alexclaber Commercial User

    Jun 19, 2001
    Brighton, UK
    Director - Barefaced Ltd
    The dB indicator lights give the relative output power (voltage technically) in dB, where 0 is max power, -3dB is half power, -10db is 10% power etc. The level knobs reduce the voltage gain the amp provides, from maximum gain down to negative gain (i.e. minus infinity) as you turn the counter-clockwise.

    If an amp provides 20dB gain, and the maximum output voltage before clipping is 100V (driving a given impedance load), then with the level knobs all the way up it will take a preamp providing 1V input to the amp to get full power out of the amp. If you turn the level knobs down to -3dB, that same 1V input will only provide 50V output. However, if your preamp is able to put out a signal of 2V the amp will increase that signal by ~17dB and produce an output of 100V. If you turn the level knobs down to -10dB the preamp will have to put out 10V to drive the power amp to full power.

    The output LEDs tell you how many volts the amp is putting out, regardless of knob position. The level knobs help you match your preamp's output levels to the power amp's sensitivity (i.e. how much gain it has).

    Alex
     
  3. 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
    Ostinato, they don't actually have anything to do with power. They just control how much the input signal voltage gets boosted. For example, in your scenario, Ostinato, if you have a 400-watt amp turned up to full gain, is it putting out 400 watts? Only if you put in enough signal to make it do so. If you put in no signal, you'll get no power--zero watts--out of the amp, except a little bit due to hiss and hum in the circuitry. If you turn the amp down a bit, the amp can still hit full power, but it'll take a comparable bit more input signal to make that happen.

    Alexclaber has a good description!
     
  4. alexclaber

    alexclaber Commercial User

    Jun 19, 2001
    Brighton, UK
    Director - Barefaced Ltd
    Most of it's culled from your various posts and/or replies to questions about my PLX. What goes around comes around!

    Just to doublecheck something: The level meters are showing the output voltage (because they move regardless of whether a load is conncted), but my understanding of output power is that as the impedance drops the maximum voltage before clipping also drops (though by a smaller amount). How do the meters know where the clipping point is and therefore recalibrate the LEDs?

    Alex
     
  5. 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
    The clip LEDs only react to actual anomalies in the output voltage (most often caused by clipping), so they don't react any differently with different loads.

    The -10, -20, and signal LEDs light at the same out voltage thresholds regardless of load; they're not meant to be precise level indicators but are to let you know approximately how much headroom you have left. So if they actually come on at, say, 21 and 11 dB below clipping when you're driving an 8-ohm load, for example, it's not any big deal.

    If the clip LED lights before the -10 dB LED does, that indicates current limiting due to a possible short circuit or excessively low load impedance.
     
  6. LiquidMidnight

    LiquidMidnight

    Dec 25, 2000
    Cool, I always wondered if the gain knobs controlled the input or the output level.
     
  7. 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
    They're in between the input section and the output section. Those two sections both have fixed gain, and the gain control is an attenuator, so it sets the overall input-to-output gain for the channel.
     
  8. Ostinato

    Ostinato Guest

    Feb 7, 2005
    Toronto ON
    Thanks for the information guys! But I'm still a bit confused, what you're saying is maximum wattage per channel can be acheived without maximum gain?

    Again, thanks
     
  9. 4Mal

    4Mal Supporting Member

    Jun 2, 2002
    Columbia River Gorge
    Maximum power is achieved at the appropriate level of gain. Preamps, cross-overs and other devices that might appear in the signal chain before the power amp have varying levels of output. The attenuator on the amp helps you set the appropriate level at the amp so that you don't over drive the amp itself. Solid state amp distortion isn't usually real pleasing and the attenuator helps you minimize it.

    In my rig, my Demeter pre sounds better with the volume set at about the mid-point of the dial than it does at 1/4. A little warmer and maybe rounder. If I set the power amp attenuator all the way up, I'm generally way too loud, so I use the attenuator to back my over all volume down to a managable level.

    If your amp wants to see 1 volt input at the power section to get to it's max rated capacity, the attenuator lets you back off a hot signal to get back down to that 1 volt. If you dump more than that 1 volt in, then you drive the amp beyond it's max rated capacity and you begin running the risk of thermal shutdown or potentially smoking something in the amp ... make sense ?
     
  10. Ostinato

    Ostinato Guest

    Feb 7, 2005
    Toronto ON
    Yes I think you've helped my brain tremendously. :) I do the same with my Alembic pre. It's usually set at 11:00 and then the amp levels are turned up to taste, depending on the room size.