I've already settled on stereo as opposed to mono,
No problem there and plenty of class-D/class-T stereo amps to choose from.
because I plan on using Sure's TK2050 Tripath amp board, good for 60x2 at 4 ohms.
Plenty of power but even 2x15W or 2X20W is quite a bit for a portable.
The case will be braced and batted plywood,
I'd recommend multi-ply like Baltic birch. It has fewer surface voids to deal with and is a bit stronger than standard ply when thickness is minimal.
I think I will use a hefty 12-14 amp hour lead acid cell, but I may consider a pricey D-based NIMH power system if the projected weight crosses thirty pounds.
You'll need (2) identical batteries 12V in series or a single 24V (pricey) one. The TK2050 chip needs 24VDC input. The higher input voltage is what allows it to make ~2x50W whereas a 2x15W or 2x20W can get by with 12VDC input.
For speakers, I'm still not sure. I might just be simple and get used 6.5" high quality car speakers off Ebay, with pro audio tweeters mounted above for extra sizzle.
Car speakers (for the most part) are designed for free-air use. That means they need a big air space to operate like a car door interior or rear deck. When you put them in a small enclosure, the bass is usually choked off. Better to use speakers designed for small enclosures like most home audio ones.
But, I fear the bass may be too meager, especially with no graphic EQ.
A fully baked speaker design won't need an EQ for every situation but an EQ is helpful if your music is poorly recorded or you want to tailor how the system sounds. You can add a car audio EQ between your source, MP3 player, etc., and the amplifier. They run on 12VDC. If you use the 24VDC amp, you need to use (2) series batteries in order to have access to 12VDC easily. Otherwise you'd need a step-down supply for it.
My other idea is to use separate 8" woofers of especially high sensitivity,
In general, the larger the woofer, the larger the enclosure.
mated to pro audio tweets for a brash sidewalk sound of yore.
Brash can be achieved by having variable tweeter attenuation using L-pads. Turn them down for normal use, turn them up for sizzle. It merely adjusts how loud the tweeter is playing. If you have an EQ in the system you can do the same thing as well as bump bass, by adjusting the extremes of the EQ from flat to boost.
This will probably require a midrange too, though, which may make the needed crossover too complex.
I have currently and have had previously, many 2-way designs with an 8" & tweeter. An 8" is totally capable of great midrange. You are right though, adding mids will make the design more complex as well as larger. The midranges (unless they are domes) will require their own isolated enclosures.
The last idea is to use small (eg. "5") woofers, tweets,
Now you are talking! I'd look for an existing 2-way, ported home speaker design and cannibalize it. If you don't have the chops to select compatible drivers, develop the crossover, & tune the woofer, etc, borrow someone else's work. There are a myriad of great sounding & now forgotten small speakers on the market. Check the completed auctions and look for what went the cheapest then hunt for another set. You'll get all the drivers (woofers & tweeters), the correct port length & enclosure volume to build your design with, and the crossovers.
plus a dual voice coil subwoofer. I have no clue which is the best option, but I'm leaning to the latter for subwoofer bragging rights plus that low-end authority that's so in vogue.
A subwoofer with further increase your enclosure size if you retain using larger main speakers. If you went in the other direction and used a mini 2" - 3" full-range driver then a 2.1 (2 FR, 1 sub) style box makes sense. However, it may not make anymore bass than a properly sorted 4" - 5.25" stereo ported version. Not if you are planning on keeping it moderately portable at least. Most of the iPod dock boxes from a few years ago were 2.1 types. That is about what you can expect for performance.
Has anyone here done this before, and if so, does anyone have any feedback on what speaker types might work best, if sensitivity is the primary concern?
I've been playing with portable audio for 20 years. I've never built a completely portable boom system but you never know what I'll do over the long winter.
:devil:
Am I barking up any obvious wrong trees?
Nope, great questions all.
Any easy way to add a graphical EQ, ideally including at least three bands?
See above.
