Fulltone OCD versions

[JHS]

Comparing the schems, the OCD is 90% VL OD and 10% mod (some part values, Gain control and RC-network is similar to the FD2).

The type of mosfet has nearly zero influence on the sound, only the internal protection diode is used for clipping. It's not the AMZ- or Mosferatu method, there are no additional diodes, the 1N914 are replaced w mosfets, that's all. The diodes clipp only at high gain levels and add some compression, you can leave them out at low gain (the dynamic range is better w/o diodes). Most of the sound and character is generated by the TL 082 chip, its the crucial part and the tantalum caps produce the clear highend and raunch.

All in all I woul'd not call it overdrive, it it nearer to a good distortion.

There's a lot room for improvements. The reaction to the guitar vol is quite bad like on the VL OD but this can be cured with a LPB-1 or other similar booster set to unity gain in front of the OCD.

JHS

[Ghandi]

hey jhs,
i allways wonderd what that little caps are?
do you think that are tantalums?
i thought that might be high quality ceramic caps!?
oh and on an old mod side mr. f told to replace the 100n cap in the fb with a 47n mylar.

cheers
ghandi

[Doug_H]

Agreed but with all the bad things you can say about Mike and his product designs the one thing he still does that other builder still don't do very well is making the pedal tough. Tour worthy. I give him credit for that alone.

Andrew

Anymore I think the most interesting aspects of a lot of the boutique gear are the paint jobs and roadworthiness/construction. Electronically speaking, there's a lot of slop out there. Not speaking of the OCD or Fuller's work per se, as I haven't seen it and have very little interest in it anyway.

Oh, and the protection-diode clipping thing was done by Aron around 8 yrs ago with his Shaka 3.

[cbriere]

All right, but the VL oD has no tone control?
What kind of tone control is used on this one,
Proco Rat or TS style? other....

[Xavier]

Perf layout for a Voodo Labs Overdrive with mosfet clippers coming

[WGTP]

IIRC there was a thread a month or so back which discussed the clipping diodes NOT going to ground, but to 1/2v. and the effect this had on the clipping, perhaps being "softer".   

If the diode in the mosfet is being used rather than the mosfet, that seems somewhat misleading.

[Doug_H]

IIRC there was a thread a month or so back which discussed the clipping diodes NOT going to ground, but to 1/2v. and the effect this had on the clipping, perhaps being "softer".   

That will depend on how well the Vr rail is filtered. I found with my tonesource (another thread) that with Vr purposely underfltered (4.7u as in the brownsource schems) and critical parts of the circuit grounded to that, it will soften up the highs in a nice way. Whereas the more typical 22u or 47u filter cap (which I'm usually inclined to use) sounded a lot harsher.

[John Lyons]

Hmmm

In the DOD OD250 ('77 version) there is no cap in the Vr.
Can someone explain the role the cap plays in the Voltage Reference?

Thanks

John

[Doug_H]

Hmmm

In the DOD OD250 ('77 version) there is no cap in the Vr.
Can someone explain the role the cap plays in the Voltage Reference?

Thanks

John

It provides voltage stability and a "solid" AC ground.

[JHS]

The tone control is a guitar style one. O-cap is a tantalum, cera cap for the 1n, rest is multilayer ceramic, maybe tantalum for the 100n in the RC network. I think Sila Mica or PIO caps where much better in terms of tone and headroom but 4 of them and the board is full.

IMHO the part and build quality on Fulltone's FXs is boutique-standard. I like that Fuller don't mount the footswitches to the board like some other B-makers do (it took a hell of time to change a crappy switch in those FX).

Connecting the diodes/mosfets to VREF will give better headroom with less compression, a different breakup and a bit topend sizzle in the VL OD and OCD. To say this is better than connecting to ground or putting them in the FB-loop is a bit unfair, the rest of the circuit is important too and connecting diodes to VREF will not work in every OD/Dist. equal good as in the VL OD.

The design of bias devider has a lot effect on the tone, a 2x10k with a 10uF sound less trebly with a sloppy bass. Increasing the 10uF to 47 or 100uF produce better highs and a tighter bass. Sometimes I increase the 10k and parallel decrease the cap value.
A additional 10-100n from + to gound increase the treble response a bit too, but also filter out the hiss implied by the protection diode (that's the reason while the small cap is there).

Adding another 100uF from + to ground will make the FX even more trebly. To decide to take  a 10uF or 47/100uF in Vref is sometimes not easy and often depends on the type of IC and the slewrate. Often I decide after a listening test to increase/decrease the caps in Vref to tailor the tone to my liking. IMHO the best way for most FX is to use a IC in the bias-devider to stabilize the circuit against load down influences and to generate a rel. sym. Voltage on the IC-rails.

