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!
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
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.
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.
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
Nice. Will do. How about the diodes? I am changing them out regardless, but will the germanium add that much more distortion?
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.
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,
Quote from: Ned Flanger on April 28, 2010, 08:12:30 PM
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 (http://www.rabbathrecordings.com/DOD250.htm) (on this schematic, the 1M resistor in the feedback loop is R5).
Hope this helps.
-Joe Hart
Awesome! Thanks everyone for the help.
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?
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...>
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
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"?
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
QuoteI tried 1n4001, 1n4148, 1n34 germanium, zener 2.4V, 3.3V, 5.1V.
Oh, come on. Try some LEDS. Maybe try some asymmetrical combinations :icon_lol:
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.
Quote from: ericohman on April 30, 2010, 04:46:36 PM
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.
Quote from: ericohman on April 30, 2010, 04:46:36 PM
#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 (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 (http://www.geofex.com/effxfaq/distn101.htm)
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?
Quote from: ericohman on May 01, 2010, 06:58:09 AM
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?
Just use a 5k pot in series with a 1k resistor and you'll have 1k-6k range.
I built a pedal based on the Dist+ and OD250 with a switch for two different values of R6 and C3. In one position, I have 4.7k and 0.047uf and in the other I have 1k and 0.22uf.
That part of the circuit changes the gain of the op-amp as well as the tone. I think you are increasing the gain (by lowering the value of R6) and also creating more of a bass rolloff in the process. FYI.
-Joe Hart
Quote from: Joe Hart on May 01, 2010, 07:13:22 AM
That part of the circuit changes the gain of the op-amp as well as the tone. I think you are increasing the gain (by lowering the value of R6) and also creating more of a bass rolloff in the process. FYI.
-Joe Hart
An increase in gain is exactly what the 1k is there for. The higher value cap adds bass as well.
Thanks for the tips!
Will try a few different caps. Also, is the TL072 a candidate that can replace the 741? Wouldn't hurt to try a few other opamps.
Quote from: newfish on April 30, 2010, 09:41:13 AM
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...>
Did you try the 1458?. Just wondering, because according to Analog Man info on his 250 mod, it's a dual version of the 741, and if you have one of the newer reissue 250's it's easy to pop one in, as they have 4558's in them. The 1458 (like the 741) is supposedly a little noisier than the 4558, but get you closer to grey spec sounds. Of course if you are building your own it's easy to just put in the 741.
I'm planning on putting a 1458 in my DOD 308 (the Yngvie one), and do the other mods to get it closer to grey specs.
Al
For reference here is an illustration I made (not to scale) by studying photos from actual gray 250 PCB's:
(http://home.comcast.net/~snmavronis/dod250grey-pcb-layout.jpg)
Quote from: ericohman on May 01, 2010, 01:08:52 PM
Thanks for the tips!
Will try a few different caps. Also, is the TL072 a candidate that can replace the 741? Wouldn't hurt to try a few other opamps.
The TL072 has a different pinout from a 741 so it will not be a direct swap. A TL071 will have the same pinout as a 741, however.
I have only tried the LM741.
I don't much about the dod250 history, the grey model etc. I breadboarded this circuit just because of its simplicity. And, I really like it, just the kind of distortion I like!
I think I also have a TL071, so I will try that.
OK. I am back with the changes I did recommended by Mark Hammer. I changed the 1M resistor to 2.2M and the 4.7K to a 3.3. Also I changed out the .047uf to a .47uf cap. And the thing sounds awesome. I did not imagine how much distortion could come out of a little box. However, there is quite a bit of buzz at max. Any thoughts on how to get rid of it?
Also, I swapped the 4558 with a 1458. The 1458 had quite a bit more distortion, almost too much. The signal would break up. The 4558 is much smoother.
One other thing, I figure I would switch the .047uf cap to a .1uf cap, thinking it would have an effect between the .47 and .047. The .1uf made it sound completely clean. I do not understand. Any ideas?
Quote from: Ned Flanger on May 01, 2010, 06:13:05 PM
One other thing, I figure I would switch the .047uf cap to a .1uf cap, thinking it would have an effect between the .47 and .047. The .1uf made it sound completely clean. I do not understand. Any ideas?
Just tried this, you must have mistaken a .01uF and .1uF or something.
I tried a .1uF and it sounded good, then tried a .01uF and it was a LOT cleaner, then I tried a 1000pF (.001uF) and the pedal was silent.
I tried a TL071 also, but it took so long to replace the LM741 to the TL071 on the breadboard that I forgot how the LM741 sounded once I fired up the TL071... my tone memory is just a few seconds :)
Is any of them known for being more noisy than the other?
