DOD Overdrive 250 Gain

[nils]

Hi there!

Today I threw together a DOD OD 250 board from spare parts. I built it around one half of a spare 4558 and with asym 1N4148's as clipping diodes after the schem from tonepad. After I tried it (at less than bedroom level though) I was wondering why it had so little distortion; somehow I expected more from it than just some mild crunch. What's the problem with my cicruit? The IC, or do I need other diodes for more OD? Any hints?

Thanks!

[vdm]

hey nils,
one thing you may not notice about that circuit and all it's similars (dist +, blue clipper etc..) is that the 741 is a key component to the sound.
if you take a stock dod 250 and remove the diodes, you will still get some pretty heavy crunch, the adding of diodes is added to push it even further into distortion (or should i say 'squish' it...)
the pedal is also very good if you can use it to push a tube amp into overdrive, though this often requires increasing the resistor after the opamp, and personally i like 50k pots. they give a fairly smooth increase in gain, and unlike the stock (on some units) value of 10k, it is a much more practical volume control.
hope this helps,
trent

[Phorhas]

Hi there... here's my 2 cents...

the silicon diodes you use can stand more voltage before clipping, more voltage than Ge diodes that is - consequently the distortion you geat from them is more OD like beacase a bigger amplified clean signal can pass throw (note that with OD you get more output in terms of volume) for an even more "open" OD sound you can use rectifier diodes, fets trannies and LEDs (try differant types with each diode for asymmetrical clipping).

you can try to increase the gain of the op-amp and use ge diode for a more of a "distortion" sound.

hope it'll help a bit.

[Ned Flanger]

I also built a Distortion+ using the 4558 and 1n914 diodes and I am very disappointed with the amount of distortion.  Naturally I am replacing the diodes with Germanium, but is there anything else I can do?  Would a 741 produce more distortion?  Any help is appreciated.

[zombiwoof]

Also, if you socketed the 4558, try a 1458 in there, it is supposed to be a dual 741.  It won't get you more distortion, that's another matter, but will be closer to the 741 in sound then  the 4558.  1458 is often used in modding the reissue 250's.  DOD also puts an extra cap in the reissues with the 4558 to cut back some of the extra high end response of the more "hifi" chip.

Al

[Ned Flanger]

Nice.  Will do.  How about the diodes?  I am changing them out regardless, but will the germanium add that much more distortion?

[Mark Hammer]

1)  Most of the signal lives in the "bass-ment".  At max gain, the 250, like it's sibling the Distortion+, rolls off at 720hz.  My hunch is that this strategy was originally adopted to avoid boosting the hum that was typical of many single-coil-equipped guitars in the 70's (although this hasn't been confirmed by undisputable evidence).  Whatever the case, use of a bass rolloff that decreases as gain is increased will get you much less clipping at max gain than you were expecting.  If you want more grind for your gain, lower the rolloff by means of a larger cap value than the stock .047uf component.  A .22uf cap or even .47uf makes more sense.  the extra bass will result in more clipping because those mids get to ride in on a much larger low-frequency wave.

2) The total distortion produced is a function of a) the clipping diodes, to be sure, b) the limits of the chip, given the supply voltage and gain, and c) the impact of the output signal amplitude on whatever the pedal happens to be plugged into.  Neither the Distortion+ nor the DOD250 were intended to be all-out fuzzes.  They were intended to provide some coloration on their own, but induce even MORE coloration from the amp.  Use of germanium diodes will, to be sure, result in more clipping at the pedal level, but will impair clipping from the amp because of the impact on output amplitude from the pedal.  There is, of course, a balance to be achieved.  If you use a trio or quartet of Si diodes, you'll get more output at max volume, but it won't be clipped as much, and there won't be nearly as much compression of the signal as you'd get from Ge or even a simple pair of Si diodes.  I find optimum to be a humble pair of Si.

3) The Dist+ and DOD250 are set to a max gain of 213.  If you're after sizzle, make the 1M feedback resistor 1.5M and the 4k7 resistor in series with the gain control 3k3.  That'll get you a maximum gain of 455, which starts to put you in the serious sizzle zone.  Dropping the 4k7 to 3k3 will demand that you increase the value of the .047uf cap, even if you currently like the bass rolloff.

