modifying the DOD 250 overdrive

[mordechai]

I was thinking about building an overdrive like the DOD 250, working from this schematic:

http://www.joehartband.com/DOD250.htm

If I replaced the 10K resistor (before the clipping diodes) with a 10KB pot, would this serve as an effective tone control?  Or would this somehow screw things up?

[Mark Hammer]

It won't screw anything up, but you may find it's of little practical use.  If you want to change the character of the clip, I would suggest playing with the ground leg of the op-amp - that segment where the Gain/Distortion control is.  Like the Distortion+, that R/C/pot path results in a bass rolloff at 720hz with max gain.  Consider use of two pots like some of the modded Proco Rats, where one additional pot is used to set the amount of full-band vs high-end drive.

[emilyandmiles]

Hmm... I'm by no means an expert, but to me it looks like the 10k resistor restricts the output before hitting the clipping diodes.

If I'm right about that (I'll be honest, I'm not usually) then lowering that value would just make the thing louder, with possibly more signal being clipped by the diodes.

If you wanted to, I think the easiest way to add a tone control would be bolting on a passive filter (like on yr guitar) onto the output.

There are some wonderfully knowledgeable folks on here who I'm sure will explain things better soon. Good luck with it

: )

[mordechai]

It won't screw anything up, but you may find it's of little practical use.  If you want to change the character of the clip, I would suggest playing with the ground leg of the op-amp - that segment where the Gain/Distortion control is.  Like the Distortion+, that R/C/pot path results in a bass rolloff at 720hz with max gain.  Consider use of two pots like some of the modded Proco Rats, where one additional pot is used to set the amount of full-band vs high-end drive.

Thanks for the suggestion, Mark.  Can you direct me to a schematic showing this modification on the Proco Rat? 

[Electron Tornado]

The Rat and the OD250 (and the Dist +) have some things in common and a lot of places where you can make changes. Using the schematic you referenced, try these:

-  C1. Increase the value here to let in more low freqs

- C4. Between the OD250 and the Dist +, one uses 10uf, while the other uses 1uf. See (or rather hear) what you like best. Maybe use a switch to make it selectable.

- R6. Decrease this to increase gain, but only up to a point or you'll get more noise.

- C3. Increase the value on this to let more lows through.

- C3/R6. Notice that the Rat has two pair of resistors and caps here. They filter different frequencies. You can change values or make them selectable with a switch or even a pot (eliminate the two resistors and use a pot to sweep between the two caps).

- R8. Maybe 10k is OK, maybe less is better, but maybe less lets more noise through.

- C5. This will filter out some highs. But try different values. Maybe you don't even need it at all.

- The Diodes. Try different types and different combinations. You may even need a rotary switch to be able to select the ones you like. Take a look at these:

http://www.muzique.com/lab/sat2.htm

http://www.muzique.com/lab/zenmos.htm

http://www.muzique.com/lab/warp.htm

- More on diodes. Maybe you would like to have them in parallel with R5 like in a Tubescreamer. Maybe you would like to have a switch to make that option selectable.


Simple is the schematic, but many are the things you can modify. Much fun will you have.

[Electron Tornado]

More advice on doing mods:

Whenever you are doing mods to a circuit, keep detailed notes. What component did you change? What was the effect? Did you like the effect it produced? These notes will help you if you want to fiddle with that circuit again in the future. These are notes you are writing from you to you. Be detailed and specific. Don't think you'll necessarily know "what you meant" sometime later. Make sure you draw up a schematic with the final values that you decided to use.

Have fun you shall!

[mordechai]

- C3/R6. Notice that the Rat has two pair of resistors and caps here. They filter different frequencies. You can change values or make them selectable with a switch or even a pot (eliminate the two resistors and use a pot to sweep between the two caps).

Thanks for the suggestions.  I particularly like the idea of using thepot as suggested above.  Question though -- could something similar also be done with the input cap instead, a la the input cap blend pot that Joe Gaggan uses on the Easy Face?  In other words, keep the fixed C3/R6, but just have the input cap sweepable via the pot to adjust the range of frequencies.  Or might this cause trouble with impedence?

