My god, what have i done?!?!?!

[Boy Howdy]

Scared ya, didn't I?

I think I may have screwed up my DOD250.

It's an '81 model. I've had it for years and never liked it. But then I found a site that had the mods and all. So I changed the input resistor to the grey model spec. I left the IC the same (an 081 something).

The whole reason I went in there though was to try to convert it to a clean boost by removing the clipping diodes. Worked. Pretty well, actually. Huge volume boost, and while not totally clean (the IC distorts of course) it was nice enough. I ultimately decided though that I preferred the sound with only one of the diodes left in place. Sounds fantastic now.

Is it okay to use only one diode? Maybe this is the source of my problem (see below).

The problem is that when I turn up the distortion level beyond about half way I start to get a sort of torn speaker sound that occurs at the end of the note. It's sort of random. Sometimes there, sometimes not. It's not a terrible sound but I'd like to not have to tolerate it. I generally run that control pretty low, it doesn't even show up at that level. It's just one of those minor annoyances.

I did play around with putting the DOD in the effects loop of my Traynor ('68 Bassmaster) which I've modded (tweaked Plexi, JTM 50). The loop is not compensated in any way, so anything that goes in there gets slammed pretty hard.

Could I have screwed up the IC by doing that? I guess I should have thought of that before, huh?

So what do ya'll think?

Thanks.

[Steve Conner]

Taking one of the diodes out is very probably what caused the torn speaker sound: it's probably blocking distortion caused by the remaining diode rectifying the signal and pumping up some coupling capacitor somewhere.

Troubleshooting 101: Put the diode back and see if the nasty sound goes away. It's not rocket science.

As a compromise, you might want to try replacing the diode you took out with a red LED. Heck, you could replace them both with blue LEDs for more output.

[Boy Howdy]

Yeah, that's what I was thinking, replacing the one silicon with a couple LEDs and all. I just haven't gotten around to opening it up again.

[Mark Hammer]

The removal of one of the diodes would have the following effect:

1) clipping one half of the waveform, while leaving the other half intact;

2) increasing the maximum output level by maybe 30%.

As for the torn speaker sound, consider that at clearly supra-threshold input levels (i.e., between the pick attack and the gain setting, the signal hitting the remaining diode is clearly above the clipping threshold), you get a smoother sound. As the note decays, you get intermittent clipping since it is applied to only one side of the waveform - essentially half as often.

There are a couple of things you can do to get a more desirable balance between cleanliness and dirtyness. One is to bump up the value of the .001uf cap in parallel with the diodes. That will shave off more top end and make the resulting sound "warmer". A value of .0033uf-.0039uf might be appropriate. If you are reluctant to remove anything from the board, you can simply tack on a .0022-.0027uf cap on the copper side of the board to the pads opccupied by the existing cap. It can easily lie flat enough to fit the available space.

A second is a little more intrusive, but permits for more variable sound. The extent to which clipping is produced depends not only on the placement/existence of the diodes, but on how low-impedance a path they provide for the signal. Placing a variable resistance in series with them will adjust the hardness or softness of the clipping produced. So, using the middle, and one side lug, of a 5k pot would introduce varying amounts of series resistance and reduce the harshness of the clipping produced, even at max gain. Note that reducing the clipping action also means that the ceiling is raised on maximum output level.

One of the quirks of the DOD250 and the MXR Distortion+ (VERY similar designs) is the change in bass as gain is increased. The gain control is tied to a 4.7k resistor and .05 (or .047uf) cap. gain is increased by making the resistance along that path (pot value plus 4.7k) smaller. Unfortunately, as you do that, the cap value has the effect of rolling off bass. At maximum gain, the bass rolloff starts around 700hz.

My gut sense about this is that the pedals (both 250 and Dist+) were introduced at a time (mid-70's) when there were very few low-noise or hum-rejecting pickup options for those on the Fender side of the fence. Since both pedals work by applying gain, if there was any 60hz hum in your signal, amplifying it by 200x was not really in your best interests. So, as the gain was increased in these pedals, bottom was rolled off such that 60hz hum was not as noticeable or annoying. The perk was that it made your signal appear to cut through a little more due to the simultaneous application of clipping, gain, and bass rolloff. Alternatively, the designers went for the tonal change, and found that the bass rolloff and hum-reduction was the perk.

If your goal is to try and using it not as a distortion but as a way of introducing either clean boost or just mildly colored boost, AND if you have a largely hum-free signal anyway, why not recoup all that lost bass? To do that, either replace the stock .05uf cap with a .22uf unit,or simply stick a suitable .1-.22uf cap in parallel with the existing .05uf unit; again, on the copper side. Don't worry about cap type; flat ceramic types are perfectly okay here.

The added capacitance, and the variable clipping hardness/softness will give you a nice booster with greater bandwidth and no speakers being torn.

[Boy Howdy]

Thanks for the reply, Mark.

I will study it tonight and see if there's anything that sounds like something I want to try. Actually, I did try replacing that .05 (C3 presumably) at one point. I liked the increased fatness, but I then put that back to its original value and tried just replacing the stock .001mf input cap with a .01mf and the difference between the two options amounted to basically the same sound so I just left the .01 in there.

I think what I will do is pick up some blue LEDs tonight and try those. If that doesn't give me what I want I can then either leave it with just the one Si or remove the clippers altogether. I tried that (removing the diodes) and that was great too. It's the fat, natural tone that I get out of the thing more than the overdrive that's important to me ... and the boost of course. But then maybe it's not okay to just remove the diodes???

The other option of leaving just the one diode works fine as long as I keep the gain control below 12:00, which is how I generally use it.

Thanks again.

[Boy Howdy]

Hmm. Well, I picked up some LEDs. Yellow ones, couldn't find blue. I put those in. I ended up with 1 yellow LED and 1 Si in series paired with the same configuration going in the opposite direction. That gets me close to the right amount of distortion for what I want.

It's the damndest thing though, that raggedy, staticky decay is still there. I used to have a Daddy O that had that same sound. And when I compare the DOD with a Dano Cool Cat Drive that I just picked up (not a bad little pedal, incidentally), The Drive seems to have a little hint of that sound too (much more subtle). Maybe it's just the nature of diode distortion???

I haven't spent enough time with the DOD in its most recent incarnation yet. I will continue to mess around with it and see if I can get it figured out.