guitar to 0/5V envelope

[Dimitree]

hi guys
I want to make a small circuit that can take the guitar input and output a variable DC voltage, from 0V to 5V (than I need to feed to a uC analog input) that represents the envelope of the signal.
My requirements are:
- no attack or decay controls, since if I need them I could handle them on digital
- 5V output protection, i.e. I don't want to produce a output > than 5V and < than 0V

I found this on my Hard disk:



the envelope looks great (red signal over the green one that is the guitar wav recording), except for the high peak at the start and the fact that its voltage is too low (I'd like to employ the full 0V to 5V range).
So what should I change?

[Mark Hammer]

I can't recommend a better envelope follower than Harry Bissell's: http://edn.com/design/analog/4344656/Envelope-follower-combines-fast-response-low-ripple

I had the pleasure to try his analog guitar synth, using this follower, and it was just about the most responsive thing I've ever used.  I have no idea of what your needs are, though, so perhaps the simplicity of what you show trumps the complexity of what Harry provided.

[PRR]

> what should I change?

The input impedance is low, 22k (and slightly nonlinear). So you want a buffer in front. This can also supply the gain to make your 5V goal.

10uFd is a huge cap, for TL072 to drive, much fatter than needed to drive the TL072 follower. However this rectifer puts a 22K bleed on the cap so you can't go a lot lower before decay gets short and ripple gets bad.

Hmmm... no reason to use 22K. TL072 will be fine at 220K, and proably 2meg for guitar application.

Attack time for the purpose may be 5mS (0.005 Seconds). TL072 output impedance (large signal) is something under 500 Ohms. Hah, that does give 10uFd. We could use a smaller cap and some series resistance so the TL072 does not work so hard.

Consider 220K input resistor and 2.2Meg feedback resistor. That gives higher input impedance, gain of 10 which will bring guitar into the 5V zone. 50mS with 2.2Meg gives a 0.022u cap. The attack resistor (where you have 100r) would be 220K for 5mS attack, or could be 22K for sub-mS attack (catch the pluck transients).

Ripple is an issue. However with a CPU in the loop you can instruct the brain to disregard small wobbles and look to the larger trends.

[StephenGiles]

Surely this is a job for the peak follower front end of an EH Space Drum, where ripple is not a problem. Someone adapted this for the LM13600 a while back