A quick "IF" regarding an OCD project

[KarenColumbo]

Dear fellows,

I just breadboarded this OCD version:



Sounding just fine after some tweaking values etc.

I particularly like the n7000 clipping - very smooth right to the end of the signal.

Now (who'd have thought?) IF I'd like to switch between MOSFET and LED clipping. (I recently purchased some fine switches - they should look just right fine in this build)


1. Which kind of LED/Diode will be "saturated" enough to not let the clipping "rip off" towards the decaying end of the signal? Which kind will clip the soonest?
2. (This is probably not a question for a quick answer) Can I "plug in" a Fetzer stage to let some JFET have it like Debbie in Dallas?

As I always am: grateful for your comments and your company

[ElectricDruid]

There's a nice discussion of the mosfet clipping over on AMZ:

http://www.muzique.com/lab/zenmos.htm

This tells us that the MOSFETs clip at about 0.7V.

There's also a nice summary of clipping thresholds in a post on this thread:

http://www.tdpri.com/threads/mosfet-vs-led-clipping.242104/

- 1N34A (germanium): ~ 340mV
- BAT41: ~ 370mV
- 1N400X series: ~500mV
- 1N4148/1N419 ("standard" silicon): ~600mV
- many transistors (including mosFET's): ~720mV

HOWEVER, there's one important thing which isn't being considered here. The OCD schematic you posted is using R9/10K to increase the impedance feeding the diodes. This helps smooth the clipping, but not all diodes will respond in the same way. Standard silicon diodes respond well to this, but it doesn't do much with rectifier diodes like the 1N400x series. So the clipping threshold isn't the only thing that will affect the tone. There's a good article about this on ESP:

http://sound.whsites.net/articles/soft-clip.htm

HTH,
Tom

[KarenColumbo]

Thanks, Druid. Aaaaand ... done. Nice rock drive now at the flick of a switch.

Now I'll dig into the jfet common source thing. Should work as a clipping stage I assume, even if there's not as lot of definite information out there.

[antonis]

Now I'll dig into the jfet common source thing. Should work as a clipping stage I assume,

Despite it's a MosFet , you can imagine it as a BJT diode-like wired arrangement..
(e.g. as a N-P-N with Collector short-circuited to Base..)

[KarenColumbo]

Now I'll dig into the jfet common source thing. Should work as a clipping stage I assume,

Despite it's a MosFet , you can imagine it as a BJT diode-like wired arrangement..
(e.g. as a N-P-N with Collector short-circuited to Base..)

Nah I wanted yet an additional clipping option with ... dunno ... a pair of J201s or something. The op amp gain stage seems to put out enough juice to push one JFET over the edge. I know that the OCD has a MOSFET clipping stage Just wanted to have a third after it and the "regular" diode clipping option I already implemented

[DavidRavenMoon]

Lose D1. You don't need it, and the pedal sounds much smoother without it. I removed it on my Joyo Untimate Drive and replaced it with a jumper.


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[antonis]

Lose D1. You don't need it,

Only if he wants symmetrical clipping..

Maybe a SPST by-pass switch should be an interesting option..

[DavidRavenMoon]

Lose D1. You don't need it,

Only if he wants symmetrical clipping..

Maybe a SPST by-pass switch should be an interesting option..

Only it doesn't really work that way. There's discussions about this.

There's a sound the OCD makes that I never liked. Removing that diode fixed it.

Also decrease the value of the gain pot to be able to get less distortion. As it stands it has very little useable range before it's full on fuss. Even with it halved past noon is maxed out.

It's nice to add switches for different clipping options. But usually you find a setting and leave it. Otherwise are you going to be bending over between songs on a gig to change settings?


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[antonis]

Only it doesn't really work that way.

Only I can't see another way than "off-setting" clipping threashold..

There's a sound the OCD makes that I never liked. Removing that diode fixed it.

I don't want to involve in personal taste mysterious sounds mysteriously vanished.. 

It's nice to add switches for different clipping options. But usually you find a setting and leave it. Otherwise are you going to be bending over between songs on a gig to change settings?

Maybe KarenColumbo doesn't suffer from waist ache.. 

[KarenColumbo]

Heh, Dear Sirs, hold your horses

I omitted this diode anyway because it said "optional" on the schematic.
It's a good point that switches are impractical in a live situation - but there's a long way to go until something I built ends up on an actual pedalboard that is used by somebody in a live situation - so at the moment I try and cram everything I can onto a breadboard to wrap my head around why something does (or mostly doesn't) what it does or doesn't.

And the thing is a season guest on my desk at the moment. When it's done and boxed it will have 2 pots (gain and volume), and 2 switches (bright/dull and od/dist). maybe I will take the distortion mode switch and have a second stompswitch instead - what with my spine and such ...

A point well taken is the dynamic range of the gain circuitry - it gets hot pretty fast, so I will have to think about that. Thanks for the hint with the pot!

What I'm trying to do next is - as mentioned - somehow splicing in a jfet gain stage as a switchable option. There's nothing like trying out something and maybe not failing.

Aaaaaand ... the way I built it it is pretty low-midsy, there's a whole lotta sparkle missing. I messed up the hp/lp combinations, I'm afraid. Well, this is something I will get sorted out eventually.

Thank you, guys, for the company!

[KarenColumbo]

Well, I finally managed to inplement a jfet gain stage into this thing. AND I managed to draw an Eagle for it (R10 is 150k as in the original below - sorry for the inconvenience).



Sounds nice. When properly balanced, the jfet stage adds a nice high gain sheen.

As you probable can see I omitted several things from the "original" OCD ...



