I have 3 OCD pedals here... 3 different revisions with 3 different Mosfet Clipping Setups...
My Rev3 pedal is one of my favorite & the Rev 6 is my least... R4 is nice too.
My question is, what is my Rev 3 doing? Ive never seen the added diode connected this way with the GATE NOT CONNECTING TO THE SOURCE BUT TO THE OTHER SIDE OF THE DIODE??.. but like I said it sounds GREAT!!!
Obviously Rev 4 & 5 are connected using the "Body Diodes"... the Rev3 I don't get & wonder if there's something tricky or cool going on but don't understand enough to see it & would like to learn?
Thanks.
(http://www.khstudio.us/DIY/OCD%20Clippings.jpg)
Bump...
Anyone?
??? I don't really know but I'll take a stab in the hopes of spawning more knowledgeable folks to chime in and correct me. It puts the voltage on the source lower than the voltage on the gate, thereby changing the amount of drop seen across the whole thing, making it asymmetrical with the other FET. Do I win a prize? :icon_biggrin:
Ha, lol... no prize but I do appreciate you helping to get the discussion started & hopefully, like you said, others with more knowledge will chime in.
Not only do I want to learn a little bit about what could be going on, I also wanted to share what I found because I've NEVER seen anyone post or talk about connecting a Mosfet like this.
Who knows, it MAY be something cool or different & as I said before, it is the best sounding (to me a some of my friends with good ears) out of the 3... It's more "Organic & Dynamic"
BTW, all of us are mostly Rock, Blues & Country players to give an idea of the "Sound" we like. Also, most of my listening tests are thru a 65 Blackface Deluxe on the Raw/Channel 1 (no verb or vibrato).
I also like it into a Marshall Plexi or JCM800 amp with Greenbacks.
I know this is an old topic, but is this relevant to the question....?
http://www.diystompboxes.com/smfforum/index.php?topic=90474.0 (http://www.diystompboxes.com/smfforum/index.php?topic=90474.0)
Your circuits disagree with my notes.
See
http://revolutiondeux.blogspot.com/2008/07/fullclone-vlod-fulltone-ocd-by.html (http://revolutiondeux.blogspot.com/2008/07/fullclone-vlod-fulltone-ocd-by.html)
These show the G+D tied together. Your schematic has G+S.
S+G is basically a typical silicone diode (Yawn). G+D is the magical mosfet clipping.
Has anybody verified the factual mosfet configuration of the OCD yet?
I'm actually dealing with the same issue in my Joyo Ultimate Drive. Schematicwise the clipping stage should be the same, but at least in my Ultimate Drives, I have two of them, the real PCB doesn't correspond to the schematic.
From the above sketches it seems to me that it could be the same with the OCD!
I've been reading a lot about diode and mosfet clippers and the issue becomes more obscure the more I read about it. Obviously it is somehow working, but the explanations don't make real sense to me.
Quote from: Bastelliese on June 04, 2017, 09:05:29 AM
Has anybody verified the factual mosfet configuration of the OCD yet?
Apparently many people - the OCD is a well documented/cloned circuit. Most of the schematics floating around are probably correct. There are quite likely some that aren't - the internet being what it is. Look for consensus.
Some Joyos have a verified manufacturing "mistake" - even on the PCB silkscreen. So it may be the pedal not the schematic that is causing your confusion. Note however that the V3 in the diagram first posted here is probably wrong.
Finally I think I got it.
What bothered me was the fact that contrary to the schematic, in the real circuit Sources+Gates are tied together and therefor the Mosfet will work as a simple Silicium diode because of the inherent body diode. Where should the magic happen, when actual Drains+Gates should be connected?
It took me quite a time to recognise that if two Mosfets are connected antiparallel (with or without Ge-diode) always the body-diodes will do the clipping job because of their lower forward voltage. In other words it doesn't make a difference whether S+G or D+G are tied together.
Back to the original post.
