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maximum watts from 120 volts

Discussion in 'Amps and Cabs [BG]' started by bizzaro, Nov 24, 2003.


  1. bizzaro

    bizzaro

    Aug 21, 2000
    Vermont
    So I read at TB somewhere that the most watts you can get from 120 volts is 1700 watts. Is this correct and if so, what is the point in getting say a QSC PLX 3402??
     
  2. For a single phase AC circuit, P=VI. Power equals volts times amps. Let's say the wall receptacle is rated 15 amps, then 15 times 120 equals 1800 watts. An amp is not 100 percent efficient, the class G power amps are slightly above 90 percent efficient if I recall correctly. 1800 times .9 equals 1600 watts.

    Higher wattage amps won't be plugged into your regular run of the mill house receptacle, they'll be plugged into receptacles from a power distribution box. These receptacles are on circuits rated for much higher current.
     
  3. bizzaro

    bizzaro

    Aug 21, 2000
    Vermont
    So if you play the Bar Circut, with 15 amps from a 120 volt line, the maximum watts available is 1800 or less? So in this application a 2000 watt amp is more than sufficient to handle all the power available?:confused: And enough head room?
     
  4. No, a 2000 watt amp won't be good. This is not comparable to an amp/speaker situation (where many people consider it acceptable for the amp to have a little more power than the speaker is rated for).

    Physics tells us that the amp cannot produce more power than the wall outlet is capable of providing. If the amp requires 20 amps to drive its rated power of 2000 watts, for example, and the wall outlet is only capable of delivering 15 amps, then the receptacle's circuit breaker will trip (hopefully...) when the amp's current draw exceeds 15 amps. If the circuit breaker doesn't trip and the current consumption keeps climbing, bad things happen, like overheated wiring in the walls.

    The devices on an electrical circuit should not exceed the current that the circuit is rated for. Let's say a hair dryer is rated 1800 watts, it pulls 15 amps at 120 volts. If you try to plug TWO of them into the same receptacle, the circuit breaker will trip (again, hopefully) because the two hair dryers are asking the wall circuit to deliver 30 amps, which is more than the circuit can deliver.

    Same way with a 2000 watt amp, it'll pull more than 15 amps: if the wall circuit is rated for 15 amps, the circuit breaker should trip..
     
  5. BillyB_from_LZ

    BillyB_from_LZ Supporting Member

    Sep 7, 2000
    Chicago
    Not to rain on the parade but molded case circuit breakers (like the 15 and 20 amp units that we're discussing) are really only intended to carry 80% of their marked rating...so the maximum continuous current that you can draw from a 15A or 20A, 120 Vac outlet is 12A and 16A respectively. Draw more on a continuous basis and you're going to be plagued with nuisance tripping...

    If you look at the charts that QSC publishes for the PLX amps, they detail the power requirements based on 1/8 maximum power (with a pink noise input and 4 ohm load) which I believe they state is a better overall approximation of the amp's power input needs when reproducing music.

    See it here....http://www.qscaudio.com/pdfs/plxspec.pdf

    The PLX3402's "power requirement" is 12A. It will draw a lot more current reproducing transients, but they're very short duration. I believe that the owner's manual has complete ratings.
     
  6. xyllion

    xyllion Commercial User

    Jan 14, 2003
    San Jose, CA, USA
    Owner, Looperlative Audio Products
    Those bigger amps were undoubtedly designed with larger venues in mind. There really is no reason to pump 2000W into your bass rig in a small bar. If you actually succeed in putting out that much power, you will either end up deaf or else your speakers are so horribly inefficient that you will be able to cook on the top of them.

    On the other hand, you may be actually using only a small fraction of the amps capacity most of the time, but having the larger amp ensures that the loud transients that will occur will be amplified cleanly.

    In general, an amp pushed to its limit all the time will wear out faster than one that is doing a fraction of the job it was designed to handle.
     
