I've underclocked and undervolted my athlon 1.33Ghz cpu to 585Mhz and to 1.30V (from ~1.75V).
Doing the calculation:
Default Watts * (Clocked Mhz / Default Mhz) * (clocked Vcore / Default Vcore)^2, gives me max 17 wattages.

Is that possible to passively cool with a heatsink like ax-7, slk-600 or a zalman flower, if it lies in the open, outside a case, with no airflow support?

At 17 watts I think it would be a good idea to have a fan of some sort. Any 5v 80mm fan should do just fine and be almost completly inaudible. Without the fan you run the risk of killing the CPU in a blaze of glory.

I read you, and fooled around some more and hit rock-bottom at
495Mhz (5.5 * 90Mhz) and 1.22 as Vcore, surprisingly it's very stable too.
Theoretically that would give 70*(495/1330)*(1.22/1.75)^2 = 12.66 watts, which ought to be in line with passive cooling..
However the formula might be flawed, so I dunno. Perhaps I should try?

Anyhow, I'm trying to minimize the amount of components needing acoustic dampening, that includes any fan IMO.
Being able to run the entire MB outside a damped box would be really nice :)

Since you're not so concerned with performance have you considered a passively cooled Via CPU?

Acording to SPCR (MikeC I think), a 5v 80L can only maybe be heard at 2 feet by someone with very good ears. A good thermalright heatsink would manage given one of these fans without the need for such drastic underclocking. If that's still too loud, you could try somthing like 3v

In order to use passive cooling, it really helps to have a heatsink that is designed explicitly for passive cooling. For example, passive heatsinks generally have their fins much further apart from one another than heatsinks intended for use with forced air, which makes convective air movement through the fins easier.

So, if you use a DIY solution made explicitly for passive cooling like bluehat then you can certainly passively cool your CPU. If you want to use a retail solution, I'd say that one of the later model Zalman flowers* would be your best bet, although they're certainly no guarantee of success.

* I believe you'd want the AlCu models rather than the Cu models, since aluminum is supposed to be more effective at dissipating heat than copper, although copper is more effective at absorbing heat. You might want to try spreading the fins out manually on it as well. Zalman's website does mention a flower cooler explicitly designed for cooling C3's passively; if you can find that one for sale, you might do alright with it.

> Since you're not so concerned with performance have you considered a passively cooled Via CPU?

The only demand I have on the CPU is that it must be capable to decode lossless encoded BONK files (a CPU hungry audio format) in realtime. If a Via C3 (only Ezra available in sweden) coops with that, then it's a very interesting choice indeed!

> a 5v 80L can only maybe be heard at 2 feet by someone with very good ears.

If it produces noise it will be in a damped case - the end
Even though that's a good and final solution, it would produce unnecessary heat which naturally is something to avoid if possible.

It depends on if the BONK format does mainly integer or float calculations. If it's integer I recon a 800MHz ezra woul be fine.

I've never heard of BONK - does it offer very high compression and audio quality? What are the file sized like?

When you say CPU intensive, how intensive are you talking? My cpmputer is currently playing a 177bps wma file with CPU utilisation hovering around 6-10% Unless it's very unusually intensive I'd be suprised if a modern processor couldn't handle it. This setup is a 2GHz athlon with only a ducted 3v fan and Zalman flower cooler, CPU is 52 degrees c, which is quite safe.

It's integer calculations. The source can be found at http://yoyo.cc.monash.edu.au/~pfh/bonk/

Comparisons to other lossless formats:
http://flac.sourceforge.net/comparison.html
As one can see, bonk doesn't perform that well in regards to cpu time. But it's reliable and has a small code base - a thousand lines or so.

I came to the conclusion that I'm going for a via and if it can't coop with bonk/flac, then I'll simply go with wav files, that it must be able to play :)

If it's integer then I'd say via. Much of the Athlon's speed comes from it's fpu.

To be honest, buying and unerclocking an athlon to run on less power is like putting a speed restrictor on a Lamboughini (sp?) for fuel economy.

Whereas the via is a new beetle. Or somthing.

That's in free air, blow it over a HS or anything and air turbulance can be heard from far away. More so on a completely open case..

Maybe he could try a panaflo @4v or less if it starts that is..