Is this the secret of the Fulltone OCD ?

[solderman]

Hi all. I have long wondered why the OCD has such glimmer over its name, Is it really that good and if it is why has no one reversed engineered it and published the findings. Yesterday I stumbled on a, in my opinion, really grate site http://www.pedalarea.com/index.htm 
In witch I found some very interesting reading.

This is quoted from the OCD part of the page.

"The overdrive/distortion portion of the circuitry is based on a simple opamp gain stage with negative feedback and with a pair of crosscoupled clipping diodes at the output to clip the signal and thus create the distortion. A similar circuit topology is used in many other distortion pedals as well (e.g. the Proco Rat), but Fulltone uses a very clever modification that significantly separates this unit from most others. First off, instead of traditional diodes Fulltone uses NMOS transistors in diode configuration (gate and drain short-circuited), which already accredites for a great difference in tone. But the biggest difference is this: In the Proco Rat the two crosscoupled clipping diodes are directly connected to ground, meaning that as soon as the signal exceeds the threshold voltage Vth of the diodes (usually 0.7V), the signal gets clipped resulting in an output signal with a peak voltage of +/-Vth independent of the input signal amplitude. This naturally creates the wanted distortion, but also loses the dynamics of the signal. In the OCD, instead of connecting the diodes to ground potential, Fulltone uses a floating bias point which is connected to the input of the opamp stage. This creates a clipping threshold proportional to the input signal amplitude, resulting in a strong dependancy of the amount of distortion created to the input signal. This is the core idea of the enormous dynamics of the OCD."


I fully understand the concept of how this is don and understand that this fetcher can be very useful in many types of effects and situations. But I don't know enough about how to design a floating bias of this kind. I assume it is something like a voltage regulated "valve" that lest the current pass if the voltage is low and opens to the diodes if it gets stronger. But how do I design this??? I assume I could use a FET and let it open at Vx but wouldn't that be more like an on/off switch instead of a smooth open and close ??

Somebody out there with more knowledge than me who can help me out??

[Steben]

Don't overshoot, my friend.
I think the fulldrive MOSfet is quite similar.

[Ben N]

Search is your friend.
http://www.diystompboxes.com/smfforum/index.php?topic=62812.0
Although not a schematic, MarkM's "OVD" layout may give you an idea of what is going on--no additioinal active elements required.

[ubersam]

...But I don't know enough about how to design a floating bias of this kind...

That "floating bias" is nothing more than the bias supply network which is common in many circuits. Look at the TS schematic, for example: http://www.generalguitargadgets.com/pdf/ggg_its8_sc.pdf - R9, R10, & C7 comprises the biasing network. The bias supply (Vref, Vr, V1/2, etc.) will be taken from the junction where all three components meet.

Read R.G.'s article about designing a bias network: http://www.geofex.com/circuits/Biasnet.htm

[solderman]

Thanks
I can see that this is nothing more than a tub screamer with MOSFET clipping. I have already built a TS 808  from Tonpad. I added an on (off) on switch to switch between standard- non- MOSFET clipping and it sounds really nice.

However I just can't let go of the thought that one could have a dist effect that work like a combination of a compressor and an envelope filter. To have the benefit of the compressor to even out the level from the pickups and then, instead of changing the frequency it kicks in more distortion the harder you strum your cords. I have built a Ge Fuzz that can do a little bit of this but the difference in level is still to big (and I don't like the sound). What I want is same level but with escalating distortion depending on how hard the strings are hit.

Possible??

//Regards
Solderman

[DougH]

From my experience, the mosfet clipping, esp with diodes in series gives you a higher Vth which IMO gives you more dynamics. From my memory anyway...

[cheeb]

To have the benefit of the compressor to even out the level from the pickups and then, instead of changing the frequency it kicks in more distortion the harder you strum your cords.

Isn't this just a really hot tube amp with a smallish speaker?

[Caferacernoc]

I also think a saturation control applied to the clipping diodes in a Dist+ diode to ground style clipper does much the same thing to retain dynamics.

[wampcat1]

the ocd actually has been reversed quite a while ago...

[DougH]

I think the "secret" was he is a builder with a reputation and at the time of its release it was the new baby all his customers wanted to hold.

These days it seems like yesterday's news... YMMV.

[Steben]

I think the "secret" was he is a builder with a reputation and at the time of its release it was the new baby all his customers wanted to hold.

While (some) holding a fulldrive II "MOSfet", which in fact is about 90% the same...

[DougH]

BTW, the MOSFET clipper in the Shaka Braddah circuit I posted above sounds really awesome. We all loved it when we were building SB3's, what was it, almost 10 yrs ago now? I still get out my Shaka Braddah and use it- it is a really good overdrive pedal.

I know I've mentioned this before, but the SB3 used to be the de facto "gateway" project for this group. Everyone in the group built one at one time or another. It was the "reference" overdrive and a lot of overdrive ideas spun off of it.

[Phorhas]

I know I've mentioned this before, but the SB3 used to be the de facto "gateway" project for this group. Everyone in the group built one at one time or another. It was the "reference" overdrive and a lot of overdrive ideas spun off of it.

Indeed. One of my early delights

[jamsden]

The clipping MOSFETs are connected to VB, not the input of the op-amp. This simply biases the MOSFETs (used as diodes in this case) to raise the clipping voltage, giving more headroom than would be accomplished by simple diodes to ground. My friend Doug and I build a similar circuit with biased clipping diodes back in the early 70's and I still have an implementation of that in my old Fender reverb unit, using an extra gain tube and the offset clipping diodes.

[GGBB]

The clipping MOSFETs are connected to VB, not the input of the op-amp. This simply biases the MOSFETs (used as diodes in this case) to raise the clipping voltage, giving more headroom than would be accomplished by simple diodes to ground. My friend Doug and I build a similar circuit with biased clipping diodes back in the early 70's and I still have an implementation of that in my old Fender reverb unit, using an extra gain tube and the offset clipping diodes.

Welcome to the forum. This topic hasn't been active in over 13 years - you might want to remember to check that before you post a reply.

[ElectricDruid]

Personally, I've got no problem with zombie threads. In some ways it makes more sense than spawning endless new threads about similar topics. In this case, it's interesting to hear someone suggest "prior art" from the early 70s. Not that I'm suggesting that the OCD pedal was necessarily stealing anyone's idea - most of the best ideas have been invented over and over again. It's still interesting to see where the first implementation might have been done, though - in this case, a Fender reverb?

[GGBB]

The OP solderman hasn't been active since 2015.

[GibsonGM]

And Jamsden just made the account same day as that nice first post...neat...

[zbt]

BOSS DS1 also connect to VB, I wonder what make it good?

[GibsonGM]

Maybe that was just simple convenience?  VB and ground are the same thing to AC, since there is a cap there that will bypass AC to gnd.