  7. BillyB_from_LZ

    BillyB_from_LZ Supporting Member

    Sep 7, 2000
    Chicago
    Come on Bob...you've never played in the small bars that I've played in.:D Or maybe you didn't play in a band with a Dictator-like 47 year old (surprisingly not deaf) guitarist who subscribed to the theory of "I want everthing louder than everything else".:rolleyes: Our home base was a crappy little 30 x 50 foot bar and most of the time I had one channel of my PLX2402 cranked into an Acme B2 and the other channel about half way up into a EV TL806 (EVM15B) cabinet. The Acme wasn't very happy and was replaced by a Goliath II.

    The scary thing was that I was the only one in the band that wore ear plugs...
     
  8. 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
    On our test benches here at QSC Technical Services we have 30- and 50-ampere AC, and some have more. That's because we do full-power tests with continuous sine waves.

    Most customers, though, use them for audio stuff, such as music, which varies in level and never stays at full power, but has short peaks that may hit full power. Most of the time, in fact, the amp is running at only a fraction of its full output power. Therefore, the amps will have much lower current demands when they are used for what they're intended.
     
  9. xyllion

    xyllion Commercial User

    Jan 14, 2003
    San Jose, CA, USA
    Owner, Looperlative Audio Products
    I dunno Billy, I think I've played with that guy, too. :D Whereabouts Chicago do you play? I grew up in Des Plaines, but now live in California.

    Bob Lee, I'm curious. Have you ever measured the power output that a typical bassist will actually use? We throw alot of numbers around on this board and I'm just wondering how much power is actually be used as opposed to just being there to handle transients. I suspect that people would be surprised at how little power they are actually using. It is also possible that I am totally wrong.
     
  10. 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
    During a typical club gig with out SR support, it's not uncommon for a bass amp to be putting out only maybe 50 to 100 watts on average, with peaks hitting 500 to 1000 watts, depending on what the amp is capable of.

    Of course, louder gigs or larger rooms require more power, while softer or smaller ones need less.

    And because loudness is a logarithmic thing, it takes a lot more power to sound just a little bit louder. And when it clips, an amp will put out more power than it is rated for. That's why when volume gets out of hand, it can easily lead to blown speakers.
     
  11. xyllion

    xyllion Commercial User

    Jan 14, 2003
    San Jose, CA, USA
    Owner, Looperlative Audio Products
    Thanks Bob, I appreciate the info.
     
  12. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    A bass player really working very hard can get up to about 1/3 power average for long enough to bother a breaker. We measured it.

    That is *very much* dependent on style, and isn't likely to be an issue for most players.

    The comment on breaker rating is true, 80% long term. But that is for lighting, motors, etc.

    Most safety tests are based on 1/8 power as a long term average in normal use. I suspect they haven't heard some of the bands I have....... but that is probably a good average.
     
  13. Eric Moesle

    Eric Moesle Supporting Member

    Sep 21, 2001
    Columbus OH
    Just an FYI:

    My rig goes through a Furman PM-8, which has an ampere meter on it, so I can see how much of a draw the rig is sucking from the wall.

    Using a QSC PLX3002, an Eden navigator preamp, and two 4x10's, pedalboard power and rack tuner, at high volume it ALL typically draws around 2 amps, and when I slap or hit peaks it sometimes blinks to 4 amps. We're talking pretty darned loud here with the 4x10's stressing their limits.

    I qualify this with a statement that I've never had the PM-8 calibrated, but I have no reason to believe that it could be THAT far off.
     
  14. Now you all know why some of us old farts say that a 100 Watt tube amp is loud enough for most gigs. :D

    Chris
     
  15. xyllion

    xyllion Commercial User

    Jan 14, 2003
    San Jose, CA, USA
    Owner, Looperlative Audio Products
    Turning 40 in April, I have been starting to develop the old fart intuition and that was my gut instinct.
     
  16. 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
    That seems about right. Is it a digital ammeter or analog?