Amiga 4000 desktop power supply unit (PSU) fan

In case anyone ever needs to know - and this is written a bit repetitively for searchability later, if someone finds it via web searches later you're welcome:

The stock Amiga A4000 desktop power supply (PSU) 12V (12 volt) fan runs at 3000 RPM ( 3000RPM ) by my measurement, moving 20-30 CFM (cubic feet per minute) of air according to what little data I can find online.

(Obviously, that's an outside-of-PSU measurement, as case affects airflow.)

It is a standard 80mm fan by measurement and can be replaced with a Noctua NF-A8 FLX 3-pin fan running on the low-noise but not lowest-noise cable, which gets you just under 25 CFM - at least in the current (2021) iteration of that fan's lineup.

This does not make the PSU run silent but does make it run a lot more quietly.

How to measure a fan's RPM without special equipment

While we're at it - see previous post - here's a decent way to measure a fan's speed without having access to specialised hardware.

1. Extract the fan and provision it for power, but do not start it yet.
   
2. Ready a sound recording device of any type, such as your phone.
   
3. Using stiff but not overly stiff tape - I used packing tape - construct an inverted T type tab and adhese the top of the T to one side of the centre portion of the rotating fan blade assembly so that the extended tab spans the radius on one side of the fan, and the tab portion sticks out, away from the fan. If any part of your assembly has any possibility of hitting any other part of the fan, you're putting it in the wrong place. The tab you've made should rotate with the fan blades, once per revolution of the blade assembly.
   
4. Using the same tape, construct a lightweight rod of, oh, 5cm length. Exact length doesn't matter.
   
5. Start the fan.
   
6. Position the rod so that it is continually struck, lightly, by the previously-made tab adhered to the fan. It should make a ticking sound, like a playing card in bicycle wheel spokes. Each tick sound will represent one (1) rotation of the fan.
   
7. Start the sound recorder, and record some number of seconds, making sure you get as light an impact on the tab you constructed as possible while making sure the tick sound remains clear. (This is so you slow the fan down as little as possible or not at all.)
   
8. Save the recording, then load it into any audio software of your preference that will let you display the waveform and zoom into a one second section thereof. Audacity is one such free software package, but there are many others.
   
9. Looking at the waveform, you will see a series of vertical lines, each one of which is a tick noise, which should be louder than background/air noise in the recording. If you do not see (or hear) this, you have a bad recording.
   
10. Count the number of these lines contained within exactly one second of time. That gives you revolutions in that second. If you are comfortable that this one second is representative, multiply by 60 for revolutions per minute (RPM). Alternatively, count across several seconds and do math accordingly. It's up to you.