If we populate the 5V buck circuit you probably could by using the voltage differential there. We rate the circuit at 2A for safety, but the buck controller is rated for 3A max. If you wanted, by changing three parts on the circuit I could probably make it output 7V constant without having to rely on differentials. I can probably test that out on Monday.
Do you know the actual power draw of the S5's fan at lower speed? If it's 3A at 12V it's probably a fair bit less than that when ramped down.
I can check that for you today. -------
7 volts stock fan draws 1 amp I ended up running delta fans on my s-5's I have an evga 1300 g2.
I plugged in a 4 pin molex tapped 2 wires to get 7.18 volts and the 2 delt's run great.
If you can figure a way to draw 7 volts the stock fan on an s-5 runs much more bearable the nasty sounding pitch drops off.
the stock fan will cool well enough at 7 volts to run the s-5 at 325 -362 freq depends on the rooms temp.
this is the final fan mod to the s-5
https://bitcointalk.org/index.php?topic=927941.0if you can get your psu to do 7 volts at 3 amps . the stock fan should work well.
see this has 2x 4 pin jacks so if you cut this off the plug you have 20x 4 pin jacks for 1 usd each, if you sold this wire with the break out board so that it was pre wired for 7 volts allowing the crazy loud oem fan on the s-5 to plug in the fan would run at a constant speed and be more endurable for the s-5 operator.
http://www.ebay.com/itm/10pcs-PC-Cooling-Fan-4-Pin-to-2x-4pin-3pin-PWM-Convert-Connector-Extension-Cable-/121088267930?pt=US_Memory_Chipset_Cooling&hash=item1c316c569a