Help with fan control

Thanks for the circuit idea Hero999. I would like to avoid direct PWM because it can create annoying acoustic noise. Your post started me thinking about parallel drive for the MOSFET. This configuration gives pure linear drive to the MOSFET.

You won't get any annoying acoustic noise from 25kHz because it's well above the human hearing range.

I do not know what causes the phenomenon, it could be distortion or switching noise or something intrinsic to a brushless fan, but first hand experience has shown that at some mark-space ratios 25kHz direct PWM will make a brushless fan produce what I can best describe as a growl or buzz. I have never encountered the growl with any of the linear fan controls I've built. This is why I want to integrate the PWM into a linear signal and control the MOSFET with linear drive.

It could be producing subharmonics but they're quite rare, normally only lower frequencies are produced, but lower frequencies are possible.

I jumped into the deep end and downloaded LTSpice to test the parallel drive circuit and was able to get a simulation running with the included models. Now I'm trying to import the Fairchild model for the FQP27P06, but the simulation won,t run due to errors. Would appreciate any help you could offer.

To be honest, I've not idea what's causing the problem, it could be the model?

I've tried simulating it but I don't have that model in my library.