Type C. OD (reworked OCD)  [documentation]

Original effect designs complete with schematics, layouts and instructions. Freely contributed by members or found in printed or online publications. Cannot be used for commercial purposes without consent of the respective owners of the copyright.
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dylan159
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Post by dylan159 »

I can't deny that I've always enjoyed the sound of the OCD, my first veroboard build, but I had a couple of issues with it which I finally decided to tackle. I don't even want to get into any business regarding the original manufacturer or his claims on this circuit, because I could always fool myself thinking I've built the voodoo lab overdrive instead.
Still, the main issue is an awful output impedance, of the order that it's not only bad on paper, but giving volume jumps even when switching pedals with decent input impedances after it, and cable capacitance influence causing a varying amount of treble loss. I had solved this by adding an output buffer, which when traces became available I found out it's what the OCD v2 does too (I had also added a capacitor before it to emulate the treble loss of a normal cable). Another issue was the linear volume control, and then in general I thought it could have used a cleanup and standard E6 values for not critical parts. So here it is:
typecod.PNG
  • Opamp. TL072, pretty much the standard for guitar inputs with the low current noise. Why use the slightly noisier 082?
  • Input stage. This is pretty much verbatim, I just made the input impedance a standard 1Meg, but also removed the input resistor because its noise gets amplified by everything, altough small in value. It's a judgement call, noise and parts saving against current limiting and protection/reliability: I still haven't hit my face against issues caused by ESD on the input, so until then i choose the former. Gain stopper was removed to make this into an even cleaner boost if wanted: the original can already be very clean, why not go all the way and make the input stage unity gain. Gain control law doesn't suffer, my spreadsheet tells me.
  • Clipping. Switched to plain 4148 diodes because the mosfets were used for the body diodes and there's not any justifiable reason to use them (a previous research I did didn't show differences at least in the VI curves), I had even made a comparison of the two and didn't hear much of a difference. They still go to vref, not because it's special :wink: but because it saves me a cap and a resistor.
  • Tone. Moved between stages so it doesn't affect output impedance. "Hey why are the resistors the same, don't you have to subtract the 10K?". As a friend pointed out, when the diodes are conducting, they're in parallel with the 10K, so that resistor doesn't count much, exactly when the tone control it's most needed. This does mean in theory that the tone control cutoff varies dynamically with the amount of clipping, which i don't think it's that bad of a thing. Both simulations and side by side comparisons say that it's pretty much like the original. Log does give a better sweep but lin works well too.
  • Output stage. Just a low gain clean boost with common values and low impedances to bring the unity gain at about noon on the volume pot and buffer the output. The gain has been chosen by having in mind headroom and an output amplitude roughly equal to guitar levels with volume at noon, starting from the assumption of a worst-case ~1.2Vpp output from the clipping stage.
    This stage has the same bass treble rolloff frequencies as the original "recovery" stage, but has been redone for low impedances and common values.
  • Volume. 10K log pot for a lower output impedance than you'll ever need and no weird interactions. Log for a smoother control.
I'm happy with how it sounds, which is to say just like before. Maybe a little less latent fizz on the note tail? It was another thing that bothered me so another win there. But also I made it my own a bit and I'm proud of that.

Vero layout:
typecod.png
Soundclips:


Extra: I was working on the plastic timbre, a Plexitone workalike, at the same time as this. Since I enjoyed that circuit but didn't want to build it separately, recognizing the similarities, I've devised a mod for the C. OD to make it sound similar with little effort.
typecodplastic.PNG
Last edited by dylan159 on 15 Apr 2021, 21:46, edited 1 time in total.
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bmxguitarsbmx
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Post by bmxguitarsbmx »

When sw2 transitions, U1B will momentarily lose bias. Assuming sw2 is a break before make. Do you get popping?

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bmxguitarsbmx wrote: 15 Apr 2021, 20:17 When sw2 transitions, U1B will momentarily lose bias. Assuming sw2 is a break before make. Do you get popping?
You're right, normally I'm concerned with those things, but it actually doesn't! Maybe the tone cap doing wonders together with the jfet op amp low bias currents.
Of course if one wants to be proper a 1M across the switch or another bias resistor are all that's needed.
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Post by plush »

dylan159 wrote: 15 Apr 2021, 20:27
bmxguitarsbmx wrote: 15 Apr 2021, 20:17 When sw2 transitions, U1B will momentarily lose bias. Assuming sw2 is a break before make. Do you get popping?
You're right, normally I'm concerned with those things, but it actually doesn't! Maybe the tone cap doing wonders together with the jfet op amp low bias currents.
Of course if one wants to be proper a 1M across the switch or another bias resistor are all that's needed.
Easier way is to add switchable parallel resistor, 75k in parallel with 33k will give 22k.

