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Power amp wattage

Discussion in 'Archived Threads 2001-2004' started by AlexKunec, Mar 31, 2002.

  1. AlexKunec

    AlexKunec Stunt Coordinator

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    wattage = voltage * current

    when a power amp delivers a higher power to the speaker does it increase its voltage or current or both?

    Is the current or voltage set at a constant rate while the other is varied?
     
  2. Peter Johnson

    Peter Johnson Stunt Coordinator

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    Both.

    An amp acts (or at least should do) like a voltage source. It provides a voltage over a load, and according to ohms law, a current is drawn.

    Example..the power amp is providing 9V RMS at say 1000Hz. Say at that frequency, the impedance of the speaker is 6ohms, then the current drawn is 1.5A

    P = V*I

    = 9 * 1.5

    = 13.5W
     
  3. Kerry Hackney

    Kerry Hackney Stunt Coordinator

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    This is a post from an earlier thread about current, impedence and amplifiers. The output voltage varies by demand.....

    watts - res - amps - volt

    500 - 8 - 7.91 - 63.25

    300 - 8 - 6.12 - 48.99

    150 - 8 - 4.33 - 34.64

    100 - 8 - 3.54 - 28.28

    50 - 8 - 2.50 - 20.00

    25 - 8 - 1.77 - 14.14

    12 - 8 - 1.22 - 9.80

    5 - 8 - 0.79 - 6.32

    4 - 8 - 0.71 - 5.66

    3 - 8 - 0.61 - 4.90

    2 - 8 - 0.50 - 4.00

    1 - 8 - 0.35 - 2.83

    0.5 - 8 - 0.25 - 2.00

    0.25 - 8 - 0.18 - 1.41

    500 - 4 - 11.18 - 44.72

    300 - 4 - 8.66 - 34.64

    150 - 4 - 6.12 - 24.49

    100 - 4 - 5.00 - 20.00

    50 - 4 - 3.54 - 14.14

    25 - 4 - 2.50 - 10.00

    12 - 4 - 1.73 - 6.93

    5 - 4 - 1.12 - 4.47

    4 - 4 - 1.00 - 4.00

    3 - 4 - 0.87 - 3.46

    2 - 4 - 0.71 - 2.83

    1 - 4 - 0.50 - 2.00

    0.5 - 4 - 0.35 - 1.41

    0.25 - 4 - 0.25 - 1.00

    500 - 2 - 15.81 - 31.62

    300 - 2 - 12.25 - 24.49

    150 - 2 - 8.66 - 17.32

    100 - 2 - 7.07 - 14.14

    50 - 2 - 5.00 - 10.00

    25 - 2 - 3.54 - 7.07

    12 - 2 - 2.45 - 4.90

    5 - 2 - 1.58 - 3.16

    4 - 2 - 1.41 - 2.83

    3 - 2 - 1.22 - 2.45

    2 - 2 - 1.00 - 2.00

    1 - 2 - 0.71 - 1.41

    0.5 - 2 - 0.50 - 1.00

    0.25 - 2 - 0.35 - 0.71

    As you can see, halving the impedence doesn't quite double the current demand because current (I) = sqrt of (Watt (W) / Res (R)) Ohms law is V = I * R... Wattage and current aren't exactly the same thing. When you reduce the resistance of a speaker you ask the amp to deliver more current or amperage.

    Let's see if I can make this more confusing... When you half the impedence, the amp will produce exactly half of the wattage at the same current. Or 50 watts at 8 ohms is 2.5 amps, 25 watts at 4 ohms is 2.5 amps. Now, an amp with a very robust voltage supply that can deliver 20 volts at both 8 ohms and 4 ohms can deliver twice the wattage at 4 ohms as at 8. Thats where you see that manufacturers rate their amps as more powerful at 6 ohms or at 4.

    Bottom line, if you want to drive a lower impedence, the amp has to be able to deliver more voltage/current and dissipate the associated heat without melting down.
     

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