Since I couldn't hear a difference I guess I should just go for the least noisy opamp between the two (LM741, TL071)
Quote from: newfish on April 30, 2010, 09:41:13 AM
Inspired by Steve's excellent 'Gray Spec' build
Who's Steve?
Quote from: Ned Flanger on May 01, 2010, 06:13:05 PM
OK. I am back with the changes I did recommended by Mark Hammer. I changed the 1M resistor to 2.2M and the 4.7K to a 3.3. Also I changed out the .047uf to a .47uf cap. And the thing sounds awesome. I did not imagine how much distortion could come out of a little box. However, there is quite a bit of buzz at max. Any thoughts on how to get rid of it?
Also, I swapped the 4558 with a 1458. The 1458 had quite a bit more distortion, almost too much. The signal would break up. The 4558 is much smoother.
One other thing, I figure I would switch the .047uf cap to a .1uf cap, thinking it would have an effect between the .47 and .047. The .1uf made it sound completely clean. I do not understand. Any ideas?
The resistor changes have made maximum gain 668, as opposed to the nominal 214 of the stock unit. Small wonder it sizzles now. The change from .047 to .1uf should have increased the distortion, not reduced it. Most likely the change in components was accompanied by a solder bridge or wire fracture somewhere.
The "buzz", I would imagine, has something to do with the sporadic clipping that comes in the decay phase of the note, where the tone intermittent goes from mostly fundamental to fundamental plus harmonics. In other words, the string is producing a signal that is largely under the clipping threshold, but occasionally
over it. That is partly a function of how much gain you have added, and what that does to the relationship between string signal and clipping threshold at different points in the note lifespan. If less gain is applied, or if the clipping threshold is raised enough (e.g., by means of more diodes or LEDs), as the string approaches the decay phase of the note, when the vibrations of the string tend to be more erratic (which is a big factor in why so many are displeased with their noise gates or envelope-controlled filters), the wonkiness of the string will still be below the clipping threshold so you won't produce or hear those little "crispy bits".
Since the goal is MORE rather than less distortion, I would recommend that you use the alternate strategy of filtering out what you'd rather not hear. So, stick a 22pf cap in parallel with the 2M2 resistor, and increase the ,001uf cap in parallel with the diodes to 3300pf (.0033uf). That will help to smooth out the crispy bits at highest gain.
Quote from: ericohman on May 02, 2010, 05:05:34 AM
Quote from: newfish on April 30, 2010, 09:41:13 AM
Inspired by Steve's excellent 'Gray Spec' build
Who's Steve?
This is steve. His ongoing Neo Classic Grey Overdrive (O.G. DOD250) has scads of interesting info and tidbits.
Quote from: Steve Mavronis on May 01, 2010, 02:46:18 PM
For reference here is an illustration I made (not to scale) by studying photos from actual gray 250 PCB's:
(http://home.comcast.net/~snmavronis/dod250grey-pcb-layout.jpg)
Quote from: Load3r on May 02, 2010, 10:42:05 AM
Quote from: ericohman on May 02, 2010, 05:05:34 AM
Quote from: newfish on April 30, 2010, 09:41:13 AM
Inspired by Steve's excellent 'Gray Spec' build
Who's Steve?
This is steve. His ongoing Neo Classic Grey Overdrive (O.G. DOD250) has scads of interesting info and tidbits.
Thanks, this thread is interesting to me too. My gray 250 clone project thread is here: http://www.diystompboxes.com/smfforum/index.php?topic=82633.0
Quote from: ericohman on May 02, 2010, 04:48:27 AM
Quote from: Ned Flanger on May 01, 2010, 06:13:05 PM
One other thing, I figure I would switch the .047uf cap to a .1uf cap, thinking it would have an effect between the .47 and .047. The .1uf made it sound completely clean. I do not understand. Any ideas?
Just tried this, you must have mistaken a .01uF and .1uF or something.
I tried a .1uF and it sounded good, then tried a .01uF and it was a LOT cleaner, then I tried a 1000pF (.001uF) and the pedal was silent.
I tried a TL071 also, but it took so long to replace the LM741 to the TL071 on the breadboard that I forgot how the LM741 sounded once I fired up the TL071... my tone memory is just a few seconds :)
Is any of them known for being more noisy than the other?
Since I couldn't hear a difference I guess I should just go for the least noisy opamp between the two (LM741, TL071)
Mistaking a cap sounds exactly something I would do. I thought I was crazy, but that makes the most sense.
Now going back to the buzz. I only get it when the knobs are max'd and I hold the guitar strings completely quite. Other than that is sounds awesome.
[quote author=Ned Flanger link=topic=20253.msg702030#msg702030 date=1272816969
Now going back to the buzz. I only get it when the knobs are max'd and I hold the guitar strings completely quite. Other than that is sounds awesome.
