Voodoo Plus - build report

[Mark Hammer]

Every now and then you get into a rut and it seems like nothing you build or try to repair works.  When that happens, folks, build a fuzz!

So that's what I did last night, just to go from idea to functioning pedal in one night, and be able to say to myself, "Yeah, I've still got it".

I took the basic design for the VooDoo Labs Overdrive pedal and changed a few things.  It was a pretty easy perfboard build.  I just realized that Gilles Caron had worked up a board layout for it, but it is small and simple enough that making a board would have made it a more complicated build than it was (although obviously a PCB is much neater).

The VDL Overdrive is essentially an MXR Distortion+/DOD 250 variation with silicon diodes and a second op-amp for a gain recovery stage.  The silicon diodes result in less overall clipping than germanium, because of their higher threshold, but the greater signal output produced by the clipping stage as a result, plus the extra gain provided by the second stage, allows you to really pummel your amp.  In this sense, it truly IS an "overdrive" rather than a fuzz, with the overdrive being achieved both within the pedal and the amp.

On mine, I went for over-the-top.  The original has a gain recovery stage with a fixed gain of about 4 and a HF rolloff around 4.8khz.  I cranked up the gain on mine to around 25, stuck in a back-to-back pair of red LEDs, and capped the HF rolloff around 6.6khz.  This effectively makes it a double clipper (Si diodes to ground in the first stage, LEDs in the feedback loop in the second).  By virtue of using LEDs in the second stage, there is considerable headroom and a HUGE output level possible.  Because there is more gain in the second stage than the stock one, you can still get a nice gentle grit and sandiness at lowest gain settings with tons of volume.

For the first stage, I went for a little more gain than the stock VDL, more in the direction of a Dist+.  The stock VDL-OD has a maximum gain in the first stage of about 11, going by Gilles' schematic redraw (Justin Philpott's drawing shows no fixed resistor to set max gain, which is a little suspect).  I used a 1meg feedback resistor, and 6k2+500k pot to ground, for a gain range of a little over 3 to well over 150.  Ouch!  The Dist+ uses a cap to ground in the first stage which rolls off a lot of the low end at the highest gain settings.  I opted for a much bigger cap (330nf)  to keep more low end and chunk in.

I'm pondering sticking in some sort of passive tone control since there is one whole heckuva lot of output to this baby (similar to the Blackfire inthat regard) and I can afford the passive loss.  One the other hand, with just a Stage-1 gain and a volume control, there is a nice range of tones available, and the unit responds well to changes in both guitar volume and tone control changes as well as pickup differences.

All in all, nothing revolutionary or distinctive sounding, but a nice illustration of what you can do by taking a basic model, stripping it down and applying some theory.  Not unlike those  fibre-glass shells I used to drool over as a kid, where you'd buy a VW bug, strip the body and replace it over top of the basic VW frame with something that looked like a Ferrari or Bricklin.

[BDuguay]

My curiosity is officially peeked.
I just realized Mark, that I've got to update my profile here to reflect my new home e-dress. Then I'm going to start pumelling you personally with what will probably be inane and mundane questions. But, like I've been told before, the only stupid question is the one that's never asked.
Brian

[jrc4558]

Cool, Mark!
Fuzzez are ok, no matter what people say!

I like that overdrive. I built one, but made some sufficient steps off the original frequencie cuts...
What do you say, do you know of any programm that would calculate the cirner frequency for the op-amp feedback loop cap-res combo?

Glad for your fun and Drive!

All the best!

[Lonehdrider]

Speaking of VW's, if I can find a reproduction hub caps cheap enough, I want to use one for a stompbox case.

Regards,

Lone

[Peter Snowberg]

LOL!

I lost a hubcap from my '58 bus so I've got three left.... maybe I won't have any left soon. :lol:

Kind of big for a FuzzFace, but classic for sure. Perfect pedal for that DeadHead in you life.

take care,
-Peter

[brett]

Thanks for the story Mark.  Just one picky remark:

The silicon diodes result in less overall clipping than germanium

I think you'll find that *per unit of volume* (ie per volt of signal), Si clips harder than Ge.  By "clipping" I mean the change in conductance per unit of volume increase.  Hard clipping is more change in conductance per volt, and vice versa.

I agree that per unit of *gain* you get less clipping with Si, because of the higher voltage threshold, but that implies comparing Ge and Si at completely different volume levels.  Bringing a Ge-clipped signal up to Si levels reveals a much softer, "less" clipped signal, as you know.

Now I don't know whether I'm adding to, or reducing confusion here.  And I'm definately getting picky in my old age...

[Doug H]

Mark, you metalhead...

It sounds like you are really having fun. I haven't done anything with an op amp for a few years. I will have to try some of these ideas out.

Your idea of cranking the gain in the 2nd stage and adding LEDs reminds me of an episode of this old sitcom I used to like (kind of a "secret indulgence") called "Men Behaving Badly". In it, one of the main characters was trying to explain why God gave women breasts. He held up his hands like he was holding a couple peaches and said, "God said, 'What the hell, let's have some fun!!!'"   (No offense intended to anyone, I just thought it was hilarious...)

