Check my schematic and some weird BB noise

[Ben Lyman]

After several other attempts at this, I have gone back to the drawing board and I think I have something pretty good this time. Starting with GaussMarkov's Voodoo Lab OD (again) http://gaussmarkov.net/layouts/vlo/vlo-schem.png and following his mod suggestions pretty closely, then looking at some other OD schematics I have this on my breadboard:


I like the subtle effect of the filter I put at the end, barely any volume loss and every position is useable, nothing too muddy or too piercing. 
Does it look like a good placement for such a filter?
Why a 2.2M pull down on the VLOD? Is a 1M okay?
What is the effect of changing the master volume pot from 100K to 500K? Good/bad? I tried the 500K because I thought it increased the overall volume, was that just my imagination telling me what I wanted to hear?
Finally, the noise... only when the circuit is bypassed do I hear it, a loud high pitch whine that goes up and down when I turn the volume and gain pots. I think this might be some kind of interference that will go away when boxed up. Have any of you experienced this on your bb before?

[Ben Lyman]

Updated breadboard... not that anyone seems to care 
I like it a lot, it reminds me of my OCD but not as fizzy when maxed out and with stronger bass+mids on the muted power chords.

[ubersam]

Finally, the noise... only when the circuit is bypassed do I hear it, a loud high pitch whine that goes up and down when I turn the volume and gain pots. I think this might be some kind of interference that will go away when boxed up. Have any of you experienced this on your bb before?

I had that happen when the input of the circuit isn't shunted to ground when bypassed. It did go away when I closed the box but I wasn't satisfied with that solution so I  fixed the bypass switching.

Updated breadboard... not that anyone seems to care 
I like it a lot, it reminds me of my OCD but not as fizzy when maxed out and with stronger bass+mids on the muted power chords.

IIRC, there was talk a while back that the OCD was based on the VLOD.

[ubersam]

Just noticed that 150K that you have at the end. If that really is a 150K resistor, that would explain the difference in volume that you heard when you switched from a 100K to a 500K volume pot. That resistor & the pot forms a voltage divider. Lets say for example that the output is 2V, with a 150K output resistor you'll get 0.8V across the 100K pot, across a 500K pot you'll get 1.5V. I usually use a 100R or a 1K in that position, 10K at the most.

[Ben Lyman]

Cool, thanks! So, I am going to look into the grounded bypass wiring. I read up on it awhile back but figured I would wait and see if people started discovering unforeseen problems with it before I tried it myself 

...and... HOLY COW! I did use a 150k and always thought it was weird that the VLOD used it, but since you mentioned it I had to double check and you are right... it's a 150r!!!   
The OCD used a 33k and 22k in parallel with a switch but I'm over pedals with switches and more than 3 knobs, less is more I always says.
Thanks again Sam for the experienced eye on my schematic, that's why I posted it.

[Keppy]

I like the subtle effect of the filter I put at the end, barely any volume loss and every position is useable, nothing too muddy or too piercing.
Does it look like a good placement for such a filter?

Changing the 150k will change the frequency response of your tone knob, so be prepared to tweak.

Why a 2.2M pull down on the VLOD? Is a 1M okay?

2.2M loads the source a little bit less. Not much different, especially with the 470k biasing resistor.

You didn't ask about the gain pot, but in my experience a gain pot in that position works best with a reverse audio taper. Sometimes I don't notice taper problems on the breadboard, so learn from my mistakes and check carefully before you box it up.

[Ben Lyman]

Thanks Keppy... and... AAARRRGGGHHH!!!! Now it's really back to the old drawing board, you are right, my patented "Super-Cool-MacAwesomeness" tone control went to shite!
I've been tweaking and just about to settle on a slightly modified RAT filter, I don't know what else to do right now, maybe take a closer look at the OCD tone filter.
Unfortunately, the largest value of reverse audio pot I have for the gain is only 5k so that's why I have the A100k in there now. Ideally I think it would be rev audio 500k or even 1M but the A100k is working okay, gets clean enough for me but the last 10% of the dial brings about a 50% increase in distortion, it'll have to do for now.

[Cozybuilder]

Only low values of C taper pots? If you have some high value A pots, then for the gain position wire an A pot backwards so that the gain increases counter clockwise. This way you can test various values of resistance to nail down the best value. Once that is determined, go ahead and order the correct C-taper pot and wire it as normal.

[Keppy]

+1, except I say just wire the pot backwards and label it "Cleanup."

The value of the pot in this case sets the MINIMUM gain, not the maximum. Unless you run your overdrives with the gain all the way down, it barely matters once you get above 50k or so.

[Ben Lyman]

Thanks again guys, sounds like a plan. I actually did wire up my FF pot backwards a while back and it worked out great, it's an unpainted, unlabeled box on my board but I call it "The Clean Face" 

[Ben Lyman]

BB noises all went away, I'm happy with the pedal. The final circuit has changed a bit since the last schemo, it started to morph backwards into a VLOD with the gain pushing into OCD territory and finally a low pass filter just like a RAT except for a .001uF instead of .0033uF. I like the more subtle tone change, it's more like a fine tuner that way.

[Kipper4]

Nice and articulate Ben. Good job.

[anotherjim]

That noise is a BB thing. There's just a bit to much contact resistance in a BB compared to hard wiring. Happens when a non-inverting amp output is returned to Vref instead of ground as here via the clipping diodes. The amp output can kick the Vref level just enough to act on the + input of the amp via the bias resistor and you have positive feedback. When a guitar is connected to the input, it's impedance is usually low enough to damp out the feedback, but it can come back when you turn it's volume control down.

[Ben Lyman]

Thanks guys. Here's the final schematic, you can see it's not much different from the VLO where it started from.
My order arrived with some reverse audio pots so I used a C500k for the gain, works great.
I kept the A500k for the master volume, is it common to use audio taper pots for that or is a linear taper more popular for a level control?

[Keppy]

I kept the A500k for the master volume, is it common to use audio taper pots for that or is a linear taper more popular for a level control?

I see a lot of both used in dirt pedals. Audio taper gives the smoothest volume changes as you turn it, but since most people want some boost out of their overdrive pedal, that means with an audio taper pot most of the rotation doesn't get used. Linear doesn't react as smoothly when you turn it, but more of the rotation will yield some amount of boost. It depends on the circuit and your taste, but I usually go for audio taper.

[Ben Lyman]

Thanks Keppy, your answer makes a lot of sense to me. I like the smoothness of the audio but, like you say, most of the time it ends up between 12 noon to 3:00. I will experiment with a linear on my next one to see how it works out. The reverse audio 500k worked out great for the gain pot, very smooth transition and easy to dial up different amounts of crunch, plus with 500k resistance it can be used as a totally (almost) clean boost.

[anotherjim]

An output pot over 100k can act with some kind of taper even if it's linear. Don't forget that the load of the next item is in parallel from the wiper. With our usual 1M inputs a 500kLin will have some noticable taper above half way where the 1M load becomes more significant, while an Audio pot would have its existing taper altered.
http://www.geofex.com/article_folders/potsecrets/potscret.htm