Hi, I'm building a power supply to run a 12vdc car stereo in my room.
The power amp is a 300w ADS Power Plate 300. At 12.5v I need a 24A
supply? This seems wrong somehow.

You may need _more_ than that. If the amp is actually capable of
creating 300 watts of clean audio output power, it's likely to
actually be drawing 500-600 watts of total power (amps are not 100%
efficient), and you may need 50 amps or more.

If the "300w" is marketing puffery, or if you're willing to run the
amp at quite a lot less than its full output capacity, the actual
needs might be a lot less than that.

I thought I'd use the xformer from
a surplus 12v battery charger with an LM317 but I have the feeling
something needs review here.

Darned right. An LM317 by itself is limited to 1.5 amperes, I think.
It's not going to cut it.

I'd appreciate any observations others
may have. Thanks for your time, pm.

You're likely to need a hefty DC supply to run an amp of this calibre
indoors.

Using a battery-charger transformer and rectifier bridge is probably a
reasonable starting place. You'll need a good-sized filter capacitor
following the rectifier bridge. You'll need a hefty set of pass
transistors - the commonest linear-supply design of this sort uses
multiple 2N3055 transistors (TO-3 cases) in parallel, with ballast
resistors, and a set of big heavy heatsinks. Take a look at the LM317
data sheet - I believe you'll find details on how to use external
transistors such as 2N3055s as external pass-elements, to boost the
LM317's current capacity. For supplies in the 25-50 ampere range I
suspect you'll need an external driver transistor or two inbetween the
LM317 and the pass transistors, to provide the necessary amount of
base drive current. You'll need to design in current-limiting
circuitry (straight limiter or foldback) to keep your supply from
frying its transistors, or starting a fire if the outputs are
accidentally shorted.

If all of this still seems a big overwhelming, you could just buy an
Astron switching power supply (see www.hamradio.com for one source),
or just decide that using an auto amp in a home setting is more
trouble than it's worth, and buy an amp designed for 110 volt operation.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
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