7805 have like 2v dropout don't they? So if you use anything less than 7v it should pass through. That's been my experience at least, I've seen 7805's pass through 6.5v before. So in theory if you use a 5v plug (which are fairly common, you can get a small 5vDC 2a center-positive 2.1mm on BGMICRO for like $4 if you're in the U.S.) it should just pass through the 7805, and actually even create less internal heat as the voltage regulation would all be external then. Of course then you're counting on the power supply providing an actual 5v and without too much ripple etc. which may or may not be realistic. But for most of my atmega projects I just use a 5v power supply if I don't have any 9v handy and I've never had it cause problems (it still runs through a 7805 so it would regulate down any stray voltages that are unrealistically high, but 6.5v has never damaged an atmega in my (limited) experience - I don't purposefully overvolt stuff but sometimes it happens ).
(edit) also 7805's operate up to something like 35v don't they? why do you specify 7-12, just because those are more common?
It has been an awful long time, but today I finally finished my MegaDrum. That is, Today I found a decent enclosure
I originally wanted an 1U 19" enclosure, but it was rather pricey and I wouldn't be able to decently cut the holes for the I/O etc, so I dropped the idea. I've been searching for other solution's ever since, until today.
I found an affordable flightcase-like tool box (23 euro's), and I decided to go for it.