Obsession with Voodoo

[gaussmarkov]

i put together a voodoo lab overdrive layout that appears on gaussmarkov.net.  i have a bunch of reasons but the one that will turn the most heads is that this circuit is rumoured to be closely related to the fulltone OCD.  the principle differences that i have seen reported through a schem by markm that was posted at freestompboxes are that

• the first-stage has had it's gain significantly increased so that it can clip the signal in DOD Overdrive 250 fashion and its high pass corner frequency has been raised to a tubescreamer level (720Hz),
• the gain pot has been moved into the feedback loop portion of the divided negative feedback,
• the diode clippers have been replaced with MOSFET clippers,
• and the gain of the second stage has been bumped up a little to push the signal through a passive low pass tone circuit.

similar changes have been applied by folks here to many other circuits, so i will leave the details to other posts on this forum.  as noted by others, the MOSFET clippers have a long history on diystompboxes.com.  i kind of like leaving the gain pot where it is in the voodoo version.  it makes the high pass corner frequency of the first gain stage go up and down with the gain so that when there is relatively little distortion and less need for attenuating lows they are there.

given the gazillions of circuits that work off of the same basic design ideas and the huge success of the OCD, the VLO seemed like a good one to repost for DIY.  as usual, you can pull the Eagle CAD files down from gaussmarkov.net and change my layout as you please.  if you come up with something you like, please share. 

all the best, gm 

[MartyMart]

http://www.aronnelson.com/gallery/v/Martys-layouts-and-photos/VooDoo_Labs_OD.jpg.html?g2_imageViewsIndex=1

Here's my old stripboard layout for the VLO.
Room for some mods and improvements since 2005 !!

MM.

[soulsonic]

gaussmarkov.net is quickly becoming one of my favorite sites.
Nice job!

[gaussmarkov]

nice Marty!  all we need is a perfboard version to complete the set.  i found one in japanese here: guitatsuku.  there's PCB link next to the second picture that goes to the vero layout itself.

another recent thread that i noticed yesterday, brought up a nice example of mods to this circuit family by a forumite.  WGTP posted in the gallery something he calls MXR YJM-DOD250Mods.  it's interesting to see how similar and how different his choices were.  his first stage is quite similar to what's in the project i cobbled together and he takes an add on in a completely different direction.  nice!

[gaussmarkov]

gaussmarkov.net is quickly becoming one of my favorite sites.
Nice job!

cheers, soulsonic.  i just checked out your blog.  great idea and a lot of work!

[gaussmarkov]

oh yeah, i was going to mention that i put a breadboard version in the IC-Based Overdrive project under Voodoo Lab Overdrive to lower the obstacles to experimentation further.

[Hamfist]

So how does Voodoo sound in relation to the DOD 250?  It's one of those that I've been planning on "getting to eventually", but I haven't yet.  I did read some HC reviews, and a couple of people mention it being too dark/muddy for humbuckers.  I almost always use low output humbuckers or P90's through a cranked Marshall 2203.  And I like pedals that add just a little more grind and not much else.  I do need something to add a little more girth to a Tele though.  Thoughts?

[gaussmarkov]

i haven't made a VLO yet, but there's some things one can see from looking at the schems.

the maximum gain in the first gain stage of the VLO is about 11 (470K/47K + 1).  the DOD OD 250 has a maximum (potential) gain of about 214 (1M/4.7K + 1) but it never reaches that gain because the signal hits the rails and starts to clip.  if you are feeding both of them a 100mV AC guitar signal, then the VLO produces about 1.1V AC from the first stage which is well within the 4.5V swing that is available from a 9V supply.  in theory, the OD 250 would produce a 21.4V AC signal if the supply allowed it to.

so one of the principle differences one will notice is the clipping by the op-amp in the OD 250.  i think Mark Hammer once referred to the DOD 250 as a faux diode clipper for this reason.  the VLO is a real diode clipper, with gentler clipping.

hard clipping means more high frequencies.  so it may be that the OD 250 is reported brighter for this reason. 

on the other hand, the VLO can attenuate lows much more in the first stage.  such op-amp voltage followers are bandpass filters with the corner frequencies calculated using the 1/(2 pi R C) formula. the lower corner frequency of the passband uses the R and C values from the resistor and capacitor in series to a constant voltage.  the upper corner frequency is calculated using the R and C in parallel in the feedback loop.  when these pedals are at max gain and the gain pot resistance is lowest they reach their highest low corner frequency, 720Hz for the VLO and 33Hz for the 250.  i guess that isn't enough to overcome the previous difference.

also, the VLO has a second stage that the 250 doesn't have.  it's another voltage follower with corner frequencies equal to 1.5Hz and 4822Hz.  the lower one doesn't matter but the upper one suggests significant attenuation of highs as well.  you could increase the upper corner frequency in the second stage by decreasing the 220pF cap in the feedback loop.  if we put in a 100pF cap, that would widen the passband to about 11KHz.  a 47pF cap would pretty much remove the attenuation of highs at this stage, with an upper corner frequency of almost 23KHz.

if you are interested in more of an explanation, i recently wrote one and posted it on gaussmarkov.net:  Op Amps 4:  Divided Negative Feedback.  there's a math appendix in the pdf doc Divided Negative Feedback and Capacitors.