The Xotic stuff f. ex. use a 10uF cap in the bias divider to soften the tone of the hifi-sounding IC.

JHS

[MKB]

IIRC there was a thread a month or so back which discussed the clipping diodes NOT going to ground, but to 1/2v. and the effect this had on the clipping, perhaps being "softer".   

That will depend on how well the Vr rail is filtered. I found with my tonesource (another thread) that with Vr purposely underfltered (4.7u as in the brownsource schems) and critical parts of the circuit grounded to that, it will soften up the highs in a nice way. Whereas the more typical 22u or 47u filter cap (which I'm usually inclined to use) sounded a lot harsher.

I'm not sure if you could get by underfiltering the reference on the VLOD, as you might couple some of the output of the first stage(through the clipping diodes) to its input, as it uses the same reference.  But if you add another Vr for the clippers, you could do anything you want with them.  BTW, not that this means anything at all, but if you add a second Vr divider and cap to the VLOD, the parts count starts getting suspiciously close to what you see in the OCD pics.

I just added a second Vr string to my VLOD build and stuck the clipping circuit on my breadboard, hope to try out some clipping stage experimentation tonight.  I also confirmed what JHS said about the diodes not having much of an effect (at least on a scope), if the level at the clippers start getting much over 0.7V or so, the second op amp stage starts to clip.

Another interesting point is that with the gain pot in the inverting leg to ground of the first stage as in the original VLOD, you have the 470K Rfb and a 220pF across it.  That gives quite a rolloff.  If you set the inverting leg to ground at 4.7K and then add the gain pot as Rfb (as indicated in the OCD mods on the Fullertone website), that would raise the rolloff frequency quite a bit at low gain settings.  I'm going to try this as well to see if this helps add clarity to the VLOD.

[Mark Hammer]

Can someone explain the role the cap plays in the Voltage Reference?

Another way to think of it is as being like an emergency battery or reservoir of current such that there is always enough to accomplish the biasing should another part of the circuit create sudden demands on the battery. 

The battery has a role akin to the wall current.  In the "good old days", when there might not be all that many electrical appliances plugged into the wall, many of them lacked either power regulation/filtering or any attention to current-consumption.  So, when your mother or grandmother or the tenant down the hall in your 3-storey walkup decided to use their Singer sewing machine, every sudden momentary surge in current consumption from the motor on that beast would translate in power line noise and static showing up on your radio or TV.  If the compressor pump on the fridge came on during the critical play of the World Series, you might have to miss a few pitches.  With everything sharing the same power, whenever one thing plugged into the same AC line needed lotsa current in a hurry, it would create an impact on the  current available to other things plugged into the same line.

The cap between Vref and ground serves the same role as a well-regulated supply now does for all those appliances in your house: it helps them get through any momentary alterations in the available current supply by smoothing and storing current from the power source.

Now, of course, if you happened to be the ONLY appliance on a desert island, then nothing ought to interfere with your access to current, right?  Right.  When we have something like a modulation pedal where the LFO has current needs that are sometimes incompatible with the audio path current needs, Vref needs to be extremely well-smoothed and stabilized so that the two portions of the circuit don't fight each other and yield ticking.  In the case of the DOD250, though, the circuit is only one lowly op-amp, so in a sense nothing is fighting anything else for mastery over the current.....I think.  There may well be some conflicts between what the V+ pin needs on a moment to moment basis and what the Vref pin needs, so while the 250 circuit likely could function without that cap, perhaps it performs a little better with it under certain circumstances.

[A.S.P.]

OMG


one of the oldest rules in EE:

"a circuit is only as good as its power supply"

[Ghandi]

the output cap is  a 2,2uF electrolytic,
the 220pF caps seem to be standart ceramics and everything else seems to be multilayer ceramic.

[A.S.P.]

wouldn`t a tantalum output cap bring more transparancy to the exit?

[Ghandi]

jep,
more transperency and a bit more raunch,

the whole pedal will sound even warmer and more transparent
if you would use some nice film caps and silva mica's for the 220pf.

I think the two 220pF caps would make the most differnce.

[A.S.P.]

ahh: I fought so!

[snoof]

Anyone want to help me trace this sucker??  I have one here on loan, and I got lots o' pics i've taken.  I'm not much on tracing double sided boards, but I can provide pics/info to someone willing to give it a go.

[aron]

>The type of mosfet has nearly zero influence on the sound, only the internal protection diode is used for clipping.

Hmmmm wonder where he got that from?

[Ed G.]

Yeah, that's what I thought also Aron.