Quote Originally Posted by ecat View Post
Why the howl ?

Basic theory of PWM control:
http://www.pcsilent.de/showInfoPage.do?name=funktionpwm

As you can see the voltage to the fan is either 0v or 12v, nothing in-between. Every time 12v is applied the fan makes a little kick which is audible to some degree dependent on the fan construction and how it is mounted. When lots of these voltage changes are applied in sequence the sound from the kicks merge into a tone with a frequency equal to the repetition rate of the voltage change aka pulse repetition frequency or prf. For fan control the prf is usually between 10Hz to 400Hz this tone is clearly audible as the howl. The easy fix for this would appear to be increasing the prf to a point beyond normal human hearing, somewhere above 20,000Hz, the problem here is such high frequencies cause heating in the fan drive electronics somewhat negating the efficiency gains pwm brings and probably shortening the useful life of the fan.
+1 for an explanation of the "PWM click!" This should be added to a PWM/fan/gen info sticky and/or put in a new thread itself.

Quote Originally Posted by dsumanik View Post
With rheobus controll you need to make sur eyour initial voltage is high enough to get the fan spinning, then you can lower your voltage a bit more and get smooth silence 8)
Yea, this initial full voltage is a great, and essential, feature. I sure wished they listed how they handle voltage on startup. Correct me if I'm wrong, but I believe the TMS-200 powers at full (pump channel too) and then goes to set voltage. What about the bigNG T-balancer and/or bigNG FanAmp?