The envelope works well, but at certain settings of the Attack, Decay, Range the effect does not taper off smoothly but gurgles turbulently. Almost sounds like I have a tremolo effect on.
Is this related to a certain component I can swap out?

Some fluttering is not unusual behavior when you have long decay and shorter sensitivity settings. That's just the amplitude of the incoming signal sinking below the peak detection of the envelope. You may be able to improve the sensitivity a bit but you will probably never completely eliminate that behavior with this circuit. Anyway, my suggestions are to either (1) replace D1 and D2 with a germanium or BAT41 Schottky diodes or (2) increase C11 from 4u7 to 10uF. One (or possibly both) might make it work better for you.

It also tends to be more noticeable when you have chords ringing out over long decay times, I find.

The search for the simple, uncomplicated, ripple-free, responsive envelope-follower continues.  Many stompbox filters use simple, but somewhat ripple-prone, rectifier circuits.  The circuit used in the Kraken/Mutron V is a little better than many, but is still engineered to be responsive and perhaps less ripple-free.

When it comes to processing guitar, the most ripple in the original signal - and consequently the greatest susceptibility to ripple in the envelope - is during the decay phase.  As the notes/strings die out, they are MUCH less stable in their actual amplitude than our perception imposes on them.  The resulting ripple can often be perceived by users as "distortion".

If one engineers the decay to be very quick, then the note, and filter sweep, will take place before the audible ripple sets in.  Of course, that only works for some tunes/styles, and less well for others.  But, as Bean astutely notes, extending the decay, by using a larger/higher value for the "averaging" capacitor will also tend to smooth out the envelope and reduce audible ripple.  Whether a longer slower decay time is matched to the picking/strumming speed one is aiming for is a whole other thing.

One of the reasons why optoisolators (LED/LDR combos) are preferred for such swept filters is because the LDRs often settle back down to their maximum resistance somewhat slowly.  As such they have a "built-in" ripple reduction quality.  Of course, the applicability of LDRs to a filter circuit depends on the rest of the circuit.  One cannot simply sub them in for whatever is currently in use.

Thanks for the replies!

I've had decent results with the GGG Mutron V, and hoped the Kraken would be an improvement, it mostly is a step up in knob-tweakability, but (for my builds anyway) less pronounced and stable of an effect.

I've also built the Ibanez "Bass Synthesizer" an interesting effect but not the envelope I'm looking for.

Quote from: Mark Hammer on March 25, 2024, 08:54:09 AMThe search for the simple, uncomplicated, ripple-free, responsive envelope-follower continues.  Many stompbox filters use simple, but somewhat ripple-prone, rectifier circuits.  The circuit used in the Kraken/Mutron V is a little better than many, but is still engineered to be responsive and perhaps less ripple-free.

When it comes to processing guitar, the most ripple in the original signal - and consequently the greatest susceptibility to ripple in the envelope - is during the decay phase.  As the notes/strings die out, they are MUCH less stable in their actual amplitude than our perception imposes on them.  The resulting ripple can often be perceived by users as "distortion".

If one engineers the decay to be very quick, then the note, and filter sweep, will take place before the audible ripple sets in.  Of course, that only works for some tunes/styles, and less well for others.  But, as Bean astutely notes, extending the decay, by using a larger/higher value for the "averaging" capacitor will also tend to smooth out the envelope and reduce audible ripple.  Whether a longer slower decay time is matched to the picking/strumming speed one is aiming for is a whole other thing.

One of the reasons why optoisolators (LED/LDR combos) are preferred for such swept filters is because the LDRs often settle back down to their maximum resistance somewhat slowly.  As such they have a "built-in" ripple reduction quality.  Of course, the applicability of LDRs to a filter circuit depends on the rest of the circuit.  One cannot simply sub them in for whatever is currently in use.

I did my best to build a "Neutron" with NOS Optocoupler but just can't get it to sound right, either misses the bias sweet spot or starts to sound overloaded, almost like feedback. I dunno...

Quote from: half_smith on April 16, 2024, 06:07:47 PMI did my best to build a "Neutron" with NOS Optocoupler but just can't get it to sound right, either misses the bias sweet spot or starts to sound overloaded, almost like feedback. I dunno...

Did you fiddle with the drive pot at all?

You have to keep in mind that the Mutron III circuit was designed for a variety of instruments, so the effective range will be different depending on what you're plugging into it.
-No matter what you plug in, distortion & feedback is to be expected when the drive pot is cranked.