Zener diode soft clipping

[POTL]

Hi all.
I recently monitored the market for parts for creating distortion pedals and realized that in addition to the J201 (or TX Japanese analogues), which have become scarce, LND (high-voltage MOSFets operating in depletion mode) have become rare and very expensive. I decided to study how modern boutique companies build their pedals. on the site PedalPCB (not advertising) I found pedal designs from Victory.
After simulating the schematics, I was very inspired by the idea of ​​their clipping.
Instead of the standard symmetrical and asymmetrical clipping modes (or a softer version with a series resistor like the Bluesbreaker circuit), we can flexibly set the limits and simulate a Cold Clipper Soldano/JCM800. Perhaps the topic is not new, but I saw it for the first time (I love discrete distortion).



perhaps it will be useful to someone. I am attaching the diagram



https://docs.pedalpcb.com/project/Bobby-PedalPCB.pdf

[SprinkleSpraycan]

Cool. Something new to tinker with!

[fryingpan]

In my experience, zener diodes don't clip very softly per se. Can they be useful though? Of course, the inherent higher clipping voltage affords more "headroom". Zener diodes should be a bit noisy but I suppose it's not a big issue in a guitar pedal.

[Umlaut]

I have seen it discussed a few times in this forum and others, but it is worth mentioning: the "discrete zener", AKA rubber diode, also makes a very interesting embodiment of the idea you mention (especially if you do not have many zener values at hand). The three resistors in series in the feedback loop allow infinite combinations: changing the middle one varies the headroom, and tinkering with the other two changes the asymmetry. Clipping can be a bit softer than with zeners. Also fun to tinker with is the dynamic bias network that can be added, be it symmetrical to asymmetrical, à la Pritchard, or the other way around à la Crate & Ampeg. Food for thought 

[teemuk]

In my experience, zener diodes don't clip very softly per se.

This. IME they are quite similar to any generic diodes - if not "harder" - in that regard. The shown circuit, however, employs two out of three stages in non-inverting configuration, which limits "clipping" gain of zener feedback sidechain to unity and hence replaces the abrupt "brickwalling" with a more gradual "fold" in the gain transfer characteristics. But true, sofness is not a zener characteristic per se. 

Can they be useful though? Of course, the inherent higher clipping voltage affords more "headroom".

Yes. Or more precisely it's the most simplest way to alter the headroom because one can just choose a part with proper zenering voltage, while altering "headroom" of conventional diodes requires added complexity of voltage dividers or suitable voltage reference supplies.

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But why a thread about circuit ideas that have been prior art since at least the early 1980's, some fourty years?
Yes, it's certainly not a new topic or idea in slightest and I suggest you take a look at, for example, SLM (Crate/Ampeg) "FlexWave / Variable Harmonics" circuits from 1987 or early millennia Traynor "Dynagain" circuits where you seen these ideas taken to a whole next level. Even in its most rudimentary form you find the asymmetric zener diode clipping already from, say, mid 1980's Hughes & Kettner amps (and they have exploited it ever since), and aforementioned "discrete zeners" (when such a wide array of different types of zener diodes didn't exist) stretch back to at least to late 1970's or early 1980's Fender and Music Man amps.