Bass guitar friendly mods for DOD Overdrive 250?

[davesisk]

Hey folks:

OK, I'm going to build a DOD OD250 as my first project, since there's step-by-step instructions here.  Cool.  I plan to breadboard it first on one of the solderless breadboards so I can make sure I'm actually capable of doing this.. , then maybe try some mods to make it a little more bass guitar friendly.

So, two mods come to mind:

1) HF rolloff:  essentially I'm thinking of using the equivalent of a passive tone control (just like what you'd find on a passive bass guitar) to roll off highs.  I'm assuming that I should be able to just wire a passive tone control between the output and ground to filter out high distortion.  (Somebody tell me if this is a totally wrong assumption.)

2) Blend clean and distortion:  I'm not sure how to go about this, so I'm looking for any advice.  I use almost entirely active basses, so I probably *could* just split the input signal without worrying about any tone-sucking, but then I do have one bass with passive mode.  I'm hoping there might be a simple mod that I could make to the existing circuit that would allow me to mix the clean and distorted signals, like maybe just adding a pot somewhere?  For instance, could I connect a variable resistor between the feedback loop to the minus side of the op amp and the final output?  Or something along those lines...  I'm hoping for a simple way, rather than having to add anything too complicated to this circuit.  (Trying to keep it newbie-friendly, since it's my first project.)  Please post some suggestions on this if you have any.

I've got a DOD OD 250 model on a Digitech GNX4, and it sounds very midrangey with a bass, not a whole lot of lows and no extended highs...by itself, it wouldn't be my first choice for a bass overdrive/distortion.  However, mixed with a clean signal and with some of the high distortion selectively rolled off, I'm thinking this might be a pretty sweet-sounding grit-box for bass!

Dave

[petemoore]

1) HF rolloff:  essentially I'm thinking of using the equivalent of a passive tone control (just like what you'd find on a passive bass guitar) to roll off highs.  I'm assuming that I should be able to just wire a passive tone control between the output and ground to filter out high distortion.  (Somebody tell me if this is a totally wrong assumption.)
  DiST+ with a treble control knob is just what I re-built, I've built many a 250 type, actually I left off the diode HF rolloff cap, I like the hash at that point, then to trim it down later with the "TC.
  2) Blend clean and distortion:  I'm not sure how to go about this, so I'm looking for any advice.  I use almost entirely active basses, so I probably *could* just split the input signal without worrying about any tone-sucking, but then I do have one bass with passive mode.  I'm hoping there might be a simple mod that I could make to the existing circuit that would allow me to mix the clean and distorted signals, like maybe just adding a pot somewhere?  For instance, could I connect a variable resistor between the feedback loop to the minus side of the op amp and the final output?  Or something along those lines...  I'm hoping for a simple way, rather than having to add anything too complicated to this circuit.  (Trying to keep it newbie-friendly, since it's my first project.)  Please post some suggestions on this if you have any.
  Well if you're breadboarding, it's maybe worth a try AFAICT, with the buffered output of the bass...etc., you might want to do some reads...Splitter Blend and articles at GEO.
  Here's an idea for the voicing, get a cheepo 7 band EQ/Boost to test before/after the distortion circuit, it may help in deciding what freqs you want boost/cut, [also how much general boost/cut] and where is a good place to do it.
  With an active pickup output to DIST+ input and low frequencies, I would guess there may be a tendancy to sound buzzy, as bass distortions sometimes/often do, attenuation of certain frequencies at the input might be hand, try the bass's controls also, it's basically placed at the input.
  Another idea is frequency specific 'mix', the clean signal path could be say bassy [because slow frequencies distortion may sound buzzy..] perhaps compression.., then voice the distortion circuit path for maybe a higher frequency portion w/distortion.

[davesisk]

Hey...thanks for the reply.  So, you're saying the very similar MXR Distortion+ might be a better choice because it has a high-frequency rolloff already?  I'll take a look at that one.

Sounds like you're saying the Dist+ rolls off highs prior to the distortion stage rather than after?  Interesting idea...I was thinking after rather than before.

