Voodoo Lab OD mods

[roknjohn]

Does anyone have mods for the Voodoo Lab Overdrive? I thought there were some a while back. I could be mistaken, but I thought they were by Mark Hammer.

[Mark Hammer]

No, but it was an easy mistake to make.  I used part of the Voodoo circuit, and called my posted schem a Voodoo Plus.  Shortly thereafter, (and without ANY pressure or even correspondance from VD or elsewhere) I felt sort of awkward about co-opting the name and changed to a different one.  So, what I posted is NOT a bunch of mods to the Voodoo Labs circuit.

[roknjohn]

Thanks for clearing that up Mark. The Voodoo Plus was exactly what I was referring to. I was able to find the renamed circuit at the Ampage site. Thank you for your assistance.

[Mark Hammer]

You're welcome.

Looking at Justin Philpott's reverse draw of it, the VL Overdrive strikes me as a decent design - essentially a Distortion+ with a much-needed gain recovery stage so you can get lesser degrees of distortion while not forfeiting a hot output.  The second stage provides a gain of about 4 with a fizz-reducing treble-trim around 4.8khz and no loss of low end (bass rolls off below 2hz).

The "problem" would seem to be in the first stage.

First, there is no gain-limiting resistor in the ground leg where the 500k gain pot resides.  Quite likely an oversight by Justin, despite an otherwise fine draw.  You should probably stick in a 1k-2k2 resistor in series with the 500k pot.  That will take the first stage from a min gain of around 2 to a max gain of around 250-450.  More than enough front-end sizzle.

Second, this pedal suffers from the same problems as the DOD250 and Dist+, and that is the loss of bottom as gain is increased.  The Low end rolloff of that noninverting stage is given by F = 1/(2*pi*R*C).  With a 0.1uf cap and a min resistance of 2k2, that yields a rolloff at max gain of 723hz, which isn't a whole lot of bottom by anyone's standards other than a mosquito's.

The solution to the drastic tone loss is to replace the 0.1uf cap with something larger, such as 1uf, which will drop that frequency down to 72hz.

Alternatively, consider doing something I attempted with the "Crank" design on my site.  Here, two successive gain stages are used with clipping distributed across the two.  Because very little resistance change is used to altere the gain in each stage, there is negligible change in bandwidth.  Of course, the difference here is that 1) you need a dual-ganged pot, and 2) hard-clipping one pair of diodes does not sound the same as moderately clipping two successive sets of diodes.  That MAY sound better to your ears, or it may not.

If such a mod interestsd you, I'd suggest the following:

a) Score yourself a dual-ganged 50k pot.  Taper is not crucial.

b) In the first stage stick a 10k resistor in series with one of the pot sections configured as a variable resistor.  Don't forget the 1uf cap, though you can probably tolerate 0.47..

c) In the second stage, put a pair of either 1N4001's or red LEDs in parallel with the 150k resistor.

d) Also in the second stage, replace the 47k ground resistor with a 15k resistor in series with the other pot section, being sure to wire up the second pot half in the same direction as the first.

Okay, what do you have now?  You have two cascaded clipping stages with some treble rolloff in each case (more in the initial stage, just the way I like it).  You have a first stage whose gain goes from just over 9 to 48, and a 2nd stage whose gain goes from 3.4 to 16.  Since gain is multiplicative, that equals a combined gain of from just over 30 to 768.  Although 30 seems high for a "minimum" gain, keep in mind that the diodes will keep that in check.  

The gain in stage one will not produce VERY heavy distortion in that stage, but it will produce clearly audible distortion.  The 220pf cap and the .001uf cap in parallel with the diodes will smooth that out a bit yield mostly lower-order harmonics.  Taking the max signal available after the 1N4148 diodes and multiplying it by 16 will most assuredly produce additional distortion.

In theory, this should produce a relatively thick-sounding, meaty overdrive that never gets thin.  It likely *won't* ever clean up all that much, but that's not really what you wanted it for, was it?

