Hi there,
since I am cut off from my work bench for the foreseeable future, I'm left with designing on paper. Doing some research into Zeners recently, I realized why back-to-back low-voltage Zeners as clippers sound as great as they do. Very dynamic and natural. If I were in the business of selling these things, I would probably call it "tube like" and for once that would be more than just an empty phrase. But seriously, such a Zener arrangement is a great way of getting sound reminiscent of FET clipping out of an opamp circuit.
Here is a little demonstration circuit:
(https://i.postimg.cc/FkFk6FS8/Gorn.png) (https://postimg.cc/FkFk6FS8)
The "woody/tinny" switch is a Z-switch. In the middle position, both legs should be connected to ground, in position 1 and 3, one or the other should be. The output volume may be a bit low as shown, unless this thing is driven with hot pickups or a booster upfront. This thing is going to be quite responsive to whatever input it is fed. One may opt to leave out D2 for symmetrical clipping or Change one or both of D1 and D3 to 3V3 Zeners to get more volume. If the circuit is not fed straight from the guitar, R1 can be lowered to a few hundred kiloohms. C5 and C6 may need adjusting by ear but the given values should be a good start.
As I said, no breadboarding or soldering for me any time soon, so here is the deal: If someone breadboards or builds this and uploads a half decent or better sound sample here, I am going to write a detailed description why it does what it does. That sound like a deal?
Cheers,
Andy
p.s. Stupid corona virus may as well be used to inspire some international cooperation instead of just interrupting it all over the place.
No takers, huh?
Well, maybe the sort of people who build nice overdrives for a pleasant guitar tone are a less adventurous bunch than I thought. Maybe millions of Tube Screamer, OCD, and Klon clones get build for reasons other than their sound. Would it help if I told you that Jimi played the Gorn Overdrive on his top secret solo album that the gov'ment does not want you to know about because they made him record it after staging his death in order to communicate with aliens from planet Swagulon-5 whose language consists of epic blues licks? I swear!
Andy
I'll clear the breaker and give it a whirl tomorrow . Pretty sure I've got a Zener kit somewhere.
I like a nice dynamic drive.
hey, im sorry,
your project seems very interesting, especially with the jimi record, but im not in to building "nice" pedals, im the kind of guy who builds pedals that will try to steal your girlfriend.
ill look for some zeners in my pile tomorrow and breadboard it up. and if i have zeners, ill give you a report on the sound.
iain
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?
Quote from: iainpunk on March 23, 2020, 06:47:48 PM
hey, im sorry,
your project seems very interesting, especially with the jimi record, but im not in to building "nice" pedals, im the kind of guy who builds pedals that will try to steal your girlfriend.
ill look for some zeners in my pile tomorrow and breadboard it up. and if i have zeners, ill give you a report on the sound.
iain
Oh boy, have I got the design for you! The last thing I actually managed to breadboard was a high gain fuzzstortion. The underlying motivation was that I have always found the Big Muff too polite, the Rat to be lacking in chest hair and the HM-2 to have too narrow a range of usable sounds. I have the schematic on paper and hope to digitize it one of these days. I originally planned to release it when I had the time to design a PCB for it but considering the current situation, I might as well release the schematic now. Stay tuned.
Quote from: idy on March 23, 2020, 10:05:12 PM
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 :icon_wink:
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".
Quote from: Fancy Lime on March 24, 2020, 04:00:50 AM
Quote from: iainpunk on March 23, 2020, 06:47:48 PM
hey, im sorry,
your project seems very interesting, especially with the jimi record, but im not in to building "nice" pedals, im the kind of guy who builds pedals that will try to steal your girlfriend.
ill look for some zeners in my pile tomorrow and breadboard it up. and if i have zeners, ill give you a report on the sound.
iain
Oh boy, have I got the design for you! The last thing I actually managed to breadboard was a high gain fuzzstortion. The underlying motivation was that I have always found the Big Muff too polite, the Rat to be lacking in chest hair and the HM-2 to have too narrow a range of usable sounds. I have the schematic on paper and hope to digitize it one of these days. I originally planned to release it when I had the time to design a PCB for it but considering the current situation, I might as well release the schematic now. Stay tuned.
i don't have any zeners under 7 volts, so im not going to be able to breadboard that overdrive of yours, but i am very very interested in a fuzzstortion. i am going to build a grind booster circuit which turns any overdriven amp in to a gain monster, with two flavours, with the same reasoning in mind as your fuzzstortion has, hahaha
"Gorn". I see what you did there. A l'il bit of Graham Chapman.
