Out of curiosity do you know of said transistorized circuit? I'm curious to see what it looks like and if I could use it for some other purposes... (one of the my hardware projects that's been sitting in the back of my head for a while is basically an audio switch; when there's audio being generated by a sound card/device/etc, the switch triggers and turns the amp on... when there's no sound for a given amount of time, switch releases and turns the amp back off. Of course it can't have any detrimental effect on the audio... that's the trick...)