Envelope trigger circuit for guitar

[R.G. Keen]

>     Can anyone recommend and/or point me to a good source (circuits or 
>     methods) for making or designing a good, reliable, and easy to use 
>     guitar envelope trigger?  I want to use my guitar to trigger an 
>     envelope generator that will drive a VCF and VCA for guitar effects.
>     
>     In the past, I've casually tried to cook up cheap and dirty triggers 
>     using an envelope follower and comparator but quickly found out that 
>     it's not a simple job.  The input signal can vary so much that you 
>     constantly need to ride the comparator trigger level to get a reliable 
>     trigger.  I was hoping to find a more adaptive trigger that would be 
>     "smart" about the trigger level.
     
Don Lancaster's "CMOS Cookbook" has a circuit for using a CMOS opamp
to ride the level of a photodiode to pick off sudden transients in 
light level, for all the reasons you note. Take a look at it.

R.G.