If you try this, let us know how it went.  There's a guy at work/school who's been giving me a lift home since my car crapped out, and I wanted to thank him.  I think I may try this one for just that purpose!

[roknjohn]

Wow, you opened my eyes to a few more possibilities than I expected. I've wanted to try this circuit for a while now and haven't gotten around to it. I was hoping on using it at the lower gain settings after hearing it on the Tonefrenzy site, but I could no longer find the demo clip when I checked it out recently. I believe Aron has a short clip of it somewhere too.

Anyhow, you've given me plenty of sideroads to explore with this. Should keep me busy for a while. This may turn into a dual mode OD/DIST device by the time I get done.

[Mark Hammer]

Well, THAT was a pretty fast perf build, if I do say so myself.  Everything on the board is wired and ready to go.  Took all of about an hour.  It's kinda late, though, and I still have to look over a government survey that somebody asked me to review for tomorrow.  So, the pots, jacks, and comments will have to wait until tomorrow evening.  Stay tuned.

[roknjohn]

Finished mine last night as well. Doesn't sound too bad either. Just a little more bright and edgy than I expected especially when the drive is cranked. Just as you predicted Mark. Didn't have a lot of time to mess with it after building it, but just for giggles,I tried swapping a few different ICs. Tried JRC TLO72, 5532, & 4558. Didn't notice anthing drastic. Then I tried a TI 4558 which I bought a couple of because someone mentioned they can make a difference in the Tubescreamer circuits. I did notice a slightly smoother response with the TI. Took away just enough edge for me to notice. I'll try to tweak some more this weekend. I think I'm gonna try some LEDs in place of the diodes.

[Mark Hammer]

Pleased you like it.

The LEDs will add some volume boost, and reduce the edginess due to their higher threshold.  It will be more in the overdrive zone and less in the distortion zone.  Alternatively, you can just add a 2nd pair of diodes in series in the 2nd stage, so that it's a 2+2 diode arrangement.  That'll put you halfway between what you have now and LEDs.

The 4.8khz rolloff in the 2nd stage has less of a tone-shaping roll and more of a straightforward hiss-reduction and gain recovery roll in the stock design.  In this case, since we are asking the second stage to clip also it is going to ADD harmonic content instead of simply preserving it.  It might be worthwhile to change the 220pf cap in the second stage to 330pf or even 390pf for a rolloff at 3.2khz or 2.7khz.  That ought to smooth it out a bit.

[Mark Hammer]

Okay, up and running, and sounds good, but modded even more.

As expected, with the extra gain and clipping in stage 2, it needs more shaving off the top, so I stuck a 390pf cap in there instead of the 220pf.

With LEDs the output is absolutely huge.  Even with the gain turned down all the way, one still has a choice of effect level that is higher than bypass.  

When I heard it, I thought it would be a perfect thank you present for a guy who has been giving me lifts home from language school since my car crapped out on me.  He's a Steve Vai afficionado, and seems to go for high gain scooped stuff (he borrowed and loved my Roseyray), so I took advantage of the huge volume boost this thing offers and stuck in the Superfuzz midscoop filter between the 2nd stage and the output level control.  Changed the 1000pf cap to 1500pf for a little more upper mids.  Even with the passive signal loss of the midscoop network, the output level is still remarkably hot.  I A/B'ed it with my Blackfire and it compared quite favourably.

I didn't have a dual 50k pot handy, so I simply went with a dual 100k.  The extra resistance has no impact on max gain, only on min gain.  I think I may stick a 39k fixed resistor in parallel with the half of the pot used in the 2nd stage to make the change in distortion over pot rotation a little smoother.

Sounds pretty decent, I must say, if a little predictable.  I used a 1458 of all things, and may try a few others but it seems to sound fine with the 1458.  Behaves very nicely in response to guitar tone knob changes and responds well to a wah in front of it (most wahs will sound fine after a distortion but not all will produce much tonal change in a distortion if placed before it).

