Gorn Overdrive: Li'l help

[Fancy Lime]

Yes, seems like we have gorn orff torpic a Bit.

[highwater]

I was working my way up to the finland anthem. I have a cap my sister brought back from a jaunt to Finland, and it has the lyrics of the Python [Monty] version printed real small around the edge of the peak.

May I ask which of the two peaks it was printed around?

[duck_arse]

I was working my way up to the finland anthem. I have a cap my sister brought back from a jaunt to Finland, and it has the lyrics of the Python [Monty] version printed real small around the edge of the peak.

May I ask which of the two peaks it was printed around?

you may, yes.

[Fancy Lime]

Not gonna happen, is it?

Cheers,
Andy

[iainpunk]

Not gonna happen, is it?

Cheers,
Andy

i dont have any zeners under 8V so im probably not going to do this exact schematic any time soon

[highwater]

Finally got my workbench semi-cleared, but the lowest zeners I have in my stash are 4.7v... are those close-enough if I double the gain and run it with a Klon-style +18/-9v supply?

[Fancy Lime]

Finally got my workbench semi-cleared, but the lowest zeners I have in my stash are 4.7v... are those close-enough if I double the gain and run it with a Klon-style +18/-9v supply?

The knee of 4V7 Zeners is not quite as soft as that of 2V7's, so the softness will not be quite as soft but still pretty soft. With 27V supply you should certainly get a nice overdrive out of this thing.

Cheers,
Andy

[aron]

I ordered some diodes. Would like to hear this clipping arrangement.
GORN - obviously from Star Trek! hahaha

[Fancy Lime]

I ordered some diodes. Would like to hear this clipping arrangement.
GORN - obviously from Star Trek! hahaha

I had to google the reference but, yes, that works too. In that light, I'm kind of surprised that the name does not seem to be taken already (according to google at least).

Cheers,
Andy

[iainpunk]

i don't get either reference 

[bluebunny]

i don't get either reference 

Either.  Or.

[anotherjim]

Shuffled of its mortal inductor. Gorn to meet its maker.

[aron]

Just waiting on Mouser....

[aron]

Well it took me long enough! But..... TL;DR - I LOVE THE GORN DIODE STRING!
I took one of my Tube screamers and took out the 1N4148 and put a socket and went to town!
I bought bat 42 and 2.7 zeners (I have no idea what the specs of the bat 42 are - I didn't even look them up).
Here are the results. The pedal was tested with distortion at 50% then 100% Volume knob was the same but you can hear how much softer the 1N4148 stock config is.


Watch out, the stock is way softer than the zeners and bat 42.


STOCK Tubescreamer:
https://www.diystompboxes.com/pedals/gorn/TubeScreamer_1n4148.mp3


bat 42:
http://www.diystompboxes.com/pedals/gorn/Gorn_Bat42_Zeners.mp3


2.7 zeners:
https://www.diystompboxes.com/pedals/gorn/Gorn_2_7_Zeners.mp3


Conclusion - for me, I'm going with the 2.7 zeners. I would add a series resistor inline with the distortion pot to get more overdrive at the highest setting.


Here is the 2.7 zener diodes with reverb. Sounds smooth and nice!
http://www.diystompboxes.com/pedals/gorn/Gorn_2_7ZenerRev.mp3

[johngreene]

Interesting diodes. So the Zeners are 2.7v in one direction and .6 in the other? More or less?
So just the two in series and reversed would be about 3.3v in each direction?
the extra BAT 42 is about .4v. So in one direction you get it in series with a zener, 3.1v, and in the other, the two zeners as before, 3.3?

Is that enough asymetry to be interesting? I have usually found a more extreme difference more interesting.... Am I missing something?

Yes. Yes you are missing something, that something being current. The actual clipping thresholds in this arrangement come out to something more like this:
BAT42: 150mV
1N4148: 400mV (as a proxy for the Zener forward, for which I could not find the relevant data)
2V7 Zener revers: 1000mV
So the asymmetry is more like 1150mV vs 1400mV. But this all changes strongly and differently for all the involved diodes with changing amplitude of the input signal, which is the main point of this whole thing. The values above are for a 0.1V input signal. With higher input signal, the thresholds go up and the clipping becomes more symmetrical. If you like, you can swap D3 for a 3V3 and/or D1 for a 2V4 (if you can get hold of one) to make it more asymmetrical across the whole range.
But like I said, detailed explanations to follow in exchange for some sound samples. Preferably with guitar and bass 

Cheers,
Andy

p.s. The tone pot can of course be a normal audio taper A10k wired in reverse, in which case custom dictates to call it "filter".

