envelope follower question

[michael]

hi all!

i'm trying to figure out Tom's preamp/envelope follower schematic

http://www.geocities.com/efm_gc/cbook/pics/apm1a_sd.gif

i only see one input and output from the envelope follower. where does it
take the envelope from? unless i understand wrong, shouldn't there be an
input signal and an envelope input?

could someone straighten me out on this?   :)

michael