For my recent sound installation, Peep, I wanted to find a small, simple amplifier I could build 7 of to amplify channels coming out of a ESI Gigaport HD+ audio interface. I ended up trying a basic version of the Little Gem amplifier from Runoffgroove - with no gain potentiometer or volume at first. The amps worked out well in tests, so I decided to use them for the installation at Stockholm Fringe Festival.
For the production version I built 7 amps on one perfboard and boxed them into a project box with cinch connectors out to the audio interface. I regulated a power supply with a LM7912 to run them off 12 volts. Instead of the 25 Ohm Rheostat, which I didn't have any luck finding online here in Berlin, I used a 100k potentiometer on the input. Not ideal, as it's obviously better to have a volume knob on the output of each channel, but pressed for time, the potentiometer worked ok for this.
The amp can drive a variety of speakers sizes from large to small. I found that a medium sized speaker, 6 inches, can get very loud with one of these amps. I didn't notice any channel bleed over when simply connecting them in parallel to the 12VDC supply.
The nice thing is that the total cost of each amp is a couple of euros, so I can build small sets of amps as needed for individual projects relatively cheaply. As small test amps for developing projects, they're perfect.