Quote from: iainpunk on March 27, 2020, 10:38:55 AM
Quote from: Fancy Lime on March 24, 2020, 04:00:50 AM
Quote from: iainpunk on March 23, 2020, 06:47:48 PM
hey, im sorry,
your project seems very interesting, especially with the jimi record, but im not in to building "nice" pedals, im the kind of guy who builds pedals that will try to steal your girlfriend.
ill look for some zeners in my pile tomorrow and breadboard it up. and if i have zeners, ill give you a report on the sound.
iain
Oh boy, have I got the design for you! The last thing I actually managed to breadboard was a high gain fuzzstortion. The underlying motivation was that I have always found the Big Muff too polite, the Rat to be lacking in chest hair and the HM-2 to have too narrow a range of usable sounds. I have the schematic on paper and hope to digitize it one of these days. I originally planned to release it when I had the time to design a PCB for it but considering the current situation, I might as well release the schematic now. Stay tuned.
i don't have any zeners under 7 volts, so im not going to be able to breadboard that overdrive of yours, but i am very very interested in a fuzzstortion. i am going to build a grind booster circuit which turns any overdriven amp in to a gain monster, with two flavours, with the same reasoning in mind as your fuzzstortion has, hahaha
Oh, have you not heard? It was my understanding that everyone had heard:
https://www.diystompboxes.com/smfforum/index.php?topic=124063.0
Quote from: Mark Hammer on March 27, 2020, 10:57:18 AM
"Gorn". I see what you did there. A l'il bit of Graham Chapman.
I just like the word. It gives me confidence. Gorn ... gorn. It's got a sort of woody quality about it. Gorn. Gorn. Much better than `newspaper' or `litterbin'. :)
Started to fear it may have been too subtle or too obscure for anyone to notice. Glad someone got a smile out of it...
Andy
it also has a tinny switch, nur?
There are some of us who probably know the Python "liturgy" better than we know the words to our respective national anthems; likely because there was a period when it was being repeated, socially, far more often than the national anthem.
A tinny switch? Ewwww!
Quote from: Mark Hammer on March 27, 2020, 11:44:25 AM
There are some of us who probably know the Python "liturgy" better than we know the words to our respective national anthems; likely because there was a period when it was being repeated, socially, far more often than the national anthem.
A tinny switch? Ewwww!
As far as I'm concerned, this may well be a better world if we replaced all national anthems with Monty Python sketches.
Quote from: Fancy Lime on March 27, 2020, 12:02:57 PM
Quote from: Mark Hammer on March 27, 2020, 11:44:25 AM
There are some of us who probably know the Python "liturgy" better than we know the words to our respective national anthems; likely because there was a period when it was being repeated, socially, far more often than the national anthem.
A tinny switch? Ewwww!
As far as I'm concerned, this may well be a better world if we replaced all national anthems with Monty Python sketches.
but i really really like the Dutch anthem, its a sarcastic/mocking song about the spanish king being an idiot (the song came about in the Dutch-Spanish 80 years war, where we defeated the spanish and kicked out catholicism)
Granted, some are funny. Sweden for example does not officially have a national anthem. There is a song that they use as such at sporting events and so on but it has no official status. The Finnish anthem has the melody of a German drinking song and the lyrics were originally a Swedish rendition of a Hungarian patriotic folk song. Now, the German national anthem... let's not talk about the German national anthem. Let's just say you can usually tell which stanza people sing in their heads when the melody plays at football games by the stupidity of their facial expression.
Andy
i just remembered that the province i live in also has its own anthem, the province is Gelderland and the anthem is called "Het Gelderlands Volkslied" which means the Glederlands Anthem, but there is a local germanic folk metal band who have also done the anthem
Haha too funny.
Sadly our village has it's own anthem too.
