DOD Overdrive 250 Questions...

[phillip]

I've going to build a nice simple distortion after building things like the Tremulus Lune and Ross Compressor, so I've chosen the OD 250.  Does anyone know the differences between the grey/yellow and yellow/black OD 250 pedals?  Tonefrenzy describes the yellow/black one as being more trebly, so the differences are probably nothing more than capacitors.

I use a Fender setup (Telecaster and Twin Amp) so I rather enjoy a good treble response...definitely don't care much for the sound getting muddy/mushy/wooly, etc.

I'm thinking about doing the mods mentioned at the bottom of JD Sleep's OD250 page at his site, changing the 0.01uF input capacitor to a smaller value (0.0022uF or 0.0047uF), changing the clipping diodes to red LEDs, and changing the 0.001uF "smoothing" capacitor on the output to ground to a larger value.

Are there any more mods for the OD250 that I should know about before I begin the project?

TIA!
Phillip

[brian wenz]

Hello Hello--
   Yeah, the input cap was changed from .001 to .01.
Brian.      P.S.--  The Ross Distortion is pretty much the same circuit.

[Fret Wire]

You can mod it into another pedal that uses the same layout, the Dist.+. Nice trebly attack to it. They're so similar.

[phillip]

Thanks for the help guys.  What should I use for the "Gain" pot?  I don't want to order a single 500K reverse audio taper pot and have to wait a month for it.  I read the build reports at Tonepad for the Dist + and some people were using 50K linear pots instead of the 1M pot that the Dist + calls for, which I believe is also reverse audio.

For the OD250, could I use the same trick, using either a 50K or 100K linear taper pot?

Phillip

[Fret Wire]

For the DOD/MXR, log sounds the best for volume. Definately use 100k. I use the 500k rev. log for the gain, it works best. Linear sounds too crowded. Log wired backwards would be the second best choice.
The gain in both pedals is max at 0 resistance, so 1M would allow the pedal to go real clean at the other end. 100k would be less clean in the 7:00 position. 500k gives a good useable spread. Both pedals have nice tones at the lower settings too. Alot of classic rock sounds are between 10:00 & 2:00 on the gain.

The original factory spec was 500K rev. log for gain, and 50k log for volume. Then, like E-H did, they subbed in 1M/10k for a while. Those were the values that got posted all over the net. You have a hard time hitting unity gain with the 10k volume pot. 50k works fine, 100k is the best way to guarantee good volume with any amp/guitar combo.

[phillip]

I guess I'll use a 500K audio taper wired backwards for the gain control...I don't know of any other sources than Small Bear for 500KC pots.  I'll definitely be using a 100KA for the volume control.  

If I could figure out how to work the math with RG's tapering resistor trick, I might be able to order a regular linear taper pot and use a tapering resistor to get the reverse audio.

Any ideas on how that would work?

Phillip

[Fret Wire]

No, it still confounds me too. If I saw one example with real values and a pot drawing, I'd get it. I see people suggest it, but never post resistor values, so I guess we're not alone.

Here's an idea. LED and pulldown resistors. First, everybody was wiring them into the off board wiring. Then people started incorporating them into their pcb's to save time wiring and splicing. You do it on your layouts. If tapering resistors work decent enough, it seems the tapering resistor could be incorporated into the pcb layout. Then, the ckt would only require a more common linear taper pot.

BTW Phillip, what size pots are you going to use? 16mm or 24mm?

[phillip]

I'm planning on using a pair of 24mm pots...I've figured that I can get a pair to fit side-by-side in a Hammond B box pretty easily.  24mm are the cheapest ones that Mouser stocks in common audio taper values.

EDIT: BTW I just ordered some more samples from TI that'll work wonderfully in the OD250...the Burr Brown OPA134PA, a single OPAMP that has the same pinout as the 741  

Phillip

[Fret Wire]

I ordered a bunch of 16mm 500kc's off Small Bear just before he went on vacation.

[phillip]

Did they arrive yet?

I have a couple of 500KA 16mm pots left over that I could use for the volume control...not sure if it would cause "muddiness" like it does in the Fuzz Face...

Phillip

[Fret Wire]

Actually it was two weeks before his vacation. I got half the order just before he left, and the other half after he got back.

[phillip]

Hmm would reducing the 10K resistor on the output to 8K2 or so increase the output a little, or would it cause problems?

Phillip

[Fret Wire]

The ckt is almost identical to the MXR. The standard output mod for the D+ is to lower the 10k to 100ohm.  If that sounds drastic, you could try in increments, 8k, 5k, 1k. The standard gain mod is to drop the 4.7k to 2.2/2.4k, and double the .047 cap to keep the same bass response. Same principal as the TS.

I've only done it to the reissues, because it's not feasible to change the 47k volume pot to 100k on those. They need it especially if you mod them to script specs. No problems.  On my builds, 100ka gives me enough volume.

The 250/D+ are a great platform for mods.

[nightingale]

phillip~
here is an interesting quote from Mark hammer about the RA pot value in a very similiar circuit:.

I have no idea why it would be necessary to use a reverse-log pot for the Dist+. Unless there is some very specific degree of distortion you need to dial in, there is absolutely nothing gained by using one taper over another. The circuit certainly does not need any particular taper to function.

All that pot tapers do is provide a particular degree of resistance change per unit of rotation. In some instances, if the audible change is such that the ear detects it as nonlinear, or if the user needs to have a certain amount of "rotation arc" reserved for a particular range of change that they want to be picky about, then taper can become important. But other than that taper is less magical than most think it is.

The few cases where pot taper is truely important seem to be:
1) Anything where you are controlling the volume. Log taper provides smoother volume changes because of how the ear works.

2) Anything where there is a defined "middle", such as balance controls, or EQ cut/boost controls, where it is important that the midpoint of rotation (or sliding) be very close to half the max resistance.

3) Anything where you continuously move the pot with your foot, such as a wah or expression pedal. In these instances, the pot taper is actually compensating for the degree to which foot movements of different angles can be articulated. You can provide fine gradations of movement when pushing forward with the ball of your foot, but only much coarser movements when rocking back on your heel.[ /quote]


also
i went with the fairchild red clipping LED's.. i was actually very surprised how smooth the clipping was.. i am also a tele guy.. it's a pretty cool  dist circuit IMO.. but i could never really get mine to be very "touch sensitive"..  like a good jfet boost, or a really nice  fuzz..
hope this helps

[Fret Wire]

Nightingale, the second paragraph (not #2) explains why the 500k rev. log works the best. First, we are accustomed to having gain go up when we turn pots clockwise, not the other direction. Second, as Mark states, the particular gain characteristics of that pedal make the log the best taper. It does have a wide range of useable gain. A different taper means that all the gain gets crowded into a small portion of the pot, making it harder to dial in sounds. On a fuzz for example, it's different. Most everyone dimes the fuzz pot, so taper is not important, just value.

[phillip]

Looks like I'm going to be socketing a lot of parts on my OD250 build!

Phillip