DOD 250 with LM308N = CRC Overdrive Preamp 308

[tubelectron]

Hi All,

I saw the big topic launched by Steve Mavronis about his Original DOD Grey 250 's work - congratulations ! - and clearly this little grey or yellow box is what I have found to be one of the most smoothest and natural sounding overdrive (except if you run at maximum drive setting, where the sound becomes raunchy, suitable for piercing 70's disco lead overdrive, à la Frantique / Gettin' Serious, for example). Today I own 2 yellow RI ones :



Last year, I acquired a Vintage RAT RI fitted with the OP07DP chip, that I found ordinary, dull sounding, and I decided to return this pedal to the '81 RAT Big Box specs, notably by changing the chip to the LM308N. The improvement was well worth and gave to the pedal a great sounding distortion - I A/B-ed it with a quick copy on breadboard to be sure : no hesitation. I also tried for the occasion the 741, modified to the 1458, the 4558 to see the differences, and ranked 1-LM308N, 2-741 and 1458, 3-4558, 4-OP07DP.

So I wondered if it was worth to test the LM308N in the DOD 250 instead of a 741, or a 4558 of the yellow RI model

And the answer is  : YES.

You even have more smoothness, and when maxxed, you get into an hard-rock sound, playing-sensitive and full of overtones !

So here's my release : the CRC Overdrive Preamp 308

The box :





The circuit :



No need to post a layout for this : see the topic about the grey 250 from Steve Mavronis, everything is there, as the LM308N has the same pinout as the 741, with only a few components to be added, which can be soldered under the PCB, like mine's. Ha ! I miss the reference numbers of the chip's pins for the 33p capacitor : it's 1 and 8 - I apologize...

An interesting variant of a simple but great design... Enjoy !

A+!

[Joe Hart]

Very interesting. What are the added components for? Smoothing out the distortion? When I have a few minutes, I'll have to take a better look at this. Thanks!
-Joe Hart

[Joe Hart]

And I love the enclosure you made! Any info on that??
-Joe Hart

[Steve Mavronis]

Disco??? 

LOL

[Joe Hart]

Disco??? 

LOL

I know that song. Maybe if you run the 250 into a clean amp? But I don't think the 250 is a very good sounding "stand alone" distortion pedal. It really needs to be pushing an already overdriven amp, IMO.
-Joe Hart

[Steve Mavronis]

Maybe if you run the 250 into a clean amp? But I don't think the 250 is a very good sounding "stand alone" distortion pedal. It really needs to be pushing an already overdriven amp, IMO.

It sounds very good into solid state Marshall amp on the clean channel though, or with some amp gain on the crunch channel for more distortion. I get nice smooth woody/tubey overdrive tones for lead and heavy chord sound for rhythm. Total bliss and I don't have the urge for anything more.

That's just me using SS as a home practice amp only. I don't gig or have a tube amp because it would be too loud. Wish I could.

[tubelectron]

Very interesting. What are the added components for? Smoothing out the distortion? When I have a few minutes, I'll have to take a better look at this. Thanks!
-Joe Hart

Have a comparison with the STD DOD250 circuit : I added nothing except the 33p required for the LM308N.

And I love the enclosure you made! Any info on that??
-Joe Hart

See the Picture topic for more info : most of my plates and logo comes from old instruments (labs, industry, etc...) that were trashed.

But I don't think the 250 is a very good sounding "stand alone" distortion pedal

The LM308N makes the difference - worth to try !

Disco??? 

Yes, Disco ! - listen at the end of this 1978 tune : there is a killing and long disco-psychedelic instrumental part, where you can here what I call a  piercing 70's disco lead overdrive.

A+!

[Steve Mavronis]

Disco??? 

Yes, Disco ! - listen at the end of this 1978 tune : there is a killing and long disco-psychedelic instrumental part, where you can here what I call a  piercing 70's disco lead overdrive.

I'll check it out. Now you're bringing back bad personal memories for me, Haha!

[WGTP]

Should the 100k resistor at the clipping diodes be 10k? 

[Joe Hart]

Have a comparison with the STD DOD250 circuit : I added nothing except the 33p required for the LM308N.

Oh, you're talking about the RI ones? Because they added/changed some stuff. But you have a 1M resistor around the Drive pot, a 27p cap by the IC, the resistor and cap going from the IC to the Drive pot are different, the pair of 220K resistors should be 22K, and the resistor before the diodes is 100K instead of 10K. But I know there are some different versions (Steve Mavronis has a great chart).

