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Stormrider 18th November 2008 08:05 PM

Slowing down AC fans?
I just got an Ashly FET-500 off ebay; the seller had a bunch of them for sale at a great price. 8 pairs of K135/J50 laterals on +/-80vdc rails for 400w/4 ohm.

However, the ~5" 120VAC fan they use to suck air through the front panel is pretty loud. Fine for the PA rack, but annoying in the HT room. The heat sinking in these amps is fairly generous, but with a sealed top/bottom the amp would get too hot with no air flow.

Is there a simple way to slow down the fan?

Rikard Nilsson 18th November 2008 08:54 PM

I would try connecting a diode in series with the fan. This removes one half wave, could do the trick.


Decker 18th November 2008 09:14 PM

AC fans
I have slowed an AC fan down by using a small transformer with dual 120v primaries as an autoformer. Connect the two 120V primaries in series, connect the fan across only one of the primaries. Don't connect the secondaries at all. Now you have half (approximately) the voltage going to the fan, slowing it down and quieting it.

wg_ski 18th November 2008 09:29 PM


Originally posted by Rikard Nilsson
I would try connecting a diode in series with the fan. This removes one half wave, could do the trick.


Induction motors don't like DC in the stator. It will slow it down, but it will run very very hot.

If there are already +/-80V rails in the amp, that puts the secondary at 56-0-56. Just run the fan off one side of the main trafo (56VAC).

richie00boy 18th November 2008 09:31 PM

Or just use a resistor.

Dr_EM 18th November 2008 09:50 PM

Isn't a series capacitor a popular way to do this? Doesn't waste heat. Must be suitably rated unit.

wg_ski 18th November 2008 10:26 PM

Choose your cap carefully. Get the circuit resonant and you'll end up with more voltage than you started with. That fan would be runnin' really fast at 100% power factor....

Stormrider 18th November 2008 11:03 PM

Thanks for the replies.

I think connecting the fan across one of the secondaries would be the easiest way to slow it down. This type of fan seems to draw around 100mA so i don't think the extra load on the transformer will be a problem. What about noise on the supply lines?

If i wanted to use a cap, how would i calculate the value?

wg_ski 18th November 2008 11:12 PM

Elementary AC analysis with a little experimentation. Measure R+jX with the motor *at speed*, then choose your cap value such that |Xc|> |2*XL|. From there, continue decreasing C (increasing |Xc|) until the voltage drops low enough to run at the desired speed.

quasi 19th November 2008 12:43 AM

I use the series capacitor method. It works very well and without heat.

I suggest you start with 0.1uF and go up or down from there to make sure the fan starts reliably and provides the air flow / noise level you want.

Make sure you use a mains rated capacitor and put a 200k resistor (2 x 100k in series) in parallel with the capacitor so that it discharges when the power is turned off.


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