DOD Overdrive 250 Gain

[Ned Flanger]

I tried a .01uf cap to kill the buzz, but it cut too much high end.  I will just put it back to the .001uf and live with it.  Other than that it turned out great.  Everyone, thanks for all the help.



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[Steve Mavronis]

Ned, love your box labeling and etch too. Good job!

[ericohman]

really nice!!

[Ned Flanger]

Thanks, now on to a nurse quacky.

[ericohman]

Without the diodes limiting outpout/adding clipping the clipping is most likely coming from your op amp hitting its voltage rails

#1 So does that mean that the amplitude peaks of the guitar reaches all up to around right under 9V? I thought the guitar was amplified to maybe around 3V at max, based on that I only got LED's to light up when playing hard... (at stock values, I also tried using a 2M7 feedback, which was too crazy for my taste...)

#2 Thinking outside the DOD250 for a minute, wouldn't a soft clipping distortion benefit from a bipolar supply +-9V so that the guitar signal can have an amplitude of around almost 18V before getting clipped by the IC? That way, the distortion could be formed just with different diodes, zeners or whatever, in the feedback for example.
Have I understood it right if a bipolar +9 and -9V supply will mean that a guitar signal can be amplified to almost 18VAC before hitting the opamp rails?

#3 Anyone have a bipolar opamp distortion circuit, as easy as the DOD250? Would love to breadboard something like that
Or could I just give up the Vref 4.5V on the DOD250, and put another battery, hooking up -9V to pin 4 of a lm741?

I read the article on gaussmarkov site that you suggested, thanks for the tip.
http://gaussmarkov.net/wordpress/index.php?s=opamps

[ericohman]

I just realised that a 50k pot is used on gauss markovs schematic, which I used. I found other schematics today that has a 500k pot for gain. Will try that now.

However, no comments on my previous post so I thought I'd just try to do something myself... I have NO IDEA if this works as intended.
Opamp LM741, what I would like to happen is that I would like the diodes to see as much voltage as from original dod250 circuit, but I want to hear how it sounds when the signal hitting the diodes has NOT been clipped.



I only have one of each for my different opamps.. so I don't want to try without asking here if it would work.

[Joe Hart]

It should work. As long as all the components are rated to handle the voltage (which they should be). I would think that in this case, the op-amp selection would make less difference in the sound of the circuit. Let us know your findings!
-Joe Hart
P.S. For what it's worth, I have amps that sound great and use diode clippers and the op-amps run at 18V. The distortion sounds great in the amp!

[ericohman]

I'll try it later tonight. Just been fooling around with the DOD breadboard a few hours so I have to rest my fingers...

That amp you mentioned, got me thinking about this thread I started a few weeks ago...
http://www.diystompboxes.com/smfforum/index.php?topic=83129.0

I guess that's pretty much the same thing, I mean, the opamps are probably not clipping the signal in that amp right?

[Brymus]

Without the diodes limiting outpout/adding clipping the clipping is most likely coming from your op amp hitting its voltage rails

#1 So does that mean that the amplitude peaks of the guitar reaches all up to around right under 9V? I thought the guitar was amplified to maybe around 3V at max, based on that I only got LED's to light up when playing hard... (at stock values, I also tried using a 2M7 feedback, which was too crazy for my taste...)

#2 Thinking outside the DOD250 for a minute, wouldn't a soft clipping distortion benefit from a bipolar supply +-9V so that the guitar signal can have an amplitude of around almost 18V before getting clipped by the IC? That way, the distortion could be formed just with different diodes, zeners or whatever, in the feedback for example.
Have I understood it right if a bipolar +9 and -9V supply will mean that a guitar signal can be amplified to almost 18VAC before hitting the opamp rails?

#3 Anyone have a bipolar opamp distortion circuit, as easy as the DOD250? Would love to breadboard something like that
Or could I just give up the Vref 4.5V on the DOD250, and put another battery, hooking up -9V to pin 4 of a lm741?

I read the article on gaussmarkov site that you suggested, thanks for the tip.
http://gaussmarkov.net/wordpress/index.php?s=opamps

OK sorry I've been busy.
Anyway,I wouldnt worry about using a bi polar design.
Search the forum your not getting a 9v swing from your 9v battery,and unless the PUs are hot I would say 1vpp max for your input signal.
The character of the DOD 250 is defined as much by the IC clipping as it is by the diode clipping,if you go to better,cleaner ICs you are going to lose the character.This may or may not be a bad thing,but it wont be true to the 250 distortion.
You might want to read Oliphant's threads about op amp design and his "original OD" they should enlighten you on what is going on.
Also read "The technology of the Tube Screamer" at GEO(by RG Keen) http://www.geofex.com/article_folders/tstech/tsxtech.htm
And then the article at GGG "cooking your own distortion" http://www.generalguitargadgets.com/richardo/distortion/index.html
These will give you more insight to opamp workings in a distortion box.
The gain in the 250 is increased as the resistance to ground is lowered by the gain pot connected to the feedback loop.
Using a 500k pot will start at around a gain of just over 2,and doesnt get crunchy until the resistance starts to get quite low.
Using the 50k pot just starts you out at a more desirable starting point crunch/gain wise around 10 to just over 100 when it reaches 1 or 2 ohms ,this final gain is set by the 4k7 resistor in series with the gain pot.
If your set on trying a bi polar supply I would use a 12v = +/-6v supply this would be better for starting with,IMO
Just read the data sheet of whatever IC you want to use and check the minimum operating voltages.
FWIW the 250 has enough swing to kick the cr@p out of an amps input stage,so anything above that would have to be attenuated and really is wasted IMO
After all here you are after distortion,if the goal was un-distorted signal gain then that would be quite different.

EDIT actually the max gain would be about 213 with a 1M feedback resistor and the gain pot reaching 0 ohms and a 4k7 resistor in series with the gain pot

[Brymus]

It just dawned on me that perhaps the DOD 250 got its name from the max gain rounded off ?

[ericohman]

Thanks for the answers Brymus. I have read those links several times, and since I breadboarded the dod250 I felt I actually learn more just by popping in components here and there to see what happens.

I'm mostly interested in what it would sound like without opamp distortion. I like the sound of dod250 and I will build one with a 5k pot in series with a 1k pot, in place of the 4.7K resistor, that's how I have it on the breadboard, the pot affects the tone pretty much in a way I like it.

However, I'm not really looking for more output of the pedal, I just want the signal to be clipped only by the diodes, just for fun really.. have no idea how it will sound compared to how the dod250 circuit sounds. I noticed that even without the diodes the dod250 has some overdrive tones.

I made a sketch, may be technical errors in it so beware...



EDIT: ...and for that, I figured I have to have more voltage so that the guitar signal can be amplified without being clipped in the opamp. And the easiest way I could think of was to use two 9V batteries hooked up like the schematic in my previous post.

[Joe Hart]

It just dawned on me that perhaps the DOD 250 got its name from the max gain rounded off ?

Wow! I'm not being sarcastic. That's mind blowing.
-Joe Hart

[ericohman]

I'm gonna try to stick two 9V batteries on the breadboard.
I hope this is what will happen, more headroom, meaning a cleaner signal, yet roughly the same signal amplitude as the original DOD250 will hit the diodes...

[ericohman]

I tried to breadboard this:



But I got no sound.. don't know why but I guess I made some mistakes because I was modding the dod250 that was already there. Will redo everything from the beginning this weekend...