Except the "current" through the diodes in the feedback loop doesn't vary that much. So while your logic may be valid, it is more applicable to diode to ground clipper than in a feedback loop of an opamp. Back to back zeners are older than dirt, and their reverse breakdown voltages vary a lot, a lot more than a small offset of .2V the BAT is going to drop it. What you are hearing is dominated by the response of the zeners, being they are so much 'slower' the response will be dominated by their reverse response. Much more than the forward drop of the BAT. So no, you are only introducing a small amount of asymmetry. Very small amount. But if you can prove it with some scope captures and examples, I am totally willing to be proven wrong.

[johngreene]

Did you know that zener diodes were used in some old chorus pedals to convert a triangle wave into a hyper-triangular one? It's true, and I think that 'artifact' plays a much more significant role in what you are hearing than the BAT shottkey diode. "back in the day" you couldn't get a zener lower than a 4.7V reverse breakdown. But now you can get them much lower, but they have a more 'fast' response. They have to. Which helps with the limited voltage swing in a guitar pedal. But again, current plays a very little part because currents tend to be quite low, they have to because they have to run off battery. But that's just my opinion, FWIW.

[Fancy Lime]

First up: the promised explanations are coming. I was about to post them but found that it was too long and rambling. I have to do some redacting to make it useful. But it's coming.

@Aron
Awesome demos, thanks! They really showcase the virtues of the Zeners, namely the smooth transition in and out of clipping, especially the latter third of the clips. Good thing you included those long chords that slowly fade. With the original tube screamer diode arrangement, there is always a bit of brittleness in the decay. With the Zeners, it sounds much more natural to me.
The strange thing is, I recently got the idea to try this because I found two 2V7 Zeners soldered together back to back in my assorted junk drawer. Judging by the soldering, it is clearly my work from when I first started building pedals many years ago. But I have no idea, what I used these diodes for or meant to use them for. The pair had never been installed anywhere. I also don't remember where I originally got the idea from. I certainly did not come up with it myself back then. There is a very good chance it come from somewhere in this forum here. If anyone knows, let me know. Or it may have been on AMZ, I read a lot of Jacks stuff before ever touching a soldering iron. Not that I understood half of it, though...

@John
Hey, nice to see you here! Haven't seen any posts by you in a while. Do you have an alarm go off when someone talks about clippers? Good to have one of the resident experts on MOSFET clipping here, since it sounds rather similar as the Zeners to my ears.
Anyway, I have to disagree on a few points. The current through a negative feedback loop of a non-inverting opamp varies linearly with the input signal. The opamp wants to keep both inputs equal so the voltage across the ground leg resistor equals the input voltage and Ohm's Law says that the current has to do the same if R in the ground leg is fixed. You are right that this variation is too small to matter audibly, *except* in low voltage Zeners. That is the thing here. The I/V relation of 2V7 Zeners around the relevant currents and voltages that we are dealing with in such a circuit is so flat that we need quite substantial voltage changes to accommodate the relative small current changes. That is why this diode arrangement reacts so well to playing dynamics, too. But this is really only the case for Zeners below 5V. And if you want to get useful clipping thresholds, 2V4 and 2V7 are pretty much your only options. Maybe 3V0.
If the BAT42 matters more than the variation of the Zeners themselves, I cannot tell you. Putting the BAT in or out made an audible difference on a test circuit I did some time ago but I did not cycle through various 2V7 Zeners to check their variance. And the datasheets are terse on information about variation of anything other than the nominal voltage. Given that low voltage Zeners are highly doped, which is a difficult to control process, I would not be surprised if the variation at very low currents was substantially bigger than the ±5% or so at 5mA that are used to determine the nominal voltage. So pending systematic investigation, I'll say you're probably right. Anyway, it may worth experimenting with whatever specific individual diodes one has at hand in the type of arrangement shown in the schematic and see what sounds good.
Oh and one more thing: What's "hyper-triangular"? Never heard that term but it sounds appropriately Star-Treky.

Cheers,
Andy

[aron]

Hi Andy, hahahaha I told John about this post. I used these zeners: Vishay Semiconductor TZX2V7A-TR (2.7 volt zeners)
I really like the way it sounds in this pedal. I need to try it in my main pedal - the Shaka, it would be interesting to hear the difference.
IMO you can clearly hear the differences in the diodes and the relative levels.

[johngreene]

I thought this discussion had a familiar ring to it.

https://www.diystompboxes.com/smfforum/index.php?topic=46134.0

[aron]

I remember reading that post about zeners.