Re: Need a new quieter fan to HD IO..
Saud, pretty much everything you have said is spot-on. Nice work.
If memory serves, the fan voltage can be calculated as follows:
V(fan) = 1.25 + (25000 / Rt) [volts]
where Rt is the resistance of the thermistor R19. The nominal 25C thermistor resistance is 10k. There is also a clamp that will limit the fan voltage to a little less than 12V regardless of temperature.
You can calculate a new nominal value of the thermistor if you need a different fan voltage, but it will also change the slope of voltage vs. temperature curve. You also need to consider fan stall, and the fact that the stall voltage will generally increase as the fan ages. The start-up circuit will get the fan spinning after powerup, but there is no stall monitoring after that.
BTW, the reason a PWM fan controller wasn't used was because when we tried it, we found that PWM current artifacts would appear in the audio noise floor and in the clock jitter residue. The analog fan controller eliminated that, although the pulsating current due to the fan's internal commutation was still a cause of concern for noise. The large LC filter (the big inductor and two big caps) minimizes the conducted noise path, however there is also a magnetic coupling mechanism that needed to be considered. We positioned the fan to minimize the magnetic coupling. This is also why we split the power supply in two pieces.
As you point out correctly, it was not a simple puzzle to solve. But at least the HD I/O didn't have all that Meitner Class-A discrete stuff the original 192 had, so it was comparatively easier to cool.
Also, the fan cost was no object: we would have gladly paid whatever for a fan that was quieter while still providing sufficient cooling for the unit to meet spec at the maximum rated ambient temperature. We auditioned many, many fans and did lots of fan listening tests before picking the one we did. But fan technology has improved, and I am sure there are better candidates today. And of course fans do age, so replacement is a valid maintenance procedure in my opinion.
DC
|