But your experience sounds very familiar. I became very frustrated with something I designed and was attempting to build that wouldn't work, even though it worked fine on the breadboard. So I took a break and built an axis fuzz. 2 transistors and all the fuzz you can handle. Nothing like a fuzz to soothe the savage soul, or something like that...

I'm doing an amp now but eventually I'll get back to the other thing.

Have fun!

Doug

[Mark Hammer]

I was trying to figure out what sort of a tone control I wanted to put on this during the bus ride home last night.  The first inclination was to look at Jack's "presence" control article at AMZ and use a modified BMP-type filter-panpot.  Then it occurred to me that I had two non-inverting clipping stages, one of which had more cumulative harmonic content than the other, and it dawned on me that I could install a panpot which varied the balance of the two stages, using a tap just after the clipping diodes in the first stage.  The first stage would be low-pass filtered first (simple 1-pole RC) to make it warmer, and the second would have some broad scoop/notch filtering.  The panpot would adjust the balance of the two sounds, but at the same time act as a sort of treble/mid/bass balance control by varying between sounds that emphasize different parts of the spectrum.  

Of course, this is a sort of luxury afforded by: a) the fact that the two clipping stages are in phase with each other, and b) the fact that the output level of the second stage is hot enough to weather the drop in amplitude that will result from a passive notch filter.  My hope is that the midpoint of a linear panpot really WILL provide equal amounts.

Brett,
I'm fully aware of potential differences in what happens when a signal reaches the critical point in diode conduction.  But don't confuse steady-state behaviour with dynamic behaviour.  Music signals vary widely (and wildly) in their amplitude.  For me, the question is "For what proportion of the full duration of the note is the signal amplitude likely to exceed the clipping threshold?"  If it's supra-threshold more of the time then you get a fuzzier (i.e., more clipped) sound.  That is separate from what shows up on the scope as one half-wave signal voltage running up against a diode.

[Doug H]

I was trying to figure out what sort of a tone control I wanted to put on this during the bus ride home last night.  The first inclination was to look at Jack's "presence" control article at AMZ and use a modified BMP-type filter-panpot.  Then it occurred to me that I had two non-inverting clipping stages, one of which had more cumulative harmonic content than the other, and it dawned on me that I could install a panpot which varied the balance of the two stages, using a tap just after the clipping diodes in the first stage.  The first stage would be low-pass filtered first (simple 1-pole RC) to make it warmer, and the second would have some broad scoop/notch filtering.  The panpot would adjust the balance of the two sounds, but at the same time act as a sort of treble/mid/bass balance control by varying between sounds that emphasize different parts of the spectrum.  

Mark, good idea. Also, take a look at the tone network on my Hot Silicon.

http://home.cfl.rr.com/dbhammond/GS_Tonebender.gif

It does a similar thing, panning between "scooped" and "warm midrange" sounds. It's pretty simple and you would have enough current drive from the 2nd stage op amp for it (no follower needed).  Having it at the end of the chain makes it very effective too.

Doug

[Mark Hammer]

Hoochie mama!

I was looking for a "morphing" type of tone control, and I do believe I found one.  As you recall, I had modified the basic Voodoo Labs Overdrive to have more gain in the first diode clipping stage, and increased the gain in the second stage so that it could drive a set of LEDs into clipping.  I wanted to have tone control that would pan from a rounder, but still full, tone to something that was buzzier but didn't lose too much bottom.

What I did was this.  I tapped the signal from the junction of the two 10k resistors between the stages and ran it through a simple RC filter of a 6k8 resistor and 10n cap to ground, giving a rolloff at just over 2.3khz.  Remember that the first stage already has some lowpass filtering in the feedback loop, plus a cap in parallel with the diodes to ground.  Between that and this additional filtering the tone is a very warm Dist+ workalike, with a real 70's chunk.

At the output of second stage I tacked on the midrange scoop of the Univox Superfuzz (the Superfuzz has a tone switch that selects between a mid-scooped sound and an equal-volume unscooped sound).  The resulting sound is a sizzlingly hot sort of death metal tone.  Remember this is not just midscooped but double-clipped with less high end rolloff than the
original VDL-OD.

I then wired up a panpot style tone control like the Big Muff so that I could pan back and forth between these two sounds.  The wiper of that pot goes to an output volume control.

What's kind of neat is the way the sound changes so much i character as you rotate the tone control.  Much more contrast than a simple round-vs-thin contrast like you find with a standard BMP-type control.  Paul, this has a lot more flexibility than the Chaos; a veritable Swiss Army knife distortion.

So, synopsis....

Two cascaded gain stages.  First one has less overall clipping and rounder tone than the second.  Second stage reclips already clipped signal for greater harmonic content, more clipping-induced compression.  Different filtering for each clipping stage.  Tone control blends outputs of each clipper for different character.

Bottomline?  There is some interesting experimentation to do with distorting a signal in two different ways, filtering to derive a unique "voice" from each that brings out its best features, and blending those voices.  I
did it with silicon diodes, LEDs and opamps, but there is no reason why it could not be done using discrete technology or hybrids.

