You CAN directly vary the speed with Aardil's controller and just unplug the thermistor. It's certainly a decent controller, but as punisher and many others have mentioned, there are alternatives.
For a 26W fan at 12V, that's just a hair above 2A. That's not a huge amount of current for hobby kits to handle. At 60% efficiency, you'd be wasting somewhere around 10W in heat, so keep that in mind. You can get higher efficiency PWM solutions, but I'm guessing that most of them trade off efficiency for "cost effectiveness".
Oh, and I'll give a hearty second to the suggestion to require the ability to vary the frequency. A frequency that works fine on some fans won't work at ALL on other fans, or it might merely sound like a banshee on crank.
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