Amp efficiency

[454Casull]

I need help understanding how amplifier efficiency affects power draw vs power output.

Let's say I require 40 Vp-p, sine wave, going into a 4 ohm resistive load; e.g. 7 amps. If the amplifier is 80% efficient, what is the required rail voltage and DC current draw?

[richie00boy]

As the amp is 80pc efficient its rail voltage must be (100/80) x (40/2) = 25V.
DC current draw will always be 7A as current cannot disappear or appear along a route with only series elements.

[454Casull]

Originally Posted by richie00boy

As the amp is 80pc efficient its rail voltage must be (100/80) x (40/2) = 25V.
DC current draw will always be 7A as current cannot disappear or appear along a route with only series elements.

Excellent, thanks.

[AndrewT]

7Apk into 4r0 requires 28Vpk, that is approximately 20Vac.
28Vpk is the same as 56Vpp not 40Vpp

[454Casull]

Originally Posted by AndrewT

7Apk into 4r0 requires 28Vpk, that is approximately 20Vac.
28Vpk is the same as 56Vpp not 40Vpp

Oops, you are correct. I accidentally doubled. I guess 40Vp-p is 3.54Arms through 4 ohms?

[woody]

I am afraid you are in error. Verry few amps will have a output swing rail to rail usualy
for a 40vpp output you will need about rail voltages of about 25 + and - . Well 20 v across 4 ohms will be 5A but when 5A is being drawn from the + supply on a voltage
peak no power is being drawn from the - supply rail. And you specified a sine wave so
your current through the output device will swing from 0A to 5A on the + part of thewave and 0 to 5A on the negative part of the cycle. But you asked for the DC curent draw well I don't think you will have any DC curent draw ( well maby 50ma or so bias ) My math isn't worth a hoot but you would probably be safe to figure a little
less than 2A average current per supply rail.

[djoffe]

This 3 page white paper may be useful...

http://updatemydynaco.com/documents/...lculations.pdf