Assuming the most left diagram to be probably wrong, I'd still like to know how the additional diode between Source and Gate/Vcc affects the circuit. As KHStudio mentioned some years ago, "Not only do I want to learn a little bit about what could be going on, ... because I've NEVER seen anyone post or talk about connecting a Mosfet like this." The same with me.
Quote from: Bastelliese on June 06, 2017, 04:36:18 PM
Finally I think I got it.
What bothered me was the fact that contrary to the schematic, in the real circuit Sources+Gates are tied together and therefor the Mosfet will work as a simple Silicium diode because of the inherent body diode. Where should the magic happen, when actual Drains+Gates should be connected?
It took me quite a time to recognise that if two Mosfets are connected antiparallel (with or without Ge-diode) always the body-diodes will do the clipping job because of their lower forward voltage. In other words it doesn't make a difference whether S+G or D+G are tied together.
Back to the original post.
Assuming the most left diagram to be probably wrong, I'd still like to know how the additional diode between Source and Gate/Vcc affects the circuit. As KHStudio mentioned some years ago, "Not only do I want to learn a little bit about what could be going on, ... because I've NEVER seen anyone post or talk about connecting a Mosfet like this." The same with me.
I'm not sure what schematic you are referring to, but all of the ocd schematics I recall seeing have g&s tied. EDIT: Never mind - shoddy brain cells.
The additional diode adds voltage drop creating asymmetry when only one leg uses a diode. With the diode in place you can reverse the MOSFET and you have the MOSFET clipping.
Quote from: GGBB on June 04, 2017, 11:50:15 AM
Note however that the V3 in the diagram first posted here is probably wrong.
Fyi it wasn't "wrong" for my V3. It was & still is my favorite (clipping style) out of the 3 I posted.
Quote from: KHStudio on June 06, 2017, 10:28:17 PM
Fyi it wasn't "wrong" for my V3. It was & still is my favorite (clipping style) out of the 3 I posted.
Well, I did say "probably" wrong. Can you post a photo? I've seen schematics where gate is tied to drain and where gate is tied to source, both with and without a diode at one end or the other, but never one where there's a diode
between gate and source (or even between drain and source for that matter). Did you find that diagram somewhere or is it your own? If you have a link to the original source of it that would be appreciated by many I'm sure.
I'd be interested in what others think about this arrangement. As far as I can tell, it fixes the gate-source voltage at the value of the diode's forward voltage which in the case of the 1N34A is about 0.3V. This is below the min g-s threshold voltage for the 2N7000 (and BS170) so the FET would be "off" just as it would be with the gate tied directly to source (Vgs=0V). So the only way the MOSFET could pass current is via the body diode - which is the standard OCD clipping. If there's anything else going on I'd love to know more.
I can be wrong, but i think the V3 not use that germanium diode. :icon_eek:
The additional diode adds voltage drop creating asymmetry when only one leg uses a diode. With the diode in place you can reverse the MOSFET and you have the MOSFET clipping.
[/quote]
Could you please say it in other words? I don`t understand what you are trying to say. I don't know, whether it's because of my poor electronic skills or due to my German-guy-style English. Expect the worst and presume both of it.
Quote from: GGBB on June 06, 2017, 11:25:27 PM
Quote from: KHStudio on June 06, 2017, 10:28:17 PM
Fyi it wasn't "wrong" for my V3. It was & still is my favorite (clipping style) out of the 3 I posted.
Well, I did say "probably" wrong. Can you post a photo? I've seen schematics where gate is tied to drain and where gate is tied to source, both with and without a diode at one end or the other, but never one where there's a diode between gate and source (or even between drain and source for that matter). Did you find that diagram somewhere or is it your own? If you have a link to the original source of it that would be appreciated by many I'm sure.
I'd be interested in what others think about this arrangement. As far as I can tell, it fixes the gate-source voltage at the value of the diode's forward voltage which in the case of the 1N34A is about 0.3V. This is below the min g-s threshold voltage for the 2N7000 (and BS170) so the FET would be "off" just as it would be with the gate tied directly to source (Vgs=0V). So the only way the MOSFET could pass current is via the body diode - which is the standard OCD clipping. If there's anything else going on I'd love to know more.
