Voodoo-1 questions

[rocker-D82]

I'm ready to realize a PCB design and a prototype of Voodoo-1. I need only to know if the schematic is right and what are the values of pots.



Other questions: what's the difference between amp1 and amp2 output jack??? What's the meaning of feedback loop through 1.6Mohm resistor?

[Mark Hammer]

Most of this schematic looks correct to me, with a few exceptions/qualifications.

1) The pots are probably all 100k, like on a Big Muff.  Certainly 100k would be fine for the gain pot, though I don't know what taper would work best for you.  The tone pot is likely 100k linear, and the output pot could probably be (though not necessarily) 100k as well.

2) The diodes are unspecified, but probably 1N4148 or 1N914.  LEDs would be probably out of the question.

3) The 70k resistor just ahead of the diodes is a very nonstandard value.  A 68k could easily replace it, although that seems just plain wrong.  In a great many similar kinds of pedals (e.g., the Distortion+ and DOD 250), the resistor value in the same location is 10k, which may make more sense of the bizarre 70k value - it's a misprint.

4) The tone cap values seem a bit off.  This is kind of an "Orman-ized" Big Muff tone control.  My gut instinct is to suggest that the true cap values are .01uf, which would make it more normal.

5) The 39 ohm resistor in the gain stage seems a little small.  In theory, it would deliver a gain of over 3800.  Yeah, like that's ever gonna happen on a 9v battery.  If you make it 390R, that still gives a max gain of 385, which seems about right for getting Rat-like tones from the cap-compensated 308.

The 1.6M "feedback" resistor may indeed be just that.  It is certainly too big to be a feed-forward resistor, and given the amount of gain in the circuit, any tiny amount of feedback would have tremendous implications.  Perhaps just as critically, the output control is located *before* this feedback point/tap, which not only makes the output control a part of the tone-shaping but seems clearly in keeping with Mayer's design philosophy, which is interactive and organic.

As far as I know, the amp1/amp2 thing is a separate buffered output.  I gather Jason drew only the actual fuzz part, and not the other electronics.

[rocker-D82]

1) [...] the output pot could probably be (though not necessarily) 100k as well.

100k linear?

2) The diodes are unspecified, but probably 1N4148 or 1N914.

In the power section I can use correctly a 1N4001?

3) The 70k resistor just ahead of the diodes is a very nonstandard value.  A 68k could easily replace it, although that seems just plain wrong.  In a great many similar kinds of pedals (e.g., the Distortion+ and DOD 250), the resistor value in the same location is 10k, which may make more sense of the bizarre 70k value - it's a misprint.

So, I use a 10k resistor. There are some other schematic by Roger Mayer to try to understand what is the right value?

4) The tone cap values seem a bit off.  This is kind of an "Orman-ized" Big Muff tone control.  My gut instinct is to suggest that the true cap values are .01uf, which would make it more normal.

Why there's 100?? When I read the schematic, I think to 100nF. But there are also 0.1uF caps (the same value!).

5) The 39 ohm resistor in the gain stage seems a little small.  In theory, it would deliver a gain of over 3800.  Yeah, like that's ever gonna happen on a 9v battery.  If you make it 390R, that still gives a max gain of 385, which seems about right for getting Rat-like tones from the cap-compensated 308.

A gain of over 3800 can clip opamp, who isn't the desiderable behaviour. I try 390R!

The 1.6M "feedback" resistor may indeed be just that.  It is certainly too big to be a feed-forward resistor, and given the amount of gain in the circuit, any tiny amount of feedback would have tremendous implications.  Perhaps just as critically, the output control is located *before* this feedback point/tap, which not only makes the output control a part of the tone-shaping but seems clearly in keeping with Mayer's design philosophy, which is interactive and organic.

I never see this feedback net. It's a strange thing!  

As far as I know, the amp1/amp2 thing is a separate buffered output.  I gather Jason drew only the actual fuzz part, and not the other electronics.

In the schematic there aren't. There's a way to understand what kind of buffer they are?