SVF zeners

this obviously had to be asked:
as you mention the possibility of leaving the zeners out for distorted resonance, would you consider it’s worth making them switchable?
i mean, with a 4 pole switch, not electronically.
or would using two discrete 2pole switches for the individual diode pairs lead to yet more variation (ok it probably would, but does that make sense?)

It’s not that hard to test that on a partially assembled board - populate the board without the Zeners, test it, then put one in place with the leads bent. Personally, I think they are magical, because you don’t have to worry about the resonance distorting when you push it up, and the resulting sine wave is very pure and can go very deep in the low-end, perfect for percussion sounds.

What each Zener do is smooth (from squarish to sine-like) one “side” of the waveform. I have no idea how the “one side clipped, one side smooth” would sound - what is obvious is that the “upper side clipped, lower side smooth” and “lower side clipped, upper side smooth” combinations would sound the same ; so there are only 3 interesting configurations.

ok, so i’ll have to figure out how to do it with 1 switch!

easy, double pole on-off-on works then

@rosch. Can you share this mod for noob people? ( with links between the switch and the zeners) I’m interested. Thanks in advance.

@rosch: me too.

haha i haven’t done the mod yet. i’ll have a look tomorrow at the schematics again to be sure i don’t talk nonsense.

sorry, i just remembered this. it should even work with a single pole double throw on / off / on:
up: both diodes
middle position: none
down: only D4 on board

You need a variable zener for maximum possibilities :slight_smile:

5v is already too much breakdown voltage for a shruthi filter. Maybe by lowering the value of the 4v7 diode you can achieve more suitable results though.

huh? i had no problems with the zeners as they are (installed, like in the build manual)

I was speaking of the link provided by punk (although I suspect this is partially a joke)

pichenettes: I have no idea how the “one side clipped, one side smooth” would sound

Well I suspect it would be rather like my SMR4 SDE ‘GERM’ flavour, which uses a single germanium diode to heavily clip one side of the waveform yet leave the other untouched. You can hear it in action here

Titus already loves it!


Just added a couple of scope screenshots on my web site showing the difference in waveform and distortion levels between Olivier’s original FLAT and my GERM filter flavours:

Hope this helps!


@pinchnettes They should sound the same in a perfect world but I would imagine pushing a membrane out is not the same as pushing a membrane in…

Speakers polarities are not even consistent between manufacturers…
Humans are sometimes able to distinguish the polarity of kick sounds played on big boomers because of the vibrations on their bodies. Otherwise, the way the ears work don’t let any doubt about that, you’re not able to distinguish the polarity of a sound. Have you ever been able to distinguish a ramp down from a ramp up within audio spectrum?

I would expect a speaker to be kicking out some air on the bass drums.

Yes. This is not a sensation related to your ear.
In the case of a filter self-resonance, I don’t think that the wind created by the sound is of any matter…

And as I said, manufacturers are not even consistent about the polarity of their systems. Some speakers have their membrane pushed when you apply a positive voltage between the plus and and the minus poles, some other do the contrary (I think JBC is like that, not sure though). The only use of the “+” and “-” conventions on speaker terminals are because you don’t want two paired speakers to go out of phase and get some phase cancellation. If you’re going to put some speakers of different brands into the same case, you have to beware about this (generally, you can verify the polarity by simply using a 9v battery).
Some power amplifiers and power amplifier chips also reverse the polarity of your signal… and you actually never know about that! Have you ever read a notice about your amplifier inverting the signal or not on your amplifier specs?
Unless you play it really loud or you put your hand close to your speaker, you can’t tell the difference :slight_smile:

@MicMicMan That was exactly my point, and no I don’t know if this does matter in this specific case, and no I’ve never heard any difference between between ramp up and ramp down. But I still would imagine that the asymmetrics in a speaker membrane movement will have more of an effect on a asymmetric waveform, ie one side zener, than on a symmetric as with a ramp (I know this might be the correct semantics but I hope you get the meaning). But I could be wrong and I’m always willing to learn!

I really don’t think asymetrics in a speaker membrane movement are that important but in case there’s some asymmetry :

  • I suppose that hifi manufacturers are doing everything they can to avoid this (since it’s a form of distortion),
  • anyway, since you can’t know in advance which is the polarity of the system your sound is going to be played on, trying to play with that is very hazardous
    So all in one, I don’t think it’s worth the effort adding a complex switching system to add the choice of polarity in clipping in such a filter.

I can see one difference though : the polarity of the oscillators signal is consistent, so maybe with some oscillator settings, the interaction between a high resonance and the original sound would be slightly different if you put the diode on one side or the other. But that’s not related to your speaker membrane.