So you want to mod your Dist+/DOD250, eh?

[Mark Hammer]

Someone requested some Distortion+ mods, so I thought I might provide what you might call "proof of concept" by building something that illustrates all the mods in one package.  It demonstrates a simple, and very flexible "extended" DOD250 for the most part, using 3 op-amps.  I call it the Boost-n-Scoop.

What you'll see below is essentially Craig Anderton's Frequency Booster grafted onto the front of a modified DOD250/Dist+, followed with a variable scoop control.  Because it is a grafted schematic, you'll pardon the inconsistent component labelling style.  The first two op-amps, used for the booster are two halves of a 4558, and the "other IC1" is a 5534, though I suppose you could use a 741.  I used a 5534 because I wasn't going for the unusual biasing in the Dist+ (three 1M resistors) so I figured I may as well shoot for lower noise and opted for the 5534 because I have dozens sitting around.

The basic idea is that pre-emphasis by resonant boosting in front of a clipping stage can create interesting resonances and notching can shift emphasis as well.  Note that this is different than simply sticking a boost/cut control in front of or following a clipping stage.  The two functions are separated here as pre-clip/post-clip EQ, and that makes a difference.



The features to note are as follows:
1) Anderton's frequency booster uses two caps to set where the resonant boost is.  You can see the range of values to consider in the schematic found either in Mr. Huge's schematic gallery here or at AMZ.  I used a .1 and .047uf cap in series for each of C1 and C2.  By wiring up a DPDT on-off-on toggle to shunt the .1uf in each pair or the .047uf, or neither, That yields nominal boosts at around 200hz, 425hz, and 625hz.  You can use other cap values but these illustrate what can be easily done.  These resonant frequencies produce tones similar to a %^&*ed wah, which lotsa folks seem to like.  You can also use a rotary switch with more caps, but the shunted-series switching is pop free.
2) The unit provides a nominal gain of two, given the resistors labelled R6 and R7.  When the SPST switch is open, it reverts to unity gain.  Note that the extra gain of 2 is multiplied by the gain in the clipping section so you end up with more push.  That helps out with other things (see below).  Note that the amount of resonant boost increases as R1+R8 gets smaller.  So you could make the resonance a little more outrageous by dropping R1 to 4k7, as well as permit greater subtlety by increasing R8 to 25k or even 50k.
3) The output of the booster is normally followed with a 10k volume pot but I went directly into the input of the clipping section here.  Note that the typical .047uf cap found in the ground leg of a DOD250/Dist+ is replaced with .22uf here to preserve bottom end at higher gain.  You will also note that the max gain is increased by use of a 3k9 fixed resistor instead of 4k7, and I've dispensed with the pointless low-gain end of the gain pot's spectrum by going directly for a 100k pot.  I used linear and it is absolutely fine.  A 22pf feedback cap is added to the 1m feedback resistor for stability and hiss management.
4) To preserve signal level, in anticipation of the loss coming from a passive scoop, I used a 4k7 output resistor instead of 10k, and a pair of silicon diodes instead of the usual back to back pair of Ge or Si diodes found in the Dist+ or DOD250.  This gives a hotter output, and also provides a higher clipping threshold.  Between the max gain of 257 in this stage, and the gain of 2 preceding it, that's a gain of x514, which is more than enough to net us seriously intense clipping.
5) The passive scoop formed by the pair of 10k resistors, .0022uf bypass cap and .1uf cap to ground is adapted from the Superfuzz.  I wired up a 100k linear pot to provide two concurrent changes.  On the one hand is varies the resistance to ground from the .1uf cap, which varies the degree of scoop.  At the same time, it varies the resistance to ground of a second cap in parallel with the clipping diodes.  So at the one extreme, it lets through more treble and cuts the lower mids, while at the other extreme it brings back the lower mids and tames the treble a bit.  Feel free to experiment with other values of the .0022uf bypass cap and the .01uf treble-cut cap to ground.  I paralleled one leg of the 100k scoop pot with a 22k fixed resistor because it doesn't take that much resistance change to lift the scoop, whereas the  variable resistance between the .01uf cap and ground needs to cover a wider resistance range.  The parallel resistor provides for the one pot to have two different values.

How does it sound?  Not especially revolutionary, and definitely not a "blues" pedal, but if you want to play Slash or Michael Shenker or work that boundary between thrash and hard rock, this is your pedal, baby.  Lotsa volume feasible, and pretty quiet too.  I imagine other kinds of voices can be found by changing the resonant frequency.  It came out pretty much exactly the way I imagined it and fired up first time (well, second, actually, once I realized I had a wire going to Vref that was supposed to go to V-).  The one I built has 4 pots and 2 toggles: one toggle for boost on/off, one for centre frequency, boost amount, gain, scoop, and volume.  A lotta flexibility in a little package.  I'll see if I can whip up some soundclips.  This is a pretty dang easy build, even using perf (which is what I used), so someone may well have one built by the time I get soundclips posted.

[petemoore]

  That^.
  Is nice.
  Thank You again Mark Hammer !
  For sending my though trains to 'let's see where was it I had...and what do I have to go for to', build this one..
  Having long loved the big D+..and seeing, well reading what you have there, it looks very nice.

[scaesic]

iv been thinking about peicing together that kind of pre-amphasis/post emphasis thing for about the lastyear, but never actually got around to thinking about it properly. awesome.

[MartyMart]

Mark, as ever superb ideas and explained like a true down to earth EE   

Bless you 
MM.

[John Lyons]

Very nice.
The Dist+ is such a simple design, easy to add some complexity and add to the tonal pallette.
Thanks for yourwork here.


John