I recall seeing that splitter/blender circuit somewhere, so I'll look it up again.  If it's simple enough, maybe that would be a decent add-on after I get the DOD250 or Dist+ working on the breadboard.  I actually already have a pretty good idea what frequency range I need to roll off, particularly if I'm going to blend the dirty signal with the clean signal.  I'm still hoping there's some very simple mod I can do to the DOD250 or Dist+ circuit to gain clean/dirty blend capability though...

Thanks,
Dave

[petemoore]

Sounds like you're saying the Dist+ rolls off highs prior to the distortion stage rather than after?  Interesting idea...I was thinking after rather than before.
  The little cap to ground at the input does that a little bit, make it bigger it'll rolloff more highs, same for the cap which isn't across the Diodes in the DOD250, rolls off highs, anywhere you want to roll off highs...just take yer smaller to very small cap, clip on some testwires, one end to ground, the other end to wherever you want highs rolled off.
  Larger cap/pot to ground makes an adjustable LP filter, which can be placed on output [where I put it] or anywhere really.

I recall seeing that splitter/blender circuit somewhere, so I'll look it up again.  If it's simple enough, maybe that would be a decent add-on after I get the DOD250 or Dist+ working on the breadboard.  I actually already have a pretty good idea what frequency range I need to roll off, particularly if I'm going to blend the dirty signal with the clean signal.  I'm still hoping there's some very simple mod I can do to the DOD250 or Dist+ circuit to gain clean/dirty blend capability though...
  Not sure what the impedance of your bass's output is, could be all you need is a separate path and summing pot, but I think you may want to have the clean path boost also, the attenuation at the mixing pot will make it unity or less, part of the charm would likely be ... clean-ish boost/dist mix available, output above unity.

[davesisk]

I couldn't find any blender project on Geofex.com (I assume that's what you meant by GEO).  However, found this here on this site...two blender/effect loop circuits:

http://www.diystompboxes.com/smfforum/index.php?topic=43947.0

Dave

[petemoore]

  As you can see the B Blend has 1 send/return opening.
  The Splitter/blend has both sides 'jacked', that makes a total of 6 jacks worth of signal assignments, IIRC there is a volume drop toward the center of the blend pot adjustments, but it does allow blending of any two parallel circuit paths, each re-patchable...put anything or something else in path red or green, watch the numerous input/output assignments, or just plugging it in correctly may require 'debugging'.
  I haven't built the B. Blender, but the pot introduces 25k to each sides signal path when in the middle position, adjustable gain on the 'bypass' side via 2k pot with 1k stop resistor might be useful.

[John Lyons]

Try this Dave

Search for the OD250. The "77 version sounds best to my and a few peoples ears.
If you make the input and output caps 1uf it will do better for bass (more Low end in and through the circuit)
The .001 or .002 cap to ground at the output will cut off some of the fizzies. You can experiment with a larger cap there if you get too much treble and fizz.

Also search for the Stupidy Wonderful Tone Control which will work to cut some treble, and will be variable. (try SWTC as well)

The B bleander should work fine as well. There is a layout in Mark M's gallery if I'm not mistaken.

John

[Mark Hammer]

Both the DOD 250 and the MXR Dist+ use a configuration where the gain of a non-inverting op-amp is set via a variable resistance between the inverting (-) pin and ground.  Looking at the schematic here ( http://www.generalguitargadgets.com/diagrams/dist_250_sc.gif ), this is the path between pin 2 and ground.  gain is increased as the resistance gets lower.  Maximum gain is set by R6 and R8.  This is x213 in the stock DOD 250 (assuming the resistors are the precise value they say they are; remember that they can be +/- 5% from that).

Unfortunately, as you decrease the value of R6+R7, that interacts with C4 to roll off the lows at the same time that the gain goes up.  With the components listed, bass is rolled off starting around 720hz at maximum gain/drive.  In the case of guitar, this lends a certain "bite" to the tone and is one of the reasons why some guitarists liked it.  As you move towards more clipping, not only do you get that extra harmonic content, but it becomes more obvious to your ears as the bass is trimmed back and the overall fundamental-to-harmonics balance is shifted.  Note that this is not just a perceptible boost for the mids and highs, similar to what you'd achieve with a post-distortion EQ (though it sort of sounds like that too), but means there is less drive in the low end as you turn up the gain.  We'll come back to that later.

