Whoa! The ESP site! I really like the site, I've built a dolby surround 'decoder' with his schematics two yrs ago... It worked really nicely (still does, just not using it no more
)...
This could be used with a relay instead of fan, so the relay can power the fan(s) on 7V for quieter operation...
And that 741 opamp, I guess if it is available locally here where I live, then it's surely available ANYWHERE... Think that price here is about 0.5 Euro or less...
Good thinking punish3r!!!