# Power ratings

Discussion in 'General Electronics Chat' started by PRS, Apr 22, 2009.

1. ### PRS Thread Starter Well-Known Member

Aug 24, 2008
989
36
You go to the store and look on the back of an amplifier or stereo system amp and you see 100 watts. I used to think that was 100 watts to the load, but lately I've come to reconsider this assumption. An amplifier has an efficiency of between 25 and 75 percent. So, in the worst case an amp rated 100 watts actually supplies 25 watts to the load and in the best case supplies 75 watts to the load. My question is, how can the consumer know the actual power to the load?

2. ### David Bridgen Senior Member

Feb 10, 2005
278
0
The power quoted for an amplifier is the power developed in the stated load.
The efficiency depends on the class of operation of the ouptut stage.

An efficiency of 50% means that the amplifier can deliver to the load only 50% of the power consumed. e.g. a 100W amplifier with 50% efficiency takes 200W from the a.c. supply when developing 100W in the load.

3. ### PRS Thread Starter Well-Known Member

Aug 24, 2008
989
36
I used to think the same. But the law requires the supply power to be stamped on the back of an amp. So once again, you buy an amp stating 100 watts and you're really getting a fraction of that.

I made an amp having 4.5 watts to an 8 ohm load. It blared all through the house -- upstairs and downstairs. If I sustained that volume my neighbors in the adjacent apartments would have complained.

4. ### Audioguru Expert

Dec 20, 2007
10,606
1,182
Many amplifier and speaker manufacturers lie about the output power.
Haven't you seen a little computer speaker supplied with a 9V/500mA wall-wart (4.5W) but the speaker is labelled 1000 Whats!?

Many car amplifiers are rated at 200 Whats. But each of its 4 outputs produces only 14 Watts at clipping for a total output of only 56 Watts.

My home stereo produces 140 Watts at low distortion. My car radio produces 288 Watts at low distortion. I turned them up pretty loud a few times but probably no where near maximum power.

5. ### PRS Thread Starter Well-Known Member

Aug 24, 2008
989
36
I like your term, Whats. Seems to me the NEC or some other Fire Protection Code book for manufacturers requires an electrical device's total supply power be put on a label. I've seen peak, peak to peak and rms specified, but that's the supply.