Start yer engines.  And yes I'll draw it up and post it.

[jrc4558]

Hi, Mark. A question: How is the noise in this VooDoo mutant that you have cooked up? Tolerable?

[Doug H]

That's really interesting because you are not only morphing between the 2 eq's, you are morphing between the 1st stg clipping sound and the 1st + 2nd stage clipping sound- if I understand it right.

Hmmm...

Doug

[Mark Hammer]

Okay, I posted the drawn up schematic and some notes.  No sound clip yet.

I'm finding a LOT more tonal flexibility than I first described.  More and more impressed with the idea of "panning for gold".  Collectively, we have tinkered with the idea of blending distorted and clean voices or filtered and straight voices, but there is a lot to be said for blending types of distortion, and for tailoring those distortion types with simple filters like one sees in the Bee-Baa, Superfuzz, and similar units.

Constantin,
I can't report on the noise factor because:
a) All my testing is done in front of the computer in a small room with flourescent lighting
b) It's not in a case and does not use shielded wire anywhere
c) The guitar pickups aren't humbucker (though positions 2 and 4 are)
d) The "amp" I listen through at midnight is a halfwatt NJM2073-based thing in a stompbox-sized case with a decent 3" speaker that lacks the sort of high end that would reveal hiss issues.  (On the other hand, what sounds great through that usually sounds pretty damn fine through the tweed Fenders in the basement).

The dual op-amp type probably doesn't matter, but since someone is going to ask, I used an old JRC4558DD pulled from a Sony TV board I bought at a surplus place recently.  I'm pretty confident that's what gives it all the tone and Sony colour.  :wink:

[Marcos - Munky]

Mark Hammer asked me to upload the schematic for his Voodoo Plus, then here is:
www.geocities.com/munkydiy/voodooplus.gif
Copy and paste in another window.

[Mark Hammer]

I did some thinking, and in retrospect thought it was a little unfair to use a name that hinted at the original VDL-Overdrive when there was unlikely to be any sonic resemblance.

So I decided to rechristen it the "Roseyray", after the actors in a movie (The Thing With Two Heads; it'll show up on Google, etc.) who were similarly dissimilar in their attitudes like the two channels of this thing, but who needed to cooperate in spite of it.

Nothing else is different about the posted schematic other than the name and writeup.  Apologies to all for the confusion.  Even deeper apologies to those who go and rent the movie based onmy comments!  :lol:

Constantin,
I'm in the process of packaging it into something that ought to provide a reasonable test of  noise levels and I'll let you know tomorrow.

[puretube]

Mark: do I see a feedbackloop around IC2 through the bridged-T & the tone-pot to the pos. input, whilst you call it "blend" between 2 stages?

[WGTP]

This sort of reminds me of the Boss OS-2 that has a "mid heavy" diode in loop, distortion that blends with a "scooped mid" diode to ground, distortion, similar to the Boss SD-1 and DS-1.  Very cool.  As stated, not only are you varying the EQ, your also varying the "character" of the distoriton.  Now I need to get the schem looked at.  

[WGTP]

There is an old Aerosmith song called "Combination" that I have know idea what the real words are, but at some point I "epiphamied" (sp) into being about the combination of things and not about the specifics.

ANYWAY, this is a nice combination of ciruits that blend together in a unique, at least to me, way.  Too many variations via diodes, etc. to even try to list.  Another nice one Mr. Hammer, but your missing a chance by not incorporating your last name into this stuff.   Maybe your saving it for the penultimate distortion.  

[Mark Hammer]

Ton,

You may be right.  There really is nothing to limit the feedback of the output of stage 2 from going back to its input.  After all, the resistors don't go in any particular direction, do they?  And there is nothing polarized between output and input that would make anything unidirectional.

On the other hand, the resistance of the blend pot remains constant in that loop regardless of setting, so I can't see the audible changes of the blend/tone control resulting from different degrees of feedback to the stage-2 input.  I'm not disputing that there may be some sort of reciprocal effect, but the value of the blend pot, as a resistor, may be sufficiently high to keep that feedback at a minimum, relative to what comes directly from stage-1.

I was operating under the assumption that the tonal changes in stage-2 came from the  Superfuzz scoop filter, but they may also stem from the feedback loop you describe.  Interesting if it did.  That would make two happy accidents instead of one.  Breadboard it and see.

WG,
I *can't* combine my name into the pedal name  Why?  Because then I would only be able to say "It's Hammerific!" about one pedal.  Keeping the names "Hammer-free" affords me a certain, shall we say, luxury. :wink:

To everyone,
This blending thing has piqued my curiosity.  For instance, imagine you had a pedal with three paths: dry, wet-1 (flange), and wet-2 (phase).  The dry is fed to the standard mixer stage, and so is the wet......BUT....you get to blend combinations of phaser and flanger as the wet signal.  Imagine the two wet signals are 8 stages of same-cap phase shift and 4 stages of Univibe cap values.  Clearly the possibilities are endless and should not be restricted just to blending two fuzztones.

Ladies and gents, put on your thinking caps and start you engines.

[puretube]

the: "PENULTIMATE METAL-ROSE HAMMER?