I couldn't have said it in a better way. I'm very interested, too.
Quote from: Bastelliese on June 07, 2017, 06:37:20 AM
Quote from: GGBB on June 06, 2017, 05:38:40 PM
The additional diode adds voltage drop creating asymmetry when only one leg uses a diode. With the diode in place you can reverse the MOSFET and you have the MOSFET clipping.
Could you please say it in other words? I don`t understand what you are trying to say. I don't know, whether it's because of my poor electronic skills or due to my German-guy-style English. Expect the worst and presume both of it.
I'll try. Adding a diode will cause it to clip less. And when only one of the MOSFETs uses a diode, the two clipping levels are unbalanced which makes the distortion asymmetrical. Any better?
How about this?
Die zusätzliche Diode fügt einen Spannungsabfall hinzu, der eine Asymmetrie erzeugt, wenn nur ein Bein eine Diode verwendet. Mit der Diode können Sie den MOSFET umkehren und Sie haben den MOSFET Clipping.
Ok, thank you, but Google translator doesn't seem to be the right remedy. Let's try the old school method, silly questions and good answers:
First of all, do you refer to the most left diode in the above drawing? Does leg mean half-wave? What does "reverse the MOSFET" mean.
Basically I know about symmetrical and asymmetrical clipping, assuming the middle drawing being asymmetrical clipping, the right one being symmetrical.
But how about the left so called Version 3?
Quote from: Bastelliese on June 07, 2017, 01:51:53 PM
First of all, do you refer to the most left diode in the above drawing? Does leg mean half-wave? What does "reverse the MOSFET" mean.
Basically I know about symmetrical and asymmetrical clipping, assuming the middle drawing being asymmetrical clipping, the right one being symmetrical.
But how about the left so called Version 3?
Yes - by leg I mean half-wave. Each MOSFET is 1 of 2 legs of the clipping arrangement. One leg clips positive signal, the other clips negative signal.
Reverse the MOSFET:
>---[ diode || ]---[ S-G D ]--->
becomes
>---[ diode || ]---[ D-G S ]--->
V3 - in my opinion - will produce the same clipping as
V6 EDIT: V4.
"V3 - in my opinion - will produce the same clipping as V4."
Why do you think so? Or is it just a feeling?
Do you mean reversing 1 or 2 mosfets?
When you have two Mosfet-clippers (D+G tied together) antiparallely connected. the result will be the same as S+G connected, because of the body diodes. They will conduct at lower voltages than the mosfet. But the diode in the left picture alters the behaviour of at least the downside Mosfet, I think.
Quote from: Bastelliese on June 08, 2017, 05:37:13 AM
"V3 - in my opinion - will produce the same clipping as V4."
Why do you think so? Or is it just a feeling?
Do you mean reversing 1 or 2 mosfets?
When you have two Mosfet-clippers (D+G tied together) antiparallely connected. the result will be the same as S+G connected, because of the body diodes. They will conduct at lower voltages than the mosfet. But the diode in the left picture alters the behaviour of at least the downside Mosfet, I think.
I explained above: http://www.diystompboxes.com/smfforum/index.php?topic=102882.msg1095614#msg1095614. But to elaborate, the extra diode only adds some voltage drop to the body diode. That MOSFET never clips in the other direction because of the anti-parallel MOSFET's body diode. So
where the gate is tied doesn't really matter. But the math I explained above suggests that MOSFET is always "off" anyway.
You would need two "reversed MOSFETs" for symmetrical clipping through the MOSFET not the body diode.
I'm sorry, but I'm still not really satisfied. Of course the Ge adds asymmetry to the clipped signal. That's undoubted. Add another Ge to the upper Mosfet and it's symmetrical again. What bothers me, is the way the Ge is connected. You are assuming that V3+V4 in the above picture will produce the same clipping, but KHStudio claims hearing a difference. Sure, there could be some differences in parts values or anywhere else in the circuit not mentioned yet. But I assume it's the manner of connecting the diode.
