# Power amp wattage

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

1. ### 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 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 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.