This thread has been extremely helpful to me. Among other things, it got me to do some more experimenting with the Proco Rat-based model on my Line 6 M13 which has led me to a wonderful Garcia-esque overdrive tone. I'll try to post a pic of my settings here tomorrow or so. For now, FYI, some info. on the OCD I found on TGP:
"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."
http://www.buildyourownclone.com/board/ ... &sk=t&sd=a
The OCD is based on the Voodoo Labs Overdrive (schematic can be found online) which itself was based on the original DOD overdrive/preamp. Voodoo Labs added an extra gain stage and extended the range of clean boost. The Fulltone OCD is purported to be using MOSFETS.
The quoted part of the OCD article describes ONLY the clipping/distortion section of the circuit. There are only a few basic principles how to create distortion: i.e. diodes connected between the signal and ground (as in the MXR dist+, Proco RAT, Boss OD-1, Dod ...), diodes in the feedback loop of an opamp (as in the Tubescreamers, Boss SD-1, most of the boutique pedals out there), and some transistor gain stages mostly used in Fuzz pedals.
Nevertheless it is very hard to make any conclusions from this, there are tons of little variations in the circuitry (asymmetric clipping, leds instead of diodes, fets instead of diodes, various different diodes, different opamps with different gains, combination of all the above....) which all affect the sound heavily. In addition most distortion pedals will have some sort of additional gain stage and/or tone control circuitry, which again have tons of options and changing the size of each capacitor for example will affect the sound strongly.
The OCD clipping section does have crosscoupled diodes at the output of a opamp gain stage, as does the Proco Rat. But, instead of connecting these to ground (=hard clipping, strongly distorting the signal and losing dynamics) the OCD uses a floating bias point with feedback from the input, thus creating a clipping threshold that is proportional to the input signal. The article does not say that the OCD is like the RAT, it merely states that both units use cross coupled diodes at the output of an opamp gain stage to clip the signal. Both pedals sound completely different and react totally different to playing intensity due to the differences in circuitry and components.
the DOD Preamp Overdrive came after the MXR Distortion+ and used a different pair of diodes and yes that did give the DOD a more raunchy sound.........
It was yellow but not mustard yellow and there was nothing in the literature to indicate it was a clone riding the fame of the MXR. I will admitt though that the DOD had something more of an attitude and the guy in Sweden that knew 53 ways of playing a sharkfinpick and also the only other owner of an electric sitar besides Janne Schaffer, had one of those DOD's allthough he had modified his with a TL071.
I don't know what the OCD consists of nor do I care but I will say I consider it my favourite Fulltonepedal.
I find dynamics great at lower distortionlevels about the level of distortion you'd get from a model 1987 Marshall
and indeed it surprised me how much like one such amp the OCD sounded........it did however mush out in quite the same way as an amp like that would when volume passes 6, while that's the nature of the beast and really I'm getting old and would need less distortion and more dynamics yikes hand me the kane and my medication,please nurse .......
As a sidemark if you'd like to push an amp like that the Carl Martin Hot Driven Boost is really good
but that's a different application.
Bosstone? who had one such in scandinavia? Not that many immortal riffs weren't recorded with one. Hey I would have liked to own one just to be different but had to settle for the Melos.
OK getting a bit technical here if you clamp an AC signal to ground or to virtualground what difference does it make?
Woohoo, that's the kind of trick question you'd get at electronic school to see if you'd understand the difference of AC /DC ........
If it amounts to anything I did find the OCD fun to play in this magical setting.........:RoCkIn
Have great fun
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