4) Gain is certaily related to amount of distortion, but fundamentally it your signal's "proximity to clip" that matters.  If the signal you hit the 250 with is VERY robust (e.g., boosted by a compressor, booster, EQ, or any combination) then you can expect the clipping stage to react more aggressively because what you have fed it does not need very much additional gain to be at the clippin threshold of the diodes.  If you feed the pedal unadulterated lipstick pickups, then your guitar is gonna need a LOT of help from the gain stage to reach the clipping threshold of the diodes with any degree of consistency.

[Ned Flanger]

Mark, thanks for the info.  Looking at the schematic, there are four 1M resistors, which 1M resistor is the feedback resistor? 

However, this brings me to another question for Mark or anyone else.  I have become quite good at building pedals from schematics and pcbs, but I truly do not know how they work.  I have read much of the wiki page and that has tons of info, some a little too much to swallow in one sitting.  I would like to start to design my own and modify my existing pedals.  What is another good resource to find out how everything works together to produce these effects.

Thanks,

[Joe Hart]

Mark, thanks for the info.  Looking at the schematic, there are four 1M resistors, which 1M resistor is the feedback resistor? 

However, this brings me to another question for Mark or anyone else.  I have become quite good at building pedals from schematics and pcbs, but I truly do not know how they work.  I have read much of the wiki page and that has tons of info, some a little too much to swallow in one sitting.  I would like to start to design my own and modify my existing pedals.  What is another good resource to find out how everything works together to produce these effects.

Thanks,

The 1M resistor is the one that is connected to the op-amp in two places. This is the "feedback loop".

Read the "Technology Of..." series. They are exceptional. Also, here's a link to a page about the DOD 250: http://www.rabbathrecordings.com/DOD250.htm (on this schematic, the 1M resistor in the feedback loop is R5).

Hope this helps.
-Joe Hart

[Ned Flanger]

Awesome!  Thanks everyone for the help.

[ericohman]

I have a DOD250 on breadboard right now. Yesterday I wired up a boxandall James tone control (2 knobs). I mounted the pots on cardboard so that I easily can hook the tonestack to a breadboard. I tried to hook it up before and aftee the opamp. Tried a few places, IIRC the sound got very fuzzlike and gated when I connected it right before the opamp, don't know why really.

I also tried it before the diodes (I used 2x 1N4001), It worked pretty well to have the tone section there.

Feedback loop:
Can I put diodes in the feedback loop on the DOD250?
Like the marshall bluesbreaker where the diodes are in the feedback loop of the TL072. I believe there's diodes in the feedback loop on the tubescreamer too?

[newfish]

For all its currently being out of fashion, other op-amps don;t sound like a 741.

Inspired by Steve's excellent 'Gray Spec' build, I also threw a quick op-amp Dist circuit together on my breadboard with a TL071 - just to see what would happen.

Not masses of gain, but makes an excellent clean boost / buffer.

Swapped the '71 out for a 741 and the difference was instant.

<i realise the '71 is J-FET based - I was just curious...>

[Joe Hart]

I know that there seems to be some debate on this, but I can clearly hear the difference between op-amps. With all else being equal, side-to-side, I can hear differences. Of course, so much else changes the sound so much more that I don't get all crazy about it. But there are definitely differences in the sounds.
-Joe Hart

[ericohman]

I went to the post office tonight to get my shipment full of diodes. I tried 1n4001, 1n4148, 1n34 germanium, zener 2.4V, 3.3V, 5.1V.

The 1n4148 and 4001 was closer than I would think, and it was hard to get a feeling for how the 1n34 sounds because the output was so low. The zener had the biggest differences, I tried 2.4V and 5.1V and both of them sounded less muddy, more treble in general, IIRC, the 5.1V sounded a bit less muddy, and more crunchy.
I did all the test on breadboard and I used symmetrical clipping (2 diodes).

I have a question #1 though, without any diodes hooked up, the output is a lot stronger, but less distortion. However, there is still fairly big amount of clipping, where does that clipping happen?