So what I'm saying is that the reissues had some changes made to them to compensate for the different ("wrong") IC. What you're describing sounds more like the original circuit. How do your changes compare to the original circuit? Or did you get the RI pedal to sound like the originals with less component changes? Just wondering.

Unfortunately, right now I don't have the time at home to try it out, but I definitely will at some point!
-Joe Hart

[Mark Hammer]

We're essentially looking at a Rat/DOD250 lovechild.  The general structure is that of a DOD250/Dist+, but the "engine" is a Rat.  Use of "too-high" gain (2128x at max) and the slew-rate-limited 308 with a 33pf compensation cap mimics the way a Rat behaves at highest gain.

Should the 100k resistor at the clipping diodes be 10k?  

I'm thinking so too.  
At max gain, when the resistance to ground is 470R, a 68nf cap gives a low end rolloff starting at just under 5khz, which is a might high.  With a 680uf cap, the rolloff would be starting just under 500hz, which is much closer to the 720hz rolloff the Dist+ and DOD250 have.

At the same time, when the Gain pot and 470R resistor total 4k7, they are effectively identical to a DOD250 max gain (213x), and have a rolloff of around 500hz, providing a little more oomph than a DOD250 will.  Using a 470R resistor to set max gain bumps the gain ceiling up to 2129x, which is Rat territory, but without the Rat low end.  The Rat uses a two-tailed gain-setting network which also provides a max gain of 268x for the bottom end, to complement the searing top end.

This unit has the searing top end, but, on paper at least, might be forfeiting the bottom chunk to get that.  The dilema is that use of the two-tailed network is only feasible if one uses the feedback resistance to set gain, as the Rat does, instead of the ground leg resistance, as the DOD250/Dist+ does.

It may well sound much fuller than I imagine, and the most desirable sounds may well be well below the gain ceiling, so loss of low-end bandwidth may be a non-issue.  I'm just ging on the basis of what I see in the drawing.

Still, good on ya for experimenting.

[tubelectron]

Should the 100k resistor at the clipping diodes be 10k? 

Yes and No.

Steve Mavronis is aware of that - AFAIR - because he had made "The DOD 250 Component Values Table" in which he mentioned that this resistor is always 10K and that's wrong (Steve, you corrected your table, AFAIR again). On the RI Yellow version, this resistor is 100K. On the gray one, it's 10K.

Here are the pics of one of my yellow RI DOD 250, all factory stock (except the knobs on one !) : look at R8.




So that's why I used this value on my CRC Overdrive Preamp 308 - I must confess that I didn't tried the 10K value on it, but when comparing my Yellow DOD RI (4558 / 100K) to a friend's Gray DOD Clone (741 / 10K), I remember having found the 100K version more bluesy and smooth, more suitable to my taste than one's friend, which is a punk-rock fan.

We're essentially looking at a Rat/DOD250 lovechild.  The general structure is that of a DOD250/Dist+, but the "engine" is a Rat.  Use of "too-high" gain (2128x at max) and the slew-rate-limited 308 with a 33pf compensation cap mimics the way a Rat behaves at highest gain.

Exactly, Mark.

[Steve Mavronis]

Should the 100k resistor at the clipping diodes be 10k?  

Yes and No.

On the RI Yellow version, this resistor is 100K. On the gray one, it's 10K.

Here are the pics of one of my yellow RI DOD 250, all factory stock (except the knobs on one !) : look at R8.

I haven't updated this chart in quite some time and it's incomplete for the early years too. I don't own a reissue yellow 250 yet, just the YJM308 so I only know that for sure. Thanks for the PCB pics. My chart is here:

http://sites.google.com/site/snmavronis/neoclassic/741overdrive/research

[tubelectron]

Have a comparison with the STD DOD250 circuit : I added nothing except the 33p required for the LM308N.

Oh, you're talking about the RI ones? Because they added/changed some stuff. But you have a 1M resistor around the Drive pot, a 27p cap by the IC, the resistor and cap going from the IC to the Drive pot are different, the pair of 220K resistors should be 22K, and the resistor before the diodes is 100K instead of 10K. But I know there are some different versions (Steve Mavronis has a great chart).

So what I'm saying is that the reissues had some changes made to them to compensate for the different ("wrong") IC. What you're describing sounds more like the original circuit. How do your changes compare to the original circuit? Or did you get the RI pedal to sound like the originals with less component changes? Just wondering.

Unfortunately, right now I don't have the time at home to try it out, but I definitely will at some point!
-Joe Hart

Ah, yes, Joe - you're right ! I have made some changes - you list them (I was lost, sorry).