A reflection of the fact we live on what was an island, surrounded by marsh bogs, back in the day.
Cut off from most places and the main access to the village was two troll bridges over the river.
The Billinghay ruffs where a gang of local tax and security lynch mob who would count your livestock in and out of the village.
Aribtrarily taxing and beating up those from outside the village who sought to date local ladies.
This was still effective after the 1950's.
Strange place. locals have been self isolating since 1100 AD.
Have a great day and dont forget to socially distance. Stay safe.
Rich
Quote from: Mark Hammer on March 27, 2020, 11:44:25 AM
There are some of us who probably know the Python "liturgy" better than we know the words to our respective national anthems
Once upon a time, in a faraway country (Batchelor, NT, Australia) I entered a talent show and introduced myself with "Ladies and Gentlemen, the national anthem of Finland", then proceeded to sing The Finland Song. When the audience figured out there was more than one verse, empty beer cans started flying, and I think some footwear too. Luckily I was able to duck under a table and finish the song from there.
In the unlikely event that there is someone out there who doesn't know The Finland Song, here it is.
We now return you to our regular scheduled programming...
Long ago and far away, Saturday Night Live overseer Lorne Michaels was half of a comedy team with partner Hart Pomerantz, and they had a brief-lived CBC television show named The Lorne and Hart Terrific Hour. One of my favorite bits of theirs concerned the newly-declared-independent Baffin Island (just to be clear, something that never actually happened). The representative from the new republic sang their new national anthem which consisted of the words "Baffin Island", repeated, and sung ceremoniously slow, to the tune of God Save the Queen.
"Ba-ffi-in Is-la-a-and, Ba-ffin Is-la-a-and, Baffn Island...etc."
It is my humble opinion that Freedonia (https://www.youtube.com/watch?v=x4OJGdtRx10) and Strongbadia (https://www.youtube.com/watch?v=zZayi1PhV80) have the best national anthems... with an honourable mention going to California (https://www.youtube.com/watch?v=jrWflCJPM4w).
Quote from: rockola on March 28, 2020, 04:44:04 AM
Quote from: Mark Hammer on March 27, 2020, 11:44:25 AM
There are some of us who probably know the Python "liturgy" better than we know the words to our respective national anthems
Once upon a time, in a faraway country (Batchelor, NT, Australia) I entered a talent show and introduced myself with "Ladies and Gentlemen, the national anthem of Finland", then proceeded to sing The Finland Song. When the audience figured out there was more than one verse, empty beer cans started flying, and I think some footwear too. Luckily I was able to duck under a table and finish the song from there.
thongs? nice one bruce.
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.
I think that overdrive is real gorn by now.
Yes, seems like we have gorn orff torpic a Bit.
Quote from: duck_arse on March 28, 2020, 10:04:34 AM
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?
Quote from: highwater on March 28, 2020, 11:22:45 AM
Quote from: duck_arse on March 28, 2020, 10:04:34 AM
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.
Not gonna happen, is it?
Cheers,
Andy
Quote from: Fancy Lime on April 03, 2020, 03:47:41 PM
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
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?
Quote from: highwater on April 04, 2020, 11:24:13 AM
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
I ordered some diodes. Would like to hear this clipping arrangement.
GORN - obviously from Star Trek! hahaha
Quote from: aron on April 05, 2020, 02:00:30 AM
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
i don't get either reference :(
Quote from: iainpunk on April 06, 2020, 09:01:33 AM
i don't get either reference :(
Either (http://www.montypython.net/scripts/wood.php). Or (https://intl.startrek.com/database_article/gorn).
Shuffled of its mortal inductor. Gorn to meet its maker.
Just waiting on Mouser....(https://s17-us2.startpage.com/cgi-bin/serveimage?url=https%3A%2F%2Fupload.wikimedia.org%2Fwikipedia%2Fen%2Fa%2Fa0%2FStarTrek-Gorn.jpg&sp=586e5eb0123e46de9065ab95f649ab30&anticache=764663)
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
Quote from: Fancy Lime on March 24, 2020, 04:00:50 AM
Quote from: idy on March 23, 2020, 10:05:12 PM
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 :icon_wink:
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.
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.
