# Amplifier Specs

Discussion in 'Electronic Design' started by Hoektoe, Aug 17, 2007.

1. ### HoektoeGuest

Can somebody please tell me how to calculate the power output of a
class AB power amp.
If the amp supply has a +-60V swing, do you use 60V or 120V in the
power calculations?

2. ### Phil AllisonGuest

"Hoektoe"

** Depends on the method of calculation of course.

A sine wave has a ratio of peak to rms of 0.7071

It also has a ratio of rms to peak to peak of 2.83

Power depend on the load resistance too.

Then maybe YOU are after peak power or music power or PMPO power or ****
knows what power.

Got the faintest clue what you are on about ??

........ Phil

3. ### Tam/WB2TTGuest

Either, but using 60V is more convenient. P=Vrms**2/R. So for +/- 60 V swing
Vrms =60/SQRT(2) = 42.4V.
Now, for an 8 Ohm load, power will be (42.4 **2) / 8 = 225W. You won't
actually get that much out because the voltage won't quite go to 60 V. 58
might be a reasonable guess.

Tam

4. ### Fred BloggsGuest

If the output swing is 60V above and below a DC of 0, please explain at
what point in time the load would ever see 120V?

5. ### D from BCGuest

Alternatively...
The max power handling of the amp is the heat dissipated by the
speakers when you almost hear distortion when playing your favorite
music.

(Assuming the speakers don't distort first.)

D from BC

6. ### Rich GriseGuest

Apparently, they haven't covered "peak-to-peak" in class yet. ;-)

Cheers!
Rich

7. ### EeyoreGuest

58V would be pretty good actually. Allowing a Vce of 2V would be reasonable but
there'll be an IR drop in any emitter resistor too. A typical value would be 0.3
ohms and with around 7A peak of load current, you'll lose another 2V there too.

And the really huge variable is power supply regulation. Audio power amps are
not normally operated from regulated supplies. Power supply 'sag' on load of 10V
would be quite normal..

All of the above would bring the full power down to ~ 150W.

Graham

8. ### CharlesGuest

Assuming a sine wave, Vp = Vp-p/2 and Vrms = Vp x 0.707