All in all, the Voodoo Labs Overdrive is still a decent design, but it also makes a nice base for hotter and more aggressive-sounding pedals.

[MartyMart]

Mark, that's a nice little "run through" of the circuit and sounds like it
would be perfect for your "driver" !
I seem to remember having mine with a 0.33 (330n) cap off the non-inverting stage, possibly another "tweak" but mostly stock, as on my
vero-layout.
Very nice OD, which I didn't want to make any "heavier"  BTW
I think I have enough OD's/Distortions now !!!!
Or can you EVER have enough ?......................... :wink:

Oh, in case you missed it, WGTP's recent design "Distron-Thrust" is a
monster !! - 2X gain controlled stages ( a dual gang 500k would be nice )
first using 3X1N4148's and second ( on mine ) 2X1N4001's, with a nice
simple "filter" which can be switched for "notched" tones etc.
John Hollis diode switching arrangement, which is a neat idea.
Lot's of sparkle, from almost "clean" to "mongoose" nastiness ! hehehe

Marty.

[Mark Hammer]

Yes, I did catch WGTP's Distron dual clipper.  Clever little twist with the diodes being flippable between ground and feedback-loop locations.  The "sparkly" capabilities would seem to stem from the midscoop placed between stages.  At lower gains this would likely result in a bit of emphasis of the high end and taming of the bite in the middle.  At higher gains this would obviusly result in serious sizzle by virtue of the reclipping occurring AFTER the midscoop.  

What I did was essentially use the same basic 3 "modules" (gain, gain, scoop) as WGTP but in a different sequence.  The both arrangements are likely to produce different sounds since the one merely subtracts from what has already been created (in the way of harmonics) and the other adds based on what has been subtracted so far.  One caveat I will note (and which I gather Distron builders have already noted) is that use of LEDs for the clipping element in the last stage, without anything else to tame the signal, results in a serious "workload" for the output level pot.  The signal is so hot that if one is trying to get just a bit of boost above bypass level, you end up only being able to use the volume pot up to about the 9:00 setting...if that.

So here is an idea....

In the original Superfuzz, from whence the midscoop filter effectively originates, switching the filter off involves reverting to a simple resistive divider (47k/10k) that provides an unaltered (tonally) signal of roughly the same volume as the scooped version.  Given the potential gain factor in the Distron, AND the tonal nuance differences between scoop-filter locations, it might be useful to stick a scoop/attenuate network similar if not identical to the Superfuzz',both before *and* after the 2nd clipping stage.  The use of two scoops might provide something vaguely amusing, the capacity to reposition the scoop-location would add some interesting tone-shaping, but more importantly it provides the option to tame the signal level when "going for the gold" and maxing out the gain/clipping in this beast.

One final query: What happens when you use diodes to clip one half-cycle in the feedback loop and the OTHER half cycle by clipping to ground after the op-amp output?  Has anyone ever tried that in either a product or experiment?

[roknjohn]

Posted this on the old forum. Didn't get much help. Hoping someone may have additional suggestions.

I finally got around to boxing everything up, but now notice a nasty fizzy sound (kinda intermod distortion like) as the notes decay to nothing, and then the distortion just drops out. Tried replacing both the 220pf caps, even increased them to 470pf, but didn't help with the fizzy problem. Swapped op amps, and cap in parallel with diodes, still no help. Double checked schematic vs. my layout too.

Anyone have suggestions?

[WGTP]

Just caught this post.  Mark's influence is all over the Distron Thrust, as I'm sure most folks have noticed.  Mark is a GREAT asset to this form.

The Mosfertu - Tim Escobedo? used the single diode (mosfet/ge) in the loop and singel diode to ground idea in 2 stages.  I have experimented with it and it is another cool variation of the multitudinous diode combinations available, especially with 2 op-amp stages.