most of which use the LM317. The latter usually does not allow 12V to be passed, as the circuit itself takes bit of power. Usualyy ~11V is the max -- about the same as 12V in real-world apps.
Mike, LM317 "drop-out voltage" is 1.5-2.25V typ. (depending on output current and temperature), therefore I wouldn't suggest this regulator when maximum airflow is required and the fan draws more than ~100mA. You may end up with only 10-10.5V output that is probably too less in some cases.
A low drop-out regulator (like LM2941) would be better. It only needs a larger output capacitor and, of course, costs slightly more than a LM317. That is why it isn't used in cheap commercial controllers.
As a perfectionist, I prefer the closed-loop (temp. controlled) solution. Well, we can't beat commercially available temp. controllers (economics of scale), but for those interested in building their own, I have designed and operated one. If SPCR is interested, I could post an article about it. It isn't that useful as it seems (as I said, similar products from China have a lcd display, might be temptingly cheap and need much less hand work) but it offers the pleasure of creativity
What do you think?