Audio Power Amplifier question

Thread Starter


Joined May 18, 2018
Hello all, I am really confused about the question my lecturer gave us. my answer to a) is 20.0 V using the P = IV formula and b) 2.50A using the same formula and converting it back to RMS. However, the other two, c) and d) confuses me.

c) How do you express it to dBu?

d) Is it using the voltage that I found in a) and use the same formula P = IV, P = I^2R, P = V^2/R to find the power?

Thank you.

An audio power amplifier has a voltage gain of 26 dB, infinite input impedance, zero output impedance, is rated at a maximum output power of 100 W into an 8 Ω resistive load, and is to be used as a headphone amplifier in a recording studio.

  1. a) What rms voltage would be measured across the amplifier output if it were delivering its maximum output power to an 8 Ω load?

  2. b) What would the rms current be through the load?

  3. c) What input signal level, expressed in dBu, would result in this maximum power to be delivered to an 8 Ω load? (this figure is also known as the input sensitivity)

  4. d) Assuming that this amplifier behaves as an ideal voltage source and thus maintains the output voltage found in part a) regardless of the value of connected load, how much power would be dissipated in the following loads?

    i) 4 Ω ii) 16 Ω iii) 100 Ω


Joined Mar 31, 2012
You need to show YOUR best attempt to answer the questions (which you've done partly). Otherwise there's no way for us to see where you are going wrong and to help you back on track.

First off, do you know what dBu is? If not, that's a good place to start.

As an aside, I'm sure glad I'm not the one wearing those headphones!

Thread Starter


Joined May 18, 2018
on c) (I think this is really wrong) I have found out that dBu is a reference level where 0dBu = 0.775V. Which I then did something like this:

P = V^2/R
V^2 = 100 x 8
V = 20√2
20√2/0.775 = 36.5dBu.

and d)
I did

i) P = 20^2 / 4 = 100w
ii) P = 20^2 / 16 = 25w
iii) P = 20^2 / 100 = 4w

Am I doing it right? Or am I really wrong? :(