How do I choose the power rating of resistors in audio power amplifier?

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.

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.

What is the reason for the chosen power rating?

Schematic:

BOM:

• So now we need to know values and the maximum permitted power rail voltages. Mar 8, 2019 at 13:13
• I have also added the complete BOM as presented on the author's website. The author further notes in the project description: "The measured power output of the prototype is about 110 watts into an 8Ω load with ±50VDC power supply rails" Mar 8, 2019 at 13:25
• Those are NOT current sharing resistors (and are also clearly NOT emitter resistors!). As to the power rating, you need to look at fault currents, especially as that design is completely lacking in any sort of over current protection so far as I can see, and you will hit 5W in a 0.22R resistor at about 5A, so 10A total in one leg of the design. 10A into 4R would be 40V so is not unreasonable, and allowing for the 50% duty a 5W part seems sane here as it is mounted in an area of the circuit prone to run hot and so will need derating. Mar 8, 2019 at 15:26
• Thanks for pointing that out(that the transistors are on the collector side). Can you tell me what that output stage topology is called for further reading? Mar 8, 2019 at 17:23
• Of course the output transistors are current-boosted emitter followers. Lookup Sziklai Pair which is an improved darlington. The max average output voltage is 35.4V which is divided by the 8 ohm speaker down to 0.48V. Then each 0.22 ohms resistor gets (0.48V squared)/0.22 ohms= 1.05W A 5W resistor will survive 4W when somebody uses a 4 ohm speaker. Jul 8, 2022 at 22:22