This is a rewrite of a post I made on the Procooling forum. It's a summary of the things I've found out about using undervolted AC fans, which are the best, how to speed control them, how much they cost, and where to get them.
In my search for quiet 120x38mm fans for a BIP II radiator in a water cooling system, I tried and discarded all the available DC fans because of their excessive motor/bearing noise. There is no quiet 120x38mm fans like the 120x25 Nexxus or Globes. I wanted a 120x38mm fan because they handle backpressure better than the 120x25's and radiators are pretty restrictive compared to just case cooling. I turned to AC fans and after trying pretty much everything but Papst AC fans (they're just too expensive), and Orion (which I couldn't find any available in less than case quantities), I settled on the Sunon 2123xst 230Vac fan undervolted to 120Vac. They are superior to every 120x25mm or 120x38mm fan I have by a large margin, and I have a lot of fans (Adda, Compuman, Zalman, Nexxus, Globe motors, Globe Fan, T&T, NTB, Papst, Panaflo old and new, Sunon DC, Silenx, Bi-Sonic and some others I can't remember). They have almost no bearing noise. As far as I know they are the best 120mm fan for airflow/noise that's available. They are even better than the Gold standard of 120x25mm DC fans, the Nexxus.
The 2123xst is easily speed Controlled with a 1K ohm potentiometer (see below for availability).
At 120Vac (125Vac in my case here) they run at about 1200rpm down to about 650rpm at the minimum pot setting. I find 1200rpm is more than sufficient for me as an upper limit. If you need higher rpm than that then put a 2.2uf capacitor in series with the fan and it will boost the top speed considerably to a brisk 2500rpm. At this point it is no longer even remotely quiet but it is moving a serious amount of air (about 85CFM free air). A 4.7uF cap will boost the top speed to a more reasonable 2100rpm, adds less heat to the pot than the 2.2uf, and fan rpm is less sensitive to line voltage fluctuations. Adding a series cap also increases the minimum RPM achievale with the pot. Unless you need the speed though I would not put any cap in as it increases the heat dissipated in the pot. Don't forget to use a non-polar cap that's rated for the voltage. These fans never get more than slightly warm running at any speed from 2500 down to 0.
The fans are here http://www.alliedelec.com/Catalog/pf.asp?FN=551.pdf
stock number 997-2123 at $9 ea
Pots are here http://www.alliedelec.com/catalog/pf.asp?FN=1233.pdf
Stock number 522-0049 at $5 ea
2.2uf caps are here http://www.alliedelec.com/catalog/pf.asp?FN=1127.pdf
stock number 612-0553 about $1
4.7uf caps are on the same page stock number 613-0500 about $2
Fan cords are here http://www.alliedelec.com/catalog/pf.asp?FN=549.pdf
Stock number 609-5642 for 3' cord straight connector for $1.2
Allied has a minimum $25 order, but 2 fans plus parts covers that.
Note that the pots are rather large diameter but they will fit into a 5 1/4" drive bay panel.
I have designed a fairly cheap and simple optically coupled circuit that allows me to control my AC fans with the programmable T-Balancer fan speed controller (or any speed controller for that matter). It works well but it's a linear solution with about 4 watts dissipated (per fan) in it's worst case so I'm working on a switched mode design to see if I can avoid the heat without generating an unacceptable amount of noise in the fan. It's neither as elegant nor as noise free as I'd like yet.
Currently I use a Criticool pump relay to turn the fans on and off with the computer
This one is convienient since I just glued some perf board to the card and added my extra control circuitry for the T-Balancer. You could also just leave the fans on all the time.
There are small 230v to 120v transformers that would allow those with 230V mains to run these setups as well.
I will add things to this thread as I finalize them such as control circuits and some real CFM/rpm numbers etc. (soon as my thermoanemometer arrives). Please feel free to ask questions.
For those watercooling who already have a pump relay these fans are not only a cheap solution, but may be the best solution available period.
I would love it if somebody else besides me would try out these 2123xst fans and leave some feedback on their experiences. I don't want to be the only voice in the wilderness.