[Elecraft] KPA 500 Fan Noise and cooling performance

[Donehrlich at q.com]

  On 6/18/2012 4:42 AM, David Robertson wrote:  ( ... a long message I 
won't include here where he improved amplifier cooling by tightening 
heatsink/right panel screws and used heatsink compound.)


I operate my KPA500 with the fan speed set to #1 continuously which 
delays the increase in the increase in fan speed once the amplifier 
starts putting out power.  My amplifier did not exceed 58 deg C after 10 
minutes at 500 watts carrier output into a dummy load and it stabilized 
at about 60 degrees in a 25 degree C room.  My right panel to heatsink 
screws (the ones under the handle) were all tight and that explains why 
my experience was better than was Dave's originally before he tightened 
his heatsink screws and added heatsink compound.  I then added heatsink 
compound to the mating surface between my heatsink and the right side 
panel.  There was no difference in cooling performance at all.  I just 
wanted to know .. and now I do .. and so do you.

The KPA500 fan is as quiet as any muffin fan ought to be but it was 
still annoying to me.  That is not an amplifier fault .. it is just that 
I have good hearing and I prefer a *very quiet shack.  For those who may 
be like me in that regard here is how I reduced the normal fan noise of 
my amplifier.  In my amplifier much of the fan noise was actually coming 
from the sheet metal of the amplifier structure which was being excited 
by the vibration of the fan which is rigidly attached to the amplifier 
structure so that fan vibration was communicated to the structure which 
then resonated and amplified the fan noise.  When I removed the fan and 
held it loosely in my hand while it was running I could feel the light 
high frequency 'buzz' produced by the rotating magnetic field of the fan 
and also, crucially, the lower frequency throb caused by a slight weight 
imbalance in the rotor. I used a small piece of sticky pad (normally 
used to mount components to a chassis, etc) and placed this very small 
weight at various points on the rotor blades until, by trial and error, 
found the 'sweet' spot which resulted in greatly reduced throb.  That 
reduced fan noise considerably.

Then, to reduce transmission of fan vibration to the amplifier structure 
even further, I mounted the fan loosely to the back of the amplifier 
using soft quarter-inch cushions between the fan and the amplifier.  I 
used light wire to do the actual attachment instead of the long screws 
that are standard.  The overall effect is considerably less fan noise 
and I am very happy about that.  The fan mounting is fragile and not to 
be recommended if the amplifier is to be moved very much but in my 
installation it is just fine.  By the way, I carefully compared cooling 
performance before and after the fan mounting modification and found no 
difference at all.  I also noticed during my testing that the direction 
of airflow makes no difference.

Don K7FJ