Zener noise - soft-knee vs hard-knee Zeners

Hi, I'm wondering about Zener noise. I read a great thread here about Zener noise measurements:

http://www.diyaudio.com/forums/parts/35821-noise-measurements-leds-zener-diodes-2.html

I also watched this great video here:

https://www.youtube.com/watch?v=WTJhIVIGvSU


I'm curious about the "soft knee" vs "hard-knee" Zeners and high-Z vs Low-Z Zeners that they talk about at 14:30.

For the standard 12V Zener that most people use for Schoeps-style circuits, is there a particular one you prefer that has a "softer" knee? I was looking at data-sheets for some 12V Zeners, and I'm not sure if I'm interpreting "knees" correctly.

I have a few TZX12Ds, but I'm seeing 1N5242B and BZX55C12 as potential options for 12V Zeners as well. I'm not sure which one I should pick for a lower noise floor. Any one have any experience with a good choice here? Or am I just splitting hairs at this point?

EDIT: I notice on the datasheets for the TZX and BZX series there's a caveat of "very low noise", but also "very sharp reverse characteristic" in the summaries.

I looked at the video, and I'm not sure what "soft knee" or "hard knee" means. Generally, zener diode behavior depends upon the "zener voltage" you ask from the part.

There are two mechanisms involved with 'zener' diodes - the zener effect, and the avalanche effect. The zener effect is dominant for diodes with thresholds below about 5-6V and the avalanche effect is dominant for diodes with threshold voltages above that. Regulator diodes using the zener effect will be vastly quieter than diodes using the avalanche effect, so maybe that's what the guy in the video was talking about. Avalanche diodes have a sharper I/V curve 'knee'. and zeners have a softer curve, and thus less regulation when used as a shunt regulator.

Still... you buy zeners according to their threshold voltage, and there's no "hard or soft knee" option - you get what physics gives you for that threshold voltage, and I can't understand how one could easily swap a low voltage zener into a circuit for a higher voltage part.

Ignoring all of that BS, if you want a quiet 12V zener, it'd be best to use two 6V zeners in series, or even three 4V zeners in series. A pair of 6V zener diodes in series is probably best, since they'll have low noise, but will still have a decently low impedance, and thus reasonable regulation. And, unlike what the guy said in the video, with "soft knee" zener effect devices, you could certainly knock that low noise down even further by slugging it with capacitance.

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