envelope follower question

[Byron G. Jacquot]

>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?

I think you've got the wrong module in mind.  

If you want 2 inputs, one for an envelope, and one for a signal to which the
envelope (or whatever other control voltage you've got handy) is applied to
control it's amplitude, you're looking for a VCA, a voltage controlled
amplifier.  They're much more common circuits than envelope followers.

An envelope follower watches the input signal, and roughly tracks it's
amplitude.  You'll get a higher CV out when you input a louder signal.  An
Envelope follower's output is used to extract the informaiton about how loud
something is.  

A common example:  when you run a guitar through a comparator (fuzz box),
you get out square waves that track the guitar's pitch very well.  But you
lose all of the dynamic content.  The waves are either running, or not
running.  If you added an envelope follower and VCA, the envelope follower
can get the dynamic variance of the guitar into a CV, then re-apply that
information to the signal using the VCA.  Instead of gated square waves,
they'll follow the dynamics of the decaying strings.

It can get more interesting when you expand to control even more modules.

Byron Jacquot