LM317 as an amplifier...

[earthtonesaudio]

I've seen several ways of using an LM317 voltage regulator for audio... primarily as a current source, but also the slightly more interesting application as a voltage follower. 

Then there are some really novel applications:
http://www.diystompboxes.com/smfforum/index.php?topic=53241.0
http://www.edn.com/article/CA185953.html

I wonder how far you could go...?

[petemoore]

  "    " from the article.
  The 317 works when theres a 4ma. or greater output load.
  You must add the ac current in the audio signal to the minimum current requirement of 4 mA. For an 8Ω loudspeaker, you need a minimum resistive load of 470Ω to avoid distortion.
  Goodness, haven't I seen a 'really high' ohm speaker >32ohms before, it's been so long since 'the ones I can't use' were tested.
  8 / 470..for an amp with no distortion, it'd be a small fraction going to the speaker, using an 8 ohmer.

[brett]

Hi
as I understand it, the current through the LM317 must be GREATER than 4mA for it to provide high quality regulation.  This might be a problem in stompboxes, but as a constant current source in a power amp, you'd usually be dealing with tens of mA up to hundreds of mA.  A low load (like 8 ohms) is totally fine as far as I can see.
cheers