What works for the goose here ain't so good for the gander.  A 6db/octave low-end rolloff at 720hz pretty much guts your lower frets/strings/notes on a bass.  Although the rolloff isn't quite so bad at lower gains, you shouldn't have to be in a position whereby you sacrifice nastiness to save bass.

There are a few solutions to this dilema.  The easiest is to pick a C4 value that leaves you with enough bass even at the highest drives.  A 220nf cap (.22uf) would provide a bass rolloff starting around 154hz at maximum gain.  As you reduce gain, that corner frequency drops until at minimum gain the rolloff begins at around 1.4hz.

Is that "better"?  Yes and no.  Clearly you have the bass back, but tucked away in that added low end is also the dreaded 60hz - home of the hum.  Ideally, you don't want to be multiplying 60hz hum by a factor of x100 or even x20.  So, can we have a configuration that gives us back the bass we need, but downplays the low end we don't want?  Yes.  The thing to remember is that the gain of a noninverting stage is set by both the feedback resistance (R8 in this case) and the "ground leg" or resistance to ground (R6+R7 here).  As R8 goes up (holding R6+R7 constant), so does gain.  As R6+R7 goes down, holding R8 constant, gain also goes up.  What if we could arrange to have R8 get bigger and R?+R7 get smaller at the same time?  And if we could, what would the advantage be for a bass player?

Well, if we could do both those things at the same time, then it might be able to cover roughly the same range of gain settings without having to change the total value of R6+R7 all that much.  Remember, that it is the the smallest values of R6+R7 in the stock version that sacrifice the most bass, and the largest values of R6+R7 in the suggested modified version that introduce the greatest risk of having accentuated 60-cycle hum.

So here's what we do.  Happily, the circuit does not care at all about the order of C4, R6, and R7.  All calculations will remain pretty much the same.  What we will do is reverse their order: C4 directly to ground, then R6, and finally R7 connected to pin 2.  But what we'll do is connect R7 differently.  In this instance, the wiper will be connected to pin 2, and one of the outside lugs connected to R6.  The other outside lug will be connected to R8 (naturally this means you break the normal connection from R8 to pin 2 and connect its free end to the pot).

What do we have now?  As we turn the pot in one direction, the resistance of the pot is added to R8 as the value of R6+R7 gets smaller: more gain.  In the other direction, R6+R7 as the R7+R8 value gets smaller: less gain.

I still need to demonstrate that this has advantages for bandwidth.  Let's say that R8 was 270k, R6 was 3k3 and the pot was the regular 500k.  Turned completely in one direction that's a gain of 270K+500k+3k3/3k3 or x234; just a smidgen hotter than stock, with a low-end rolloff starting around 212hz at max gain.  Not perfect, but much better for bass than 720hz.  Turned completely the other way we have a gain of 270k+500k+3k3/500k+3k3 or x1.5.  We can use all of that 500k in the feedback loop, but we don't need all of it in the ground leg.  Let's say we placed a 15k fixed resistor (R12) in parallel with one side of the pot - the part between pin 2 and R6.  Since we have the pot and resistor in parallel, the maximum resistance possible is constrained by the added resistor, at least for that half/leg of the pot.  The other leg is free to reach maximum resistance, but the paralleled leg will never go any higher than 14k6.  This would make our minimum gain [270k+14k6+3k3]/[14k6+3k3] = 16.8.  Obviously more than the x3 found in the stock unit, but unlikely to sound dramatically less clean and something you can easily trim back with the volume control.  More importantly, with a maximum possible value of R6+(R7+R12) = 17k9, our minimum rolloff at lowest gain is now 40hz. 

That's not our 60hz goal, but remember that as we start to turn up the gain, that rolloff point will go upwards.  Moreover, if we drop the value of C6 a bit (if it isn't doing so already), we can probably filter out a bit more extreme low end and hum.

Note that you will be unlikely to want a .001uf cap for C7 all the time.  Consider placing a .0027 and .0033uf cap in series and using a SPST toggle to shunt the .0022 cap.  That would give you one tone setting where much more of the high end is rolled off so as to get bass growl, and another setting which is just a bit tamer than the stock for when the gain isn't up all that high.  Finally, since the modded version will allow more drive from the low end as you turn up the gain, you should actually be able to achieve MORE fuzz from the bass than a stock unit...when you want it.