Electronic – How to choose the power rating of resistors in audio power amplifier

audiopower amplifiertransistors

I read a bit about audio power amplifiers and struggle to understand the power rating of the emitter (i.e. current sharing) resistors in power amplifiers.

I refer to this project.

[EDIT: schematics and BOM below: all credits and copyright go to buildaudioamps. com)]

According to the BOM, the resistors R31-R34 are chosen to have a 5W rating (wirewound type).
I have done some simple Spice simulation and I see that at the maximum (reasonably usable) input voltage, the power dissipation across a single emitter resistor is only about 500mW, which is to be expected because of their low value.

Somehow I have the feeling, that the guy has an idea of what he is doing.

Can anybody tell me a reason for the chosen power rating?

Schematic:

enter image description here

BOM:
enter image description here

Best Answer

110 watts into an 8 ohm load implies an RMS current of 3.71 amps. That current, through a single 0.22 ohm resistor implies a power of 3.025 watts. Considering that this power is shared by 4 resistors, the average power per resistor is 0.76 watts.

But then one has to also consider the biasing of the output stage and this will cause a certain amount of DC current to pass though the resistors so maybe 1 watt per resistor might be dissipated if it's a class AB stage and possibly a couple of watts if it isn't properly biased i.e. it is closer to a class A stage.

The zobel network (C16 and R33) on the output might also add a few tens of milli watts to each resistor at lowish frequencies and a few hundred milli watts at high audio frequencies.

So you could be looking at maybe 2 watts to 2.5 watts for each resistor and therefore choosing a 5 watt resistor seems like a little bit of overkill considering that I would opt for more like a 3 watt resistor.