- the 220K instead of 22K were intended to lower the power drawn to the battery, because this pedal was intended to be housed in a 1590A box, with a small 12V battery (like on my FuzzRite Ventures). The current lowered from 615µA to 375µA, so it was pertinent with a small battery. I don't think it changed the sound significantly.

- the 1M was added to give a bit more spread in the Drive control (I love the 50/50 adjustment, that's all...)

- the 27p limits fizz, as usual - in the RI there's a 22p

- the other values have been tailored to my taste : a smoothe overdrive suitable to play blues on a clean amp (SF Fender, Boogie MKIIA...) with my 335.

- you're right : my schematic is close to the Yellow RI model than the Gray Vintage one

How do your changes compare to the original circuit? Or did you get the RI pedal to sound like the originals with less component changes? Just wondering.

I speak about the comparison between the Yellow RI and the CRC, not the Gray vintage original... I had a gray original, and I remember it was more "cutting" than the RI. The CRC 308 compared to the vintage clone of my friend sounds more smooth but with the same sustain, probably because the IC itself is more driven and clipped with the 100K resistor than with the 10K.

It may well sound much fuller than I imagine, and the most desirable sounds may well be well below the gain ceiling, so loss of low-end bandwidth may be a non-issue.  I'm just ging on the basis of what I see in the drawing.

And you're perfectly right, WGTP.

A+!

[tubelectron]

At max gain, when the resistance to ground is 470R, a 68nf cap gives a low end rolloff starting at just under 5khz, which is a might high.  With a 680uf cap, the rolloff would be starting just under 500hz, which is much closer to the 720hz rolloff the Dist+ and DOD250 have.

Mark, I do not see any 680µF capacitor in my RI DOD nor in my 1980 D+... It's a 56nF there, so even less than the 68nF I use.

It may well sound much fuller than I imagine, and the most desirable sounds may well be well below the gain ceiling, so loss of low-end bandwidth may be a non-issue.  I'm just ging on the basis of what I see in the drawing.

Sorry, Mark ! - Yes, your analysis is the right one - From min to 2:00, it's grinding blues starting from clean to nearly full. From 2:00 to  4:00, density increases to hard-rock voicing, after 4:00 to the max, a lead mid-resonant peak appears, giving easy overtones. So it's an interesting array of possibilities, but the main range is 11:00 to 13:00 for me.

A+!

[WGTP]

Yep that's a 100k resistor.  The highest I recall seeing on these type distortions is 10k.  The Rat has a 1k.

I would think that would soften the diode clipping distortion, but make the op amp distort more by working harder.

Also, in conjunction with the 1n cap at the diodes, the low pass filter is a factor of 10 lower than with the 10k resistor, so that is rolling off some highs.  This filter and the highpass filter at the feedback loop are probably producing the mid resonant  peak.  :0

Whatever works for you.  

[tubelectron]

Yep that's a 100k resistor.  The highest I recall seeing on these type distortions is 10k.  The Rat has a 1k.

I would think that would soften the diode clipping distortion, but make the op amp distort more by working harder.

Also, in conjunction with the 1n cap at the diodes, the low pass filter is a factor of 10 lower than with the 10k resistor, so that is rolling off some highs.  This filter and the highpass filter at the feedback loop are probably producing the mid resonant  peak.  :0

Whatever works for you. 

That's it, WGTP.

For those who haven't the schematic of the RI Yellow version, here it is, traced directly from one of mine. I have 2 of them, an early RI and a late RI, and these are exactly identical :



Along with the schematic of my 1980 MXR Distortion +, equally traced directly from the unit.



A+!

[jagermonster]

Very interesting, I definitely intend to give this a try, I absolutely love the 250...built one not too long ago that I'm very happy with, bigger output cap and MOSFET clipping. I also love the Rat, seems this is something I MUST do.

My main question: what does the 27pf add to the RI variant of the 250? I know it isn't present in the "grey" spec. Wondering if it would do anything to improve the 250 I mention above.

[Steve Mavronis]

My main question: what does the 27pf add to the RI variant of the 250? I know it isn't present in the "grey" spec. Wondering if it would do anything to improve the 250 I mention above.

It's there to tame the high treble of the stock 4558 chip. The originals with a 741 don't have it. Swap a 1458 in (dual 741) and you don't need it either.

[jagermonster]

That was fast! Thanks, I guess I don't really need it, as I'm running a 741. I've also seen some 250 variants with 33pf or other low-value caps across the (+) and (-) sides of the clipping diodes. What does this accomplish? Is this simply to pass high frequencies without clipping them?