We also agree that there is no Mosfet-clipping in the OCD circuits, all being body diode clipping.
The body diode of a N-channel Mosfet is located between drain and source, drain being cathode. What about the gate? Why is it normally connected to source, couldn't it be left open and what was the differnce?
I may very well be wrong, but you can't just claim I am wrong without backing up that claim with evidence. The scientific method is what we use to test a hypothesis, which in this case is "hearing a difference." Use the scientific method to validate the claim: There are a limited number of ways that current can flow through the two clipping arrangements according to the terminals connected to signal. Make a list of them all and then apply physics and math along with the known properties of the diodes and MOSFETs to test whether or not they actually will pass current from an amplified guitar signal with no DC bias present. If you have access to a scope, use that too. I applied known math to known properties to make my conclusion. I invite anyone to prove me wrong or show me that there are other properties I didn't consider - I already stated that a few days ago.
EDIT: You actually can claim anything you want :). But your claim has no merit unless you back it up with evidence.
Excuse me, I did not want to be overbearing nor question your knowledge. I didn't mean to proof you're wrong or even offend you I'm sorry for that. I just wanted to keep the topic open to get futher beyond and my impression was that you aren't that sure either. Please keep in mind that I´m writing in a foreign language which I'm not used to. So there might be some 'between the lines' content or connotation that's not intended. I'm sorry for that!
please, somebody, draw the circuit out. let's see what it is we are working with, if only the clipping section. as far as I understand this arrangement, having read an explanation of the correct wiring for mosfet clipping, the external series diode opposes the internal mosfet diode (the body diode you are all referring to?), so it can't conduct. I can't remember the correct gate connections, tho, I was hoping someone else would point to the discussion in question.
Well, I might not be the perfect guy to explain the circuit and mosfet clipping, but I'll try my best to keep the discussion going.
First of all here's a schematic, which seems to have most common concensus:
(https://s24.postimg.org/4xktwzegx/ocd_schem.gif) (https://postimg.org/image/4xktwzegx/)
For me the most helpful resources were the thread 'Understanding MOSFET clipping' at freestompboxes.org and the website of amzfx http://www.muzique.com/lab/zenmos.htm which is a real good starting point to get into this stuff.
But I'll try to explain in my own words.
In circuits like OCD and similar, e.g. Joyo Ultimate Drive, where two Mosfets in anti-parallel wiring form the clipping stage, it doesn't matter whether the drains+gates or the sources+gates are connected and therefore the mounting direction isn't of interest. This is because of the forward voltage of the body diodes being far lower than the gate/source threshold of the mosfets. Therefore the body diode will conduct and clip the signal. The body or inverse diode is intrinsically formed between source and the substrate and between drain/substrate. Actually there are two body diodes per Mosfet, but usually the source/substrate diode is internally shorted out. When we talk about the body diode we refer to the drain/substrate diode and since substrate and source is shorted out it's referring to the diode between source and drain. With drain being the cathode of the diode assuming we have n-Channel Mosfets. The body diode behaves like a simple Silicium diode with a forward voltage of about 0,7V.
For OCD and similar circuits this means, there is no magical Mosfet clipping at all.
After all I think most of the unique sound of UD/OCD comes from the overdriven TL082.
Nevertheless I have to admit that I recently saw a video on youtube, where a guy demonstrated his modded Boss DS1. He replaced the original clipping diodes D4/5 with Mosfets, IF520 I suppose. The only other alteration he did was to change the tone capacitor to 22nF, I think. And this mod altered the sound drastically. Assuming the above written to be true, this would say the sound difference only comes from altering the tone cap value. This seems also unlikely.
As far as I know there are three ways of getting Mosfet clipping (drain+gate connected):
-Using just 1 Mosfet. The body diode and the "Mosfet-diode" are anti-parallel causing an asymmetrical clipping.
-Using 2 Mosfets in series, opposite direction. One half-wave will pass the Mosfet and the body diode of the second one, the first body-diode and second Mosfet will block current. For the second half-wave vice versa. This is symmetrical clipping.
