[AMPS] Interesting Characteristic of DC "Brushless Fans"

[Kim Elmore]

My AL-80A had a noisy AC fan.  The noise was mainly AC motor noise, rather 
than fan-blade or air noise  (hum/buzz/whine). The amplifier case made a 
very good sound board for the fan motor,  so, I decided to replace it with 
a DC "brushless" fan.  Perhaps it's common knowledge, but I learned that 
these gadgets don't possess a DC motor of any kind. Instead, they use an 
inverter to generate AC at somewhere around 15 kHz (so I'm told).  This is 
rather obvious when I look at my new fan, because a circuit board clearly 
makes up the back side of the motor.

After installing it, I found that, sure enough it's quiet.  On 20 and 10 m, 
it gets even quieter because RF gets into the inverter and causes the fan 
to nearly stop.  The AL-80A has a big hole in a divider between the RF deck 
and the PS side.  The fan moves air through this hole, conveniently located 
almost squarely on the tube, so the fan is exposed to lots of RF. I'll try 
putting some aluminum window screen over that hole to shield the fan, and 
see if that helps.  The problem only occurs when output is above 500 W or 
so, so I'm hoping that this will be enough to do the trick.

Kim Elmore, N5OP


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