1. first of all I ditched the mosfet clipper and put 914s in place, they sound "warmer" to my ears.
2. I left out R11 and the switch S2 since the LP/HP switch only switched between "very dull" and "just dull" (see below)
3. VR2 ("Tone") was left out since I found it didn't do anything useful to the overall sound it just took away the little treble there was
4. I put in an OPA2134PA instead of the TL082 - no really audible difference but I love those

Well - it sounds a bit "dull" - it's direct, it rocks and it metals with the jfets on but I'm missing something - it sounds like from under a (very light) blanket.

Is there a point in the circuit where I could splice in an EFFECTIVE treble blend/switch? Should I review those R4/C4 and R6/C6 filters? What would you do, if I may ask your opinion yet again?

[KarenColumbo]

Hm,

seems that those jfets (I tried j201s and 5457s) do have a life of their own and my idea of "splicing them in" doesn't work as expected because they are kinda a pedal within another pedal. They are particularly noisy, the distortion "rips off" at the very last decay of long notes. Seems those two kinds (opamp and "pseudo tubes) don't mix well - and they are at the wrong place within the circuit.

Should I reference them to 4.5 Volts instead of ground? I'll never know

At least this journey was a partial success

However.

I will get rid of this portion of the circuit and have another opamp gain stage instead.

Still no idea about these missing treble frequencies.

[duck_arse]

R17 - is it really 2k2? and R21 - is it really 2k2? your jfet vol pot should probably go to V/2, because it will mess the bias of the second stage when switched in.

[DavidRavenMoon]

Only it doesn't really work that way.

Only I can't see another way than "off-setting" clipping threashold..

You have to look at the voltage drop involved. It basically removes that one MOSFET from the circuit. If you want asymmetrical clipping, try replacing the MOSFET with the GE.

But honestly the pedal sounds better with just the two MOSFETs.

There's a sound the OCD makes that I never liked. Removing that diode fixed it.

I don't want to involve in personal taste mysterious sounds mysteriously vanished.. 

Here's an idea. Build the damn pedal and see for yourself. You've never actually tried a real OCD, have you? Typical backseat poster. Lol.

It's nice to add switches for different clipping options. But usually you find a setting and leave it. Otherwise are you going to be bending over between songs on a gig to change settings?

Maybe KarenColumbo doesn't suffer from waist ache.. 

Maybe your snark quality sucks? You're going to bend over and change settings between songs? That's lame.

Or you only play in your bedroom? Lol.

Just find a sound you like and hard wire it.


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[KarenColumbo]

Only it doesn't really work that way.

Only I can't see another way than "off-setting" clipping threashold..

You have to look at the voltage drop involved. It basically removes that one MOSFET from the circuit. If you want asymmetrical clipping, try replacing the MOSFET with the GE.

But honestly the pedal sounds better with just the two MOSFETs.

There's a sound the OCD makes that I never liked. Removing that diode fixed it.

I don't want to involve in personal taste mysterious sounds mysteriously vanished.. 

Here's an idea. Build the damn pedal and see for yourself. You've never actually tried a real OCD, have you? Typical backseat poster. Lol.

It's nice to add switches for different clipping options. But usually you find a setting and leave it. Otherwise are you going to be bending over between songs on a gig to change settings?

Maybe KarenColumbo doesn't suffer from waist ache.. 

Maybe your snark quality sucks? You're going to bend over and change settings between songs? That's lame.

Or you only play in your bedroom? Lol.

Just find a sound you like and hard wire it.


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I hereby confess: I actually do play only in my living room. On stage I play the keys. But stompboxes for keyboarders ain't in demand recently. So ...

Anyway I've already rebuild the OCD to the point where i started. WITHOUT D1. On my other breadboard there's something like BWs take on the Mu Amp. After a well deserved vacation I will let them grow into each other somehow.

As to "just hardwire the sound you like": not a bad idea at all, but a bit of balancing usability vs. pedalboard space would be advisable. But there's still the second stompswitch waiting on the idea horizon.

[ElectricDruid]

I hereby confess: I actually do play only in my living room. On stage I play the keys. But stompboxes for keyboarders ain't in demand recently. So ...

Stompboxes with keyboards is great! Fender Rhodes has often had many pedals used on it - phasers particularly, but also waa-waas and all sorts else. I love a nice flanger on a synth sequence. Rich ramp waves really give a flanger something to "bite into". And then there's acid techno. Where would that be without the 303 and a dirt box?

offtopic2cents,
Tom

[KarenColumbo]

I hereby confess: I actually do play only in my living room. On stage I play the keys. But stompboxes for keyboarders ain't in demand recently. So ...

Stompboxes with keyboards is great! Fender Rhodes has often had many pedals used on it - phasers particularly, but also waa-waas and all sorts else. I love a nice flanger on a synth sequence. Rich ramp waves really give a flanger something to "bite into". And then there's acid techno. Where would that be without the 303 and a dirt box?

offtopic2cents,
Tom

Problem is: I'm one of those Funk players that rely heavily on sustain pedal action. A Wah pedal or even just a stompbox would break my leg groove irreparably

[GGBB]

Here's a simple clipping option. Note the drain-gate connection (not gate-source):



You can use an on-on-on DPDT switch to toggle between the standard OCD clipping, symmetrical true MOSFET clipping, and asymmetrical MOSEFT / body-diode clipping. The diodes can be anything, but low Vf (germanium) is probably best since the MOSFET Vf is already ~3V.

[KarenColumbo]

Interesting snippet, thanks. What I wanted, though, was to merge diode (or mosfet) clipping and tube-analogy transistor overdriving. But I think I'm half way there already

[antonis]

What I wanted, though, was to merge diode (or mosfet) clipping and tube-analogy transistor overdriving.

An off-topic different design could be to include diode or mosfet in NFB loop of a "conventional" BJT..
(in a Big Muff style like below..)