-Using 2 Mosfets in anti-parallel plus 1 diode in series to each Mosfet. The additional diodes must be oriented against the body diodes to block them and therefore with cathode to drain or anode to source. This will give symmetrical clipping with slightly higher g/s-threshold. Of course it can be made asymmetrical by adding another diode for one half-wave.
Now let's return to the original post by KHStudio and his drawing.
As mentioned earlier, all 3 versions show normal diode clipping. I own two Joyo Ultimate Drives and in the schematic gates and drains are tied together, whereas on the real PCB the gates and sources are connected. But as we learned earlier, this doesn't make any difference. I suppose this being the same for the OCDs.
The schematic on the right shows a symmetrical diode clipping arrangement, long known from circuits like Tubescreamer.
The middle one (V4) has a Germanium-diode in series to one of the body diodes, thus creating an asymmetrical clipping stage, known for example from Boss SD1.
Here the gates and sources are providently tied together, to prevent the Mosfets from unwanted gate/source current flow and stable conditions.
As GGBB considers, and I basically agree with him, the left V3 seems to be very similar to V4 and therefore should produce asymmetrical clipping.
But the question is still the same. Why is the diode connected in this uncommon way?
Everybody who's interested in Joyo Ultimate Drive, should read the thread 'Joyo - JF-02 Ultimate Drive "2010" MISTAKE' on freestompboxes.org.
Good night
Sascha
Quote from: Bastelliese on June 09, 2017, 06:57:53 AM
Excuse me, I did not want to be overbearing nor question your knowledge. I didn't mean to proof you're wrong or even offend you I'm sorry for that. I just wanted to keep the topic open to get futher beyond and my impression was that you aren't that sure either. Please keep in mind that I´m writing in a foreign language which I'm not used to. So there might be some 'between the lines' content or connotation that's not intended. I'm sorry for that!
Not offended - sorry if I cam across that way. The point I'm trying to make is - let's not imagine something magical is occurring, let's use principles of math, physics, and electronics to determine what is happening. Electrons obey the laws of physics - always.
Quote from: Bastelliese on June 09, 2017, 03:24:24 PM
Why is the diode connected in this uncommon way?
That's a great question.
What we don't know:
- the source of the diagram
- the source of the OP's pedal that implements "V3" clipping
- whether the "V3" pedal is an original OCD or a clone
- if a clone, whether it was a production pedal or pcb or a hobbyist pcb/vero/perf
What we do know:
- the original OCD V3 has been reasonably well documented and does not match the OP's "V3" diagram
- apparently not one single other report of an OCD (original or clone) having this style of diode connection
- this could not be an assembly error on a production pcb - you can't connect the diode like that by misplacing or misorienting components as happened in some of the Joyos - the actual traces of the pcb would have to change
Based on that, I'd argue there is no evidence to believe OCDs or production clones were ever made like this. We have one person's claim and absolutely nothing more to support it, versus solid evidence that V3 isn't connected this way. I will however accept that it could be someone's own creation.
I am not saying the OP is lying - I am sure he/she believes they are correct.
I am not saying the OP is stupid - I don't know him/her but I assume they are human, and, like every human I know, makes mistakes on occasion.
I am not saying that the pedal doesn't sound different/better to the OP - I'm suggesting that it's probably not the diode that is the reason. Lets suppose for a moment that the pedal actually does have the diode connected that way. Then, if my analysis of the physics is correct, it would clip the same as "V4" anyway, so there must be another explanation for the difference that the OP hears - it's not the diode.
So in answer to the quoted question - what I am suggesting is -
the diode isn't normally ever connected that way. If it does exist, it does so either by a layout mistake in someone's DIY project, or by intentionally trying to be clever or misleading, or by a misunderstanding of how it is normally connected.
My vote goes to mistake - either on the part of the OP, or on the part of the pedal builder. If the OP or anyone else can clear up the unknowns I listed, I might be swayed to believe otherwise.