Speed control for 120vac fan that does not add noise

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The capacitor should work, but is complicated to size values. Another thing that works pretty well is a pair of identical fans and a switch to drive them in series or parallel. Running in series cuts the per motor drive voltage in half which usually constitues a serious reduction in speed and very low energy loss.
 
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Series cap and series fan connections are good solutions. Do NOT use or even try 70v transformer, it WILL burn up at 120v, and insulaton is not rated for AC line use. 240v to 120v stepdown transformer or autoformer rated at twice the load you are running will operate at low flux levels(=cooler) with 240v winding connected to 120v line and fan connected to 120v output winding.
 
Novi, I think there are two issues here:

1) the device is not made for 110 Volts + surges. This is a hazard. You or a loved one may be electrocuted.

2) the device is made for musical signals with a much higher average frequency than your mains. Long story short: higher frequencies need a smaller transformer for the same power rating than lower frequencies, so for 60 Hz, your device will have a much reduced power rating.

and in good Monty Python style, one more.

3) music Watts have a much lower energy density than fan Watts. The latter draws a constant current, whereas music doesn´t, so the Watt ratings cannot be compared.

No there are actually 4.

4) the impedance or load your fan presents to the net will actually decrease when it turns slower. I can imagine a scenario where with the fan turning very slowly, you will draw even more current through the device than what it was made for. So yes, it can set also set your house on fire.
 
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