[Mark Hammer]

The input cap on the DOD250 has less influence on the resulting tone than the suggested "Rat" mod will.

Electron Tornado has some very good ideas listed.

the 10k resistor before the diodes serves several purposes:

• the behaviour of diodes depends on the current hitting them, though I gather it may take more substantial changes in the 10k current limiting resistor than we can tolerate here to produce anythng audible
• the HF rolloff produced by the cap in parallel with the didoes will depend on the value of the 10k resistor.  Change it and you'd need to change the cap value
• the output level of the unit will depend on the combined resistance of the resistor before the diodes and the pot; making it larger will reduce the overall max output level

Really, the simplest change with the most impact in this basic design is to use Si rather than Ge diodes (already done) and raise the value of the LF-limiting cap adjacent to the Drive pot from .047uf to somethng greater than .1uf.

I will also add that the unit responds well to a SWTC add-on tone control.

[mordechai]

I think I will give the C3/R6 mod a try with a pot selecting between two caps to ground.  However, I'm not sure where to place this pot.  On the original schematic, the C3 comes before R6, which itself goes into the 500K pot R7 to ground.  Would I set up two caps in parallel coming off of the signal from R5, with a pot replacing R6, with lug 1 going to one of the caps, lug 3 going to the other, and lug 2 wired to go to the R7 500K pot?  Something like this:

           .............C3A
           |             |
| ----R6pot         |---------------R5
|          |            |
|         ..............C3B
|
500KB (R7)

If I'm not too far off on this, what value pot would you suggest for R6?  and I am guessing something like .0022uF for C3A and .1uF for C3B...sounds reasonable or am I off on useful values here?

[Mark Hammer]

Since it uses the ground leg to set gain, note that gain will increase the smaller the resistance to ground.  I would suggest using a 100k for the Gain/Drive pot, and a 10k-25k for the pot you label R6.  R5 can be dropped from 4k7 down to 3k3.

For cap values, with the suggested pot and R5 values, I'd recommend .01uf and .22uf.

Let's do some math.....

Suppose R6=10k and R5=3k3.  If we move the R6 wiper all the way over to the .22uf side, and reduce R7 to zero ohms, we have a ground leg of 3k3/.22uf, and a second parallel ground leg of 13.3k/.01uf.  The first has a low-end rolloff of 219hz at max gain, and a max gain (using the standard 1M feedback resistor) of 304x.  That'll give you some nice grunt.

The second parallel ground leg, via the .01uf cap will provide a boost, starting around 1.2khz, and a gain of 76x (which means you won't really notice it against the full-range boost).

Okay, let's rotate the R6 pot the other way.  At max gain (R7=0 ohms), there will be a full-range gain of 76x, with a low end rolloff of 54hz.  Meanwhile, there will also be a boost of 304x for content above 4.8khz.

Naturally, as the resistance of R7 is increased, the discrepancy between what each of those two ground leg paths results in will be reduced.  If  R7 = 50k, then whether there is an additional 10k in front of the the smaller or larger cap does not matter quite so much.  However,as R7 resistance gets reduced and gain goes up, along with treble push potential, adding ina tone control like the SWTC becomes that much more appealing.

[Electron Tornado]

Here's what I did when I built my Dist + / OD250. Referencing the schematic from earlier:

R5 and R7 are stock, but for C3/R6 I have a toggle switch between the stock values and another pair that is 0.44uf and 1k.

C4 is also 4.7uf. It is a happy medium between 1uf and 10uf (too bright and too dark, respectively)

The diodes are on a rotary switch since I put several different combinations in there.

I added a 0.001uf cap to ground before C1.

Pedal sounds great.

[mordechai]

What differences did you note with the addition of the .001uF cap at the front end of the circuit?

