Beginner doubt - Voodoo Lab Overdrive

[lepra85]

Hi everyone,
I was viewing the schematic of the Voodoo lab overdrive and some doubts came to me.
Unfortunately I had a hard time trying to upload the image of the schematic without success.
The link: http://gaussmarkov.net/layouts/vlo/vlo-schem.png

Doubts:
1)
at the very beginning of the circuit there is the input capacitor (C1) in series R2. I understand that if R2 is to limit the current of any Popping 3DPT Switch issue must be before the capacitor.
what is the Function of this resistor?
why this resistor is after the capacitor and not in the other way?

2)
At the output of the first Amplifier is R6. I understand that this resistor is to limit the output current of the OA when the clipping diodes are in "On" state.
Does this resistor form a filter with C4?
in case i want to use a 100k Pot for this filter, Should i put it before or after the diodes?

3)
Just after the Diodes there is R7.
what is the Function of this resistor?

4)
The volume Pot value is 100k.
There is any diference in use a 10k Pot? I dont see a significant voltage drop using this value neither any noise issue.

Thanks in advance and sorry for my english (is not my native language)

[Mark Hammer]

Since the VLOD is two cascaded non-inverting op-amp stages, R7 does for stage 2 whatever R2 does for stage 1.

As for making the lowpass filter variable, any variable resistance should come between Rg and R7, with C4 going from the junction of R7 and the new variable resistance.

If you look at the high-end rolloffs for the two stages, though, you'll note that there isn't gobs more top end to roll off.  Stage 1 rolls off, starting around 1.5khz and stage 2 starts rolling off around 4.8khz.  Certainly the added harmonic content that comes from the clipping will be noticeable, but any single-pole lowpass filter on top of that may have a very subtle action.

You may be better off simply using the design, as is, and wiring up a 3-position toggle to add another 220pf, or 680pf in parallel with the stock 220pf in stage 2 to move the rolloff down by one or two octaves.

[MrStab]

Hi Lepra, welcome to the forum. sorry if my english is confusing (even other native-speakers tell me it is)!

if R2 is to limit the current of any Popping

R1 is there to limit popping, R2 is there to protect the opamp input from the outside world, e.g too-high voltage, C1 failure etc.

Does this resistor form a filter with C4?

yes. 1/(2*π*R*C) = 1/(2*π*10,000*0.000000001) = 15,915.5Hz. So not a filter you need to worry about, because the roll-off frequency is so high up. It's probably there to kill radio interference, though i'd expect that sort of thing to be earlier in the signal path.

in case i want to use a 100k Pot for this filter, Should i put it before or after the diodes?

as i said above, this seems to be a "functional" filter and not a "tone-shaping" filter. i doubt it'd do any harm to put a pot there, but you need to consider that the initial frequency will be higher than most amps can deliver. you should use a reverse-log pot. i'd put it just after the diodes, though i doubt it matters.

Just after the Diodes there is R7.
what is the Function of this resistor?

I'm not actually sure. The only reason i can think of is to protect IC1B from the diodes shorting, but even then, that would just mute the pedal. If it has some tonal value, it's not obvious to me.

There is any diference in use a 10k Pot?

As this pot is immediately after a buffer (hence low-impedance), a 10k pot is probably fine. say your volume pot is at 50%: if you have a 100k pot, there's 50k impedance in series with your signal (=treble loss). if it's only 10k, you only have 5k in the way. the total resistance between the signal path and GROUND via. the pot is less of an issue with low-impedance signals (but still something you should search the forum for - there's loads of detailed posts on volume pot values). Having said all that, it could be that the designer of the circuit just thought it sounded better with 100k.

[MrStab]

1.5khz

that seems more realistic. i've really screwed up that equation. listen to Mark, not me. lol

nvm, wrong part

[lepra85]

Mark/MrStab, Thanks for your answers.
One only more question:
Regarding the C1-R2 series conection.
do you know the reason why C1 is before R2 and not the other way (R2 before C1).

Mostly in the schematics i saw  the Capacitor is first, but in a few (e.g. MXR distortion III) are in the other way.
I wonder if there is any difference.

thanks again

[MrStab]

as far as i know, you can do either. i prefer putting R2 AFTER C1, however, because i'm worried in case the capacitor discharges into the opamp. i have nothing to base that fear on, though.

[duck_arse]

in a series circuit, component order don't matter. so C1&R2, R9&C6, R10&C7, C3&R5&GAIN pot, doesn't matter which is first in line.