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Post by dylan159 »

plush wrote: 15 Apr 2021, 20:35
dylan159 wrote: 15 Apr 2021, 20:27
bmxguitarsbmx wrote: 15 Apr 2021, 20:17 When sw2 transitions, U1B will momentarily lose bias. Assuming sw2 is a break before make. Do you get popping?
You're right, normally I'm concerned with those things, but it actually doesn't! Maybe the tone cap doing wonders together with the jfet op amp low bias currents.
Of course if one wants to be proper a 1M across the switch or another bias resistor are all that's needed.
Easier way is to add switchable parallel resistor, 75k in parallel with 33k will give 22k.
Stay away from me with those nonstandard values! :shock: at that point adding 10K in series to the 22K or 68K in parallel to 33K.
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Post by bmxguitarsbmx »

Interesting! So, the tone cap holds the charge (bias) long enough for transition! Ha. I love it:)

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bmxguitarsbmx wrote: 15 Apr 2021, 20:52 Interesting! So, the tone cap holds the charge (bias) long enough for transition! Ha. I love it:)
Haha I don't but if it works it works
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Post by dylan159 »

Improved the Peak switch. Thanks for pointing it out guys.
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Post by plush »

dylan159 wrote: 15 Apr 2021, 20:45

Stay away from me with those nonstandard values! :shock: at that point adding 10K in series to the 22K or 68K in parallel to 33K.
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Post by dylan159 »

plush wrote: 15 Apr 2021, 22:10
dylan159 wrote: 15 Apr 2021, 20:45

Stay away from me with those nonstandard values! :shock: at that point adding 10K in series to the 22K or 68K in parallel to 33K.
Image
Half and half. :D I do hate nonstandard values thrown around but the suggestion is sound. 68K in parallel to the 33K is actually very very close to 22K, even more than the series alternative, so I went for that instead.
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Post by plush »

dylan159 wrote: 15 Apr 2021, 22:15 Half and half. :D I do hate nonstandard values thrown around but the suggestion is sound.
75k is a standard value for e24 (1-5%).
I could have assumed that you meant e12. But, seriously, who uses 10% these days?

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plush wrote: 15 Apr 2021, 23:59
dylan159 wrote: 15 Apr 2021, 22:15 Half and half. :D I do hate nonstandard values thrown around but the suggestion is sound.
75k is a standard value for e24 (1-5%).
I could have assumed that you meant e12. But, seriously, who uses 10% these days?
Tolerance isn't directly related to series. Sure, if you want more values you need the accuracy, but all resistors come in 1% these days as long as they're metal film.
As to why I like to limit myself with E6 most of the time, it started as a way to make my circuits more easily prototypable by only stocking up common values, but then I thought this would extend to others as well, making it more accessible to everyone. And it's not a compromise! Most of the time it's just a matter of scaling things to get the same result, other times they're values that don't matter at all. In this case 68K gets closer to the target than 75K anyway (22.2K vs 22.9K).
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Post by plush »

dylan159 wrote: 16 Apr 2021, 08:33
plush wrote: 15 Apr 2021, 23:59
dylan159 wrote: 15 Apr 2021, 22:15 Half and half. :D I do hate nonstandard values thrown around but the suggestion is sound.
75k is a standard value for e24 (1-5%).
I could have assumed that you meant e12. But, seriously, who uses 10% these days?
Tolerance isn't directly related to series. Sure, if you want more values you need the accuracy, but all resistors come in 1% these days as long as they're metal film.
As to why I like to limit myself with E6 most of the time, it started as a way to make my circuits more easily prototypable by only stocking up common values, but then I thought this would extend to others as well, making it more accessible to everyone. And it's not a compromise! Most of the time it's just a matter of scaling things to get the same result, other times they're values that don't matter at all. In this case 68K gets closer to the target than 75K anyway (22.2K vs 22.9K).
Oh, now I understand your logic, thanks for clarifying.
Good luck with your future experiments.

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Post by TWSpedals »

Instead of the tone switch and 2 resistors why not replace them with a 50KC pot so you can adjust the cut off frequency of the tone stack beyond just 2 positions. Arguably cheaper as a result, and lowers part count too.

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Post by dylan159 »

TWSpedals wrote: 02 May 2021, 00:37 Instead of the tone switch and 2 resistors why not replace them with a 50KC pot so you can adjust the cut off frequency of the tone stack beyond just 2 positions. Arguably cheaper as a result, and lowers part count too.
Because the difference is subtle enough as it is I'd say. Not really cheaper, since knobs cost. You can just omit it and make it 3 control, choosing between cutoff or attenuation for tone.
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Post by TWSpedals »

dylan159 wrote: 02 May 2021, 07:35
TWSpedals wrote: 02 May 2021, 00:37 Instead of the tone switch and 2 resistors why not replace them with a 50KC pot so you can adjust the cut off frequency of the tone stack beyond just 2 positions. Arguably cheaper as a result, and lowers part count too.
Because the difference is subtle enough as it is I'd say. Not really cheaper, since knobs cost. You can just omit it and make it 3 control, choosing between cutoff or attenuation for tone.
Or alternatively have a RAT style filter control. It's what I'd do personally.

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Post by dylan159 »

TWSpedals wrote: 05 May 2021, 13:27
dylan159 wrote: 02 May 2021, 07:35
TWSpedals wrote: 02 May 2021, 00:37 Instead of the tone switch and 2 resistors why not replace them with a 50KC pot so you can adjust the cut off frequency of the tone stack beyond just 2 positions. Arguably cheaper as a result, and lowers part count too.
Because the difference is subtle enough as it is I'd say. Not really cheaper, since knobs cost. You can just omit it and make it 3 control, choosing between cutoff or attenuation for tone.
Or alternatively have a RAT style filter control. It's what I'd do personally.
That's what I call variable cutoff instead of variable attenuation with fixed cutoff as here and in guitars. They're slightly different and have different impact on output impedances, but here both will work well.
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Post by dylan159 »

I've found an error in not noticing the 33K stays connected in the original, so all there's to do is to lower the 68K to 22K bringing the tone control back to where it was. LP mode wasn't affected, HP should be more different. Updated schems and layout:
typecod.PNG
typecodplastic.PNG
typecod.png
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