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
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.
I thought this discussion had a familiar ring to it.
https://www.diystompboxes.com/smfforum/index.php?topic=46134.0
I remember reading that post about zeners.
Quote from: Fancy Lime on April 15, 2020, 04:58:33 PM
Oh and one more thing: What's "hyper-triangular"? Never heard that term but it sounds appropriately Star-Treky.
https://www.diystompboxes.com/smfforum/index.php?topic=108221.0
Alright, so here is the promised long winded explanation. I tried to make it accessible for beginners. If questions remain, feel free to ask. I don't have to tell those who disagree with my reasoning that they are welcome to express their disagreement, because they will anyway. But please don't hold back, I may be wrong about everything. Anyway:
What does an opamp do?
An opamp tries to keep the non-inverting and inverting input at the same voltage at all times by varying its output accordingly. What the output needs to do in order to make that happen and whether that is possible at all, depends on the configuration of components around the opamp. The negative feedback loop generally allows the opamp to keep the inputs equal and therefore to act as a linear amplifier. Let's look at the basic non-inverting amplifier topology:
(https://i.postimg.cc/5X6JJccv/noninv.png) (https://postimg.cc/5X6JJccv)
We shall call the voltage at the inverting input Vi, the voltage at the non-inverting input Vn, and the voltage at the output Vo. The feedback resistor Rf and the ground resistor Rg form a voltage devider between the output and the inverting input. Because the transfer function of the voltage divider is: Vo = Vi*((Rf+Rg)/Rg) , and we know that Vi = Vn, we can calculate the amplification of the stage. To nobody's surprise, we can rearrange the last equation to the more familiar form of the non-inverting opamp transfer function: Vo = Vn*(1+Rf/Rg).
That's all great but not really news to anyone. The important point to make here is, that the voltages at the inputs are always the same at all times (assuming we stay within the limits of the opams capabilities), no matter what resistors we put in. That means, Vi is independent of Vo and completely determined by Vn. So how do the output and the inverting input communicate if the voltage of the latter is independent of the former? Current!
What is a resistor?
One way of looking at a resistor is as a translator between voltage and current. An ideal resistor (and for our purpose we can assume resistors to be ideal) exhibits Ohmic Behavior, i.e. it follows Ohm's Law: V = R * I, for the entirety of it's specified use conditions. Non ohmic devices still follow Ohm's Law but only "locally" in an I/V plot; this will be important later. It is no coincidence that if we plug Ohm's Law into the transfer function of the non-inverting opamp amplifier (or the inverting one, for that matter), we find that the currents through Rf and Rg are the same. This is how the opamp keeps the inverting input the same as the non-inverting. It pushes exactly as much current through Rf as is "lost" to ground via Rg (we will assume that the input currents of the opamp are very small compared to the currents going through the resistors and therefor need not be considered). And Ohm's Law tells us what voltage the output needs to assume for that to happen.
So what? And what about diodes?
If we replace the feedback resistor with a pair of diodes, what changes and what remains the same? We will ignore the parallel resistance from the gain pot for this discussion because it is not important for an understanding of the principles. Vi is still the same as Vn, so the voltage across Rg is still Vn. Because Ohm's Law still applies to Rg, the current that the opamps output needs to supply to the inverting input via the negative feedback loop also remains the same. But diodes are not ohmic resistors, so what voltage swing the output needs to perform in order to supply that current is different.
What is a diode?
An ideal diode is something that blocks all current if the voltage across it is less than the threshold (aka. breakdown) voltage but lets all current pass above that threshold. This is how we mostly think about diodes: A perfect insulator or a perfect conductor, depending on the voltage across it. We are all aware that this is not *really* true. However, the significance of the error we make by this simplification is *very* different for different diodes and different circuit topologies. The region where it is usually most interesting for our purposes, is the so called "knee", the transition from mostly blocking current to mostly letting it through. In this knee region, the current through a diode is still a function of the voltage across it, just not a linear one as Ohm's Law would be. If we look at infinitesimally small changes in voltage, we can derive an inclination from the I/V curve, which is nothing other than an inverse resistance because Ohm's Law tells us I/V = 1/R. So we can look at a diode as a variable resistor that changes it's resistance as a function of the applied voltage: the resistance becomes smaller as the voltage becomes larger. Interestingly, the change of "local resistance" seems to be a linear function of voltage for extended regions along the current axis, at least for Zener diodes in forward breakdown. The resistance of the diode, which changes with voltage and current, is called Dynamic Resistance. We will be coming back to that. The total equivalent resistance of a diode is the dynamic resistance integrated from I=0 (or V=0) to the current (or voltage) of interest.