[/quote]
If you've increased the gain significantly you can expect to be amplifying any noise that much more as well. Changing the value of the cap in parallel with the diodes might help some.
I'll try that and let you know.
Quote from: ericohman on May 02, 2010, 04:48:27 AM
Quote from: Ned Flanger on May 01, 2010, 06:13:05 PM
One other thing, I figure I would switch the .047uf cap to a .1uf cap, thinking it would have an effect between the .47 and .047. The .1uf made it sound completely clean. I do not understand. Any ideas?
Just tried this, you must have mistaken a .01uF and .1uF or something.
I tried a .1uF and it sounded good, then tried a .01uF and it was a LOT cleaner, then I tried a 1000pF (.001uF) and the pedal was silent.
I tried a TL071 also, but it took so long to replace the LM741 to the TL071 on the breadboard that I forgot how the LM741 sounded once I fired up the TL071... my tone memory is just a few seconds :)
Is any of them known for being more noisy than the other?
Since I couldn't hear a difference I guess I should just go for the least noisy opamp between the two (LM741, TL071)
The 741 and 1458 are older IC's, and are a bit noisier than the more modern chips. The reason most guys go for those chips is that they are the ones that get the sound of the original 250's. If you don't care about that, you can use one of the lower noise modern chips, but you probably won't get the exact sound of the old gray 250's.
Al
Okay. I'll swap back to lm741 and see if I can hear a difference.
I've seen various schematics on dod 250. The one I have on breadboard was without a cap in parallel with the diodes. I tried what Mark said, putting a cap in there, and it made a nice difference, much smoother. I have seen schematics with caps in parallel with the diodes. Anyone care to link a verified gray spec schematic for reference?
Quote from: Ned Flanger on May 02, 2010, 12:16:09 PM
Quote from: ericohman on May 02, 2010, 04:48:27 AM
Quote from: Ned Flanger on May 01, 2010, 06:13:05 PM
One other thing, I figure I would switch the .047uf cap to a .1uf cap, thinking it would have an effect between the .47 and .047. The .1uf made it sound completely clean. I do not understand. Any ideas?
Just tried this, you must have mistaken a .01uF and .1uF or something.
I tried a .1uF and it sounded good, then tried a .01uF and it was a LOT cleaner, then I tried a 1000pF (.001uF) and the pedal was silent.
I tried a TL071 also, but it took so long to replace the LM741 to the TL071 on the breadboard that I forgot how the LM741 sounded once I fired up the TL071... my tone memory is just a few seconds :)
Is any of them known for being more noisy than the other?
Since I couldn't hear a difference I guess I should just go for the least noisy opamp between the two (LM741, TL071)
Mistaking a cap sounds exactly something I would do. I thought I was crazy, but that makes the most sense.
Now going back to the buzz. I only get it when the knobs are max'd and I hold the guitar strings completely quite. Other than that is sounds awesome.
Is it in an enclosure?. Putting it in an enclosure may reduce the noise some, with the shielding the enclosure affords. Stock circuit 250's are noisy when you get the gain and volume up high, so with mods to increase the gain you're going to get even more noise, as was noted before.
Just for general knowledge to those who haven't had any experience with these pedals, the 250 is really an Overdrive, in stock form it doesn't have a lot of gain, so don't expect it to give you a great deal of distortion. Most people that use them use them in front of an already overdriven amp, to goose up the gain (it's really good in front of a cranked non-master Marshall). Of course, if you mod it for more gain, you're making it something different entirely.
Al
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.
(http://img29.imageshack.us/img29/449/p5020031.jpg) (http://img29.imageshack.us/i/p5020031.jpg/)
Uploaded with ImageShack.us (http://imageshack.us)
Ned, love your box labeling and etch too. Good job!
really nice!!
Thanks, now on to a nurse quacky.
Quote from: Brymus on April 30, 2010, 06:30:09 PM
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
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.
(http://djoman.dyndns.org/labs/bipolar-dod250-inspired_v001.gif)
I only have one of each for my different opamps.. so I don't want to try without asking here if it would work.
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!
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?
Quote from: ericohman on May 05, 2010, 04:06:10 PM
Quote from: Brymus on April 30, 2010, 06:30:09 PM
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
It just dawned on me that perhaps the DOD 250 got its name from the max gain rounded off ?
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...
(http://djoman.dyndns.org/labs/bipolar-dod250-question01.gif)
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.
Quote from: Brymus on May 06, 2010, 04:05:24 PM
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
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...
(http://djoman.dyndns.org/labs/rails.gif)
I tried to breadboard this:
(http://djoman.dyndns.org/labs/bipolar-dod250-inspired_v001.gif)
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...