DOD Overdrive 250 Gain

[Electron Tornado]

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.

[Joe Hart]

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

[Electron Tornado]

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.

[ericohman]

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.

[zombiwoof]

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

[Steve Mavronis]

For reference here is an illustration I made (not to scale) by studying photos from actual gray 250 PCB's:

[Electron Tornado]

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.

[ericohman]

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.

[Ned Flanger]

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?

[ericohman]

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)

[ericohman]

Inspired by Steve's excellent 'Gray Spec' build

Who's Steve?

[Mark Hammer]

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.

[Load3r]

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.

For reference here is an illustration I made (not to scale) by studying photos from actual gray 250 PCB's:

[Steve Mavronis]

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

[Ned Flanger]

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.

[Electron Tornado]

[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.

[Ned Flanger]

I'll try that and let you know.

[zombiwoof]

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

[ericohman]

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?

[zombiwoof]

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