OK, I finished off my "other stuff" and had some electronic playtime.
Have a look at this schematic:
It uses a 555 timer to create a PWM signal at pin 3 of the 555.
That signal drives the base of the 2N3904, and out of the collector of the '3904 we have an "open collector PWM signal" which can connect to the brown wire of the fan and will adjust the speed of the fan.
Power for this circuit can come from the 24v PSU which drives the fan, but to reduce the 24v down to something which the 555 can tolerate, there is a 7812 voltage regulator.
To test this, I lashed up the 555 and associated timing components on a breadboard, and looking at the output of the 555 (pin 3) on the oscilloscope I saw this:
With the 10k pot at one end of its travel there was a pulse train with a 10% duty cycle.
Turning the 10k pot to nearly the other end of its travel:
An 86% duty cycle pulse train.
With the 10k pot at the end of its travel:
A 99% duty cycle, but the frequency has shifted from 25kHz to 18kHz, not really a problem, but that is simple 555 circuits for you.
So, as an electronics beginner, will this do what you want?
ps, I am no musician, practically tone deaf.
JimB