Author Topic: OCD - 3 different revisions with 3 different Mosfet Clipping Setups...  (Read 10203 times)

KHStudio

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.

« Last Edit: May 25, 2013, 02:29:08 AM by KHStudio »

KHStudio

Bump...

Anyone?
« Last Edit: May 25, 2013, 02:38:44 AM by KHStudio »

GGBB

 ??? 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:
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KHStudio

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.

ch1naski

Re: OCD - 3 different revisions with 3 different Mosfet Clipping Setups...
« Reply #4 on: October 06, 2013, 02:14:34 PM »
I know this is an old topic, but is this relevant to the question....?
http://www.diystompboxes.com/smfforum/index.php?topic=90474.0
Mockingbird wish me luck.

gjcamann

Re: OCD - 3 different revisions with 3 different Mosfet Clipping Setups...
« Reply #5 on: October 06, 2013, 10:37:16 PM »
Your circuits disagree with my notes.
See
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.

Bastelliese

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.

GGBB

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.
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Bastelliese

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.

GGBB

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.
« Last Edit: June 06, 2017, 10:47:07 PM by GGBB »
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KHStudio

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.

GGBB

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.
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GiovannyS10

I can be wrong, but i think the V3 not use that germanium diode.  :icon_eek:
That's all, Folks!

"Are you on drugs?"
-ARSE, Duck.

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Bastelliese

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.

Bastelliese

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.

GGBB

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.
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Bastelliese

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?

GGBB

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:

Code: [Select]
>---[ diode || ]---[ S-G D ]--->

becomes

Code: [Select]
>---[ diode || ]---[ D-G S ]--->

V3 - in my opinion - will produce the same clipping as V6 EDIT: V4.


« Last Edit: June 07, 2017, 07:08:47 PM by GGBB »
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Bastelliese

"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.

GGBB

"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.
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