#2 Also, what sets the "quality" of intermodulation. I noticed it sounds pretty bad, even at low distortion levels, for example when plucking a major third, open d-string and f# at fret 2, 6th string, etc... Is that because of the opamp?

#3 Is there any pedal that is known for "sweet sounding intermodulation"?

[Brymus]

Without the diodes limiting outpout/adding clipping the clipping is most likely coming from your op amp hitting its voltage rails
As the signal output exceeds the rails its clipped,where it hits and exceeds is left flat.
Gausmarkov has an excellent tutorial explaining this better
Not sure about your intermodulation however the 741 is suppose to have a pronounced high roll off the higher the gain(due to the low slew rate IIRC),and with out clippers limiting it this is probably more noticable

[Tony Forestiere]

I tried 1n4001, 1n4148, 1n34 germanium, zener 2.4V, 3.3V, 5.1V.

Oh, come on. Try some LEDS. Maybe try some asymmetrical combinations 

[ericohman]

I tried assymetrical clipping using 1n4001 yesterday but didn't like it as much. Going to try some LEDs tomorrow. Just have to buy a few first I only have white ones at home. 

[Electron Tornado]

The 1n4148 and 4001 was closer than I would think, and it was hard to get a feeling for how the 1n34 sounds because the output was so low. The zener had the biggest differences, I tried 2.4V and 5.1V and both of them sounded less muddy, more treble in general, IIRC, the 5.1V sounded a bit less muddy, and more crunchy.

I have a question #1 though, without any diodes hooked up, the output is a lot stronger, but less distortion. However, there is still fairly big amount of clipping, where does that clipping happen?

A pair of 1N34s, I believe, are what is used in the Dist +, and possibly also in the OD250 (they are both almost identical circuits). When using a pair of them, you can expect a lower output because they are clipping more of the signal than a pair of 1n4148s or 1n4001s, so less signal gets through.  When you remove any clipping diodes, the signal is not being clipped and more signal gets through, hence it's louder.

Turn the gain up toward max and the op amp itself will start to clip. That clipping will still be present even with clipping diodes in the circuit, but the diodes are clipping more than the op amp, so what you hear has more to do with the diodes.

[Electron Tornado]

#2 Also, what sets the "quality" of intermodulation. I noticed it sounds pretty bad, even at low distortion levels, for example when plucking a major third, open d-string and f# at fret 2, 6th string, etc... Is that because of the opamp?

#3 Is there any pedal that is known for "sweet sounding intermodulation"?

Here is a bit about intermodulation from a Wikipedia page about the Fuzzbox: http://en.wikipedia.org/wiki/Fuzzbox  :

As clipping is a non-linear process, intermodulation will occur, leading to the generation of an output signal rich in extra harmonics of the input signal. Intermodulation distortion also produces frequency components at the various sums and differences of the frequency components of the input signal. In general, these components will not be harmonically related to the input signal, leading to dissonance. To reduce unwanted dissonance, simple power chords (root, fifth, and octave) are often used when using fuzzboxes, rather than triads (root, third, and fifth) or four-note chords (root, third, fifth, and seventh).


Read this article for more on distortion:   http://www.geofex.com/effxfaq/distn101.htm

[ericohman]

Thanks!

I have now been tested a lot of things on the circuit and I start to get a feeling of what kind of mods suits me.
The only thing I really think makes a big difference to the better, is the R6 resistor.

Schematic can be found here:
http://www.rabbathrecordings.com/DOD250.htm

I also tried a pf value cap in parallel with the 1M feedback resistor but it didn't hear much difference, at least not with a value of 25pF like on gaussmarkov's site. Tried a few other values but the only difference I could hear was to the gain.

However, back to the R6 resistor. I tried a few values and settled with a 3.3K resistor and put it in parallel with the 4.7K (R6) and the pedal had a lot more treble now. Good with the normal channel on my jtm45 clone amp. I also accidentally put a 10R resistor in there, beware, crazzzyy noises!!

Is it hard to wire up a pot as a variable resistor, in place of R6, and make the pot only go between, say, 2k and 6k. Can I wire resistors directly on the pot lugs somehow to make the sweep min-max be in 3.3k-6k territory?