What does that mean for our opamp circuit?
If we think of the diodes as a variable resistor and put that in place of Rf from earlier, we can understand the implications of clipping diodes and their characteristics for the output voltage of the circuit. At low Vo, Rf is large, so the amplification is large. As Vo increases due to the large amplification, Rf is getting smaller, making the amplification factor smaller as well. This rounds of the peaks of the waveforms. Huston, we have clipping!
What is special about low voltage Zener diodes?
Zener diodes actually exhibit two different kinds of breakdown in forward direction. At high voltages, the so called avalanche breakdown is dominant. Avalanche behavior is pretty close to that of an ideal diode, with high resistance before the trheshold, a sharp knee, and after the knee, the current that can pass through the device is almost completely independent of the voltage across it (i.e. very low remnant resistance). This is the characteristic that I always considered "typical Zener bahavior" and it is what makes these things so useful as voltage regulators. However, at Zener voltages below about 5V, the behavior is very different. Here, actual Zener breakdown dominates, which is a far cry from ideal diode behavior. That is why low voltage Zeners are actually pretty bad as voltage regulators unless they are operated at a constant and well defined current. You can see the dependece of the characteristic shape of the I/V curve in Fig. 14, taken from the Vishay datasheet of the BZX84 series Zeners:
(https://i.postimg.cc/V5RmBksz/bzx84-Fig14.png) (https://postimg.cc/V5RmBksz)
https://www.vishay.com/docs/85763/bzx84v.pdf
The fundamental curve characteristic trends are the same for other Zener series because they are dictated by the physics of the breakdown mechanism. As you can see in the figure, the "nominal Zener voltage" is taken at 5mA. The 8.2 Zener breaks down at 8.2V no matter if the current pushed through is 2mA or 35mA. But the 2.7 Zener varies a lot. At 2.7V it can pass 5mA, but at 2.0V it can already pass 2mA, and at 3.5V it can pass 45mA.
But which of these numbers is relevant for us?
If we assume that our ground resistor Rg is 1kΩ, and we have a 100mV signal at the non-inverting input, then the opamp will make it so that the same voltage is present at the inverting input and our old friend Georg tells us that 0.1mA flow across Rg. And as we have seen earlier, the same amount of current needs to flow through the diodes. Well and good but the voltage corresponding to 0.1mA in the I/V plot is pretty impossible to read from Fig. 14. Luckily, the Vishay datasheet also has the dynamic resistances plotted in Fig. 3. This plot shows us that the dynamic resistance of the low voltage Zeners is pretty high at down to 0.2mA. If we extrapolate the curve for 2.7 a bit down from 0.2mA to 0.1mA, we land somewhere close to 1kΩ. If these data would extend another two orders of magnitude below where they do, we might even be able to calculate the static resistances at the relevant currents and thereby extend the I/V plot to where we need it. With the data we actually have, we can only reason quantitatively. So let's do that.
(https://i.postimg.cc/p5yGFCXh/bzx84-Fig3.png) (https://postimg.cc/p5yGFCXh)
Let's define the dynamic resistance a little more concisely. It is essentially an application of Ohm's Law to an infinitesimally small section of the I/V or V/I graph. The former formulation gives us inverse resistance, which is a bit cumbersome, so let's take the latter formulation. Then we get:
ΔV/ΔI = r
Wherein the Delta denotes local change and lowercase r is the dynamic resistance. What that means is, that if we have a large r, then we need a lot (relatively speaking) of change in voltage across the diode in order to achieve a given change in current. In Fig. 3 we see that for the 2.7 and 3.6 Zeners, the dynamic resistance is fairly large (especially compared to the medium voltage Zeners in Fig. 4) and it does not drop off as steeply as for the 4.7, 5.1, and 5.6 models. This is relevant because our input signal is dynamic. Assuming again Rg=1kΩ, we get a current demand through the diodes of 0.1mA at 100mA input signal (about "average" for single notes, very approximately, of course) and 1mA at 1000mV (about peak voltage of a chord). Becuse r stays high in this range, we need a big ΔV throughout relevant guitar input signal conditions to satisfy the current conditions of the opamp. And this is what makes the clipping knee soft. But it is different than using a resistor in series with a pair of diodes because the resistor is not dynamic. With the Zeners we sneak into clipping very gradually. With the series resistor, we would still have a sharp knee but a "hill on the plateau" above it.