[Electron Tornado]

What differences did you note with the addition of the .001uF cap at the front end of the circuit?

I checked my notes, and it looks like I actually used a 0.005uf for that cap. That cap shows up in some of the schematics I found for the Dist + and the OD250, with values of 0.001uf and 0.0022uf. Now here's where I can illustrate the importance of taking good notes I mentioned earlier. All my notes say is what I actually used, but not why I used that particular value. The 0.005uf may have been the one that sounded best to me, or it may have simply been the closest value I had on hand at the time, and it sounded fine. 

Try different values and see what you like - and make notes about what values you tried and how they sounded. But don't let that one component hold you up. Throw in there whatever you have that's close, get the circuit working and then start to tweak it. OR - leave it out and it might sound good to you without it.

[WGTP]

http://www.aronnelson.com/gallery/main.php/v/WGTP/MXR_YJM_DOD250Mods.jpg.html?g2_imageViewsIndex=1

[mordechai]

What differences did you note with the addition of the .001uF cap at the front end of the circuit?

I checked my notes, and it looks like I actually used a 0.005uf for that cap. That cap shows up in some of the schematics I found for the Dist + and the OD250, with values of 0.001uf and 0.0022uf. Now here's where I can illustrate the importance of taking good notes I mentioned earlier. All my notes say is what I actually used, but not why I used that particular value. The 0.005uf may have been the one that sounded best to me, or it may have simply been the closest value I had on hand at the time, and it sounded fine. 

Try different values and see what you like - and make notes about what values you tried and how they sounded. But don't let that one component hold you up. Throw in there whatever you have that's close, get the circuit working and then start to tweak it. OR - leave it out and it might sound good to you without it.

I will certainly take notes ET -- and might post a build report too if anyone is interested.  In adding that small cap-to-ground, though, I am assuming I should put it in between the 1M to ground at the start of the circuit and the standard input cap, right?

[mordechai]

Let me bounce another idea off of your collective wisdom: if I use symmetrical silicon or germanium diode clipping, the signal may need a boost stage at the end of the circuit, as some of you have suggested.  But if I -- A) use asymmetrical clipping; B) use LEDs; and C) decrease the value of the 4.7K resistor going to the drive pot (as Mark Hammer has suggested) down to 3.3K -- would these combined mods not compensate the signal strength, increase output and even compensate for the less-intense LED clipping by allowing the op-amp to clip a bit more?  I'm less interested in a compressed distortion tone and more interested in getting a good, dirty growl with enough output so that I don't have to crank the volume knob too much to hit the front end of my amp.  I also like to feed a fuzz face signal into an overdrive pedal, and I'm hoping that this would allow for the combined signal not to get too muddy and retain some note definition...

[Steve Mavronis]

What differences did you note with the addition of the .001uF cap at the front end of the circuit?

I checked my notes, and it looks like I actually used a 0.005uf for that cap. That cap shows up in some of the schematics I found for the Dist + and the OD250, with values of 0.001uf and 0.0022uf. Now here's where I can illustrate the importance of taking good notes I mentioned earlier. All my notes say is what I actually used, but not why I used that particular value. The 0.005uf may have been the one that sounded best to me, or it may have simply been the closest value I had on hand at the time, and it sounded fine.

I will certainly take notes ET -- and might post a build report too if anyone is interested.  In adding that small cap-to-ground, though, I am assuming I should put it in between the 1M to ground at the start of the circuit and the standard input cap, right?

I believe the original '76/77 era DOD 250 with the missing 10uF polarized cap had a 2.2nF filter cap to ground at the beggining like mentioned above. On the later boards you can see the empty holes for it near the input - but located after the .01uF input cap instead of before it. I'm working on revised version of my '79/80 era grey 250 layout with extra spots for an input filter cap to ground (before the regular input cap) and for an op-amp feedback cap if needed depending on which IC is used. I love the baseline sound of the pedal so just looking for whatever helps reduce the noise signature even if somewhat an improvement. Good luck on your project and experimentation.