"Normal" diodes also have a dynamic resistance. But it is usually desirable for most diode purposes, that this be as small as possible at the targeted operating currents and, more importantly, drops off as steeply as possible around the threshold voltage. The other notable exception to this behavior apart from low-V Zeners are germanium diodes. This is one of the main reasons that prompted development of Si diodes as a better alternative. Unfortunately, Ge diodes are plagued with a bunch of other problems, as well, like enormous temperature effects, reliability issues, huge fabrication tolerances, and an utter lack of detailed datasheets for types that are still obtainable today. So I would not design with Ge diodes in this day and age if I can help it.
You can hear the soft clipping transition effect very nicely in Arons sound samples, especially the latter third of each. With the standard diodes, there is this somewhat brittle sizzling as long sustained notes drop out of clipping. With the Zeners, the clipping fades into the background much more seamlessly and naturally. You can also hear that the Zener arrangement reacts more strongly to playing dynamics, although this is better experienced by playing yourself than by listening to a recording.
Some design tips:
When using 2V7 Zeners as clippers, know your currents and choose your circuit impedances accordingly. In a non-inverting opamp stage, use 500-5000Ω of resistance for the ground leg, else you get too high thresholds or too little current variation for the Zener effect to matter. Same goes for the input resistor when using this arrangement in an inverting opamp stage. By strategically placing additional caps and resistors in the feedback loop and the ground (or input) leg, you can vary the impedance independently of the frequency response. This will affect the frequency dependent clipping characteristics of the Zeners more than it would normal diodes. This may be worth experimenting with but I have not tried it.
Cheers,
Andy
Quote from: Fancy Lime on April 16, 2020, 01:16:35 PM
Alright, so here is the promised long winded explanation. I tried to make it accessible for beginners. If questions remain, feel free to ask. I don't have to tell those who disagree with my reasoning that they are welcome to express their disagreement, because they will anyway. But please don't hold back, I may be wrong about everything. Anyway:
nice write up, 10/10, would watch again
- Iain
One more thing I forgot to mention: The gain pot has of course some influence on the currents and therefore on the Zener clipping. That influence is different when using a gain pot in the ground leg, compared to one in the feedback loop. We will leave it as an exercise for the reader to figure out how exactly :icon_wink: Or you can just try it, I guess.
Andy
This mod sounds good with my tiny Champ! Even though the Champ is bright, the pedal kicks in and there's no more "fizz" like the usual 1N4148 clipping. Very nice!
Here's my build:
(http://www.bouron.org.uk/marc/gorn033.jpg) (http://www.bouron.org.uk/marc/gorn032.jpg)
I just gave it a very quick test with my Champ-a-like and it's very versatile - thanks Andy. I just need to turn my "tinny/woody" switch around... :icon_redface:
In case anyone finds it useful, this is the vero layout I used:
(http://www.bouron.org.uk/marc/pedals/gorn-vero.png)
Extremely useful, indeed..!!! :icon_eek:
Love that Gorn on that pedal!!!!
I might "accidentally" build one of these instead of something else in my queue. :icon_wink:
Be careful, Eric! :icon_eek:
Quote from: bluebunny on July 27, 2020, 11:18:55 AM
Be careful, Eric! :icon_eek:
The danger has passed. Turns out, I just don't have the parts. I have the op amp and the scrap of veroboard, but I don't seem to have anything else (diodes and pots, mainly) that I would need at the moment. I have zener diodes in every available voltage above 3.0V though. :icon_neutral: