DOD Preamp 250 mod

[imguitardan]

I'm a novice who just completed my first mod (The Ruetz Rat).  Now I'd like to mod my Preamp 250.  It looks really simple inside.  Let me preface by saying that I know nothing and can't even read a schematic.  I was hopeing that I could change a few things (caps/op amp) to get less highs and more mids (I'm looking for a more natural growl/less shrill tone).  Any recommendations.  I'd like to make this pedal more Rat-like.  (After trying many boutique pedals, the Rat is still my favorite distortion/OD).  Not more gain, just more "girth".  Thanks for any help.

[petemoore]

1rst I think that's DOD 'OD'250.
 A DISTortion+ Type. [diode from SP to ground clipper].
 Find the cap across the clipping diodes, replace with *slightly larger value.
 or add a small value cap from Signal path to ground...say near or at output...input?
 Rummaging around the reads may take some time, but it will make it much easier for us, and I mean you too.
 Your unit can easily be tuned, consider building it's competitors with some sockets...
 GGG I think has the 250 schem, Dist + ...

[phillip]

You could try half of the "more gain/more bass" mod.  On pin 2 of the IC there's the 4K7/0.047uF series.  If you want a fatter sound, you could increase the value of the 0.047uF capacitor to 0.068uF or so.  That'll add more bass.

You could also increase the value of the input capacitor a little or increase the value of the 0.001uF capacitor that's in parallel with the clipping diodes to 0.0022uF or 0.0033uF.  Each of those changes will shave some high end off.

Phillip

[Mark Hammer]

Removed.  Victim of acidental double-click.  Same content as succeeding post.

[Mark Hammer]

Phil's recommendation is spot on.  The way this circuit works on both the DOD 250 and the MXR Distortion+ is such that as you increase the gain, you lose bass.  At max gain in both these units, you lose a lot of content under 150hz.  Increasing the .047 cap in question to .068uf or even .22uf will remedy that.  I'd recommend the larger value, but that's me.

Both the Rat and the 250/Dist+ use what is called a noninverting op-amp as the gain stage for producing the clipping.  These are easily identified in schematics by the following:

1) The signal "enters" the chip via the "+" input pin.  There is usually , but not always, a resistor to ground from that pin.

2) There is what is referred to as a "feedback" resistor between the output of the chip, and the "-" input pin.  This may also be accompanied by a capacitor in parallel and some other components from time to time.

3) There is a resistance between that "-" pin and ground.  It usually connects to ground through a capacitor.  The capacitor *could* come "first" or second.  Both work.  That rresistance can also be two or more separate paths, each with a resistance and cap.

The gain of the stage will depend partly on the ratio of the feedback resistance to the resistance to ground.  Think of those two resistances as being the two halves of a volume control, each resistance being one side of the wiper.  When more of the signal from the output is fed back to the input, the gain of the chip is reduced.  When that feedback is bled to ground, less of it makes it through to the input, and the gain is increased.

Think of noninverting op-amps as being like a car that "idles" at 4000RPM+.  The only way to keep the car from crashing into stuff all the time is to keep your foot firmly on the brakes, and ease up on them every time you want to go a little faster.  So faster = less brakes rather than more gas.

Since the amount of "brakes" applied is the relationship between the two resistances, making one bigger (feedback) or the other smaller (R to ground)  essentially lifts the foot off the brakes by providing less feedback to the input.  Here's the important part: The Rat adjusts gain via the feedback resistance, where the 250/Dist+ adjust gain via the R to ground.

Why is this important?  Because of how it affects the rolloffs.  As noted above, on the 250 the bass cuts out as you turn up the gain.  This is because you are making the R to ground smaller, and when combined with the .047uf cap, smaller resistance values produce a higher rolloff frequency on the bass end.  On the Rat, however, the gain is increased by varying the feedback resistance.  In this case, increasing the feedback resistance, in combination with the feedback cap, lowers the rolloff on the treble end.

So, in one you don't lose any treble as you max the distortion, BUT you lose bass.  On the other, you don't lose bass, BUT you lose treble.  Both of these have their advantages, but if you want the sound of a Rat, then there are some particular changes you need to do.  The first of these is to put the distortion pot where the feedback resistor goes, and the replace the distortion pot's old location with a fixed resistor.

So how would you "Rat-ify" a 250?

First, change the .047uf cap noted easrlier to a .22uf cap.  If you can't fit a .22uf in there, .1uf will be an audible improvement, though not as good as we'd like.  Use .1uf in a pinch.  

That cap will be in series with a 4k7 resistor.  The 4k7 resistor normally goes through the distortion pot to ground.  It will now go directly to ground. Whilke you're there, replace it with a 3k9 resistor.

There will be a 1meg resistor between the output and "-" input of the chip, with a 25pf cap in parallel.  That resistor should be replaced with a 10k resistor in series with a 1meg pot.  The pot can be linear or log.  Both will work.  The cap should be changed from 25pf to either 39pf or 47pf.

The 10k resistor in series assures that the unit will never have a gain lower than 3.  This will not be enough gain to produce clipping but will be enough to provide SOME reasonably clean gain should you want it.  At max gain, the unit will have a gain of (1meg+10k+3k9)/3k9 = 260, which is roughly what the Rat has.

The increased feedback cap will mean that at min gain (R = 10k), there will be no audible treble loss, at max gain, though, the treble will be rolled off starting around 4040hz with C = 39pf, and 3352hz with C= 47pf.  Although this seems low, bear in mind the unit produces a LOT of treble sizzle the more it distorts.  Trimming back on the treble with more gain should allow both of these to be meaty with a bit of bite.

That's it.  A Rat-ified 250.   It will lack the tone control, and it will lack the quirks of the LM308 used in the Rat, and there is one other aspect I won't go into, but the basic aspect of keeping bass and losing treble as gain is increased will be there.  If you want ALL the bells and whistles, we can talk after you get this part done.

[petemoore]

http://bryant02.home.att.net/schematics/od250schem.gif
 Is this the schem?
 Not to be stickly, but *4k7...From pin 2 going left...I see a .05uf to *47k to 0k-500k V/R to ground.
 Nicely Ratified OD article Mark, thank you.

[dosmun]

You can also check this out

http://www.analogman.com/dodmod.htm

[phillip]

http://bryant02.home.att.net/schematics/od250schem.gif
 Is this the schem?
 Not to be stickly, but *4k7...From pin 2 going left...I see a .05uf to *47k to 0k-500k V/R to ground.
 Nicely Ratified OD article Mark, thank you.

Ack...my schematic has a couple of errors...will fix ASAP

Phillip

[Mojah63]

The Rat mods sound cool   I have a few Yellow 250's and I've seen different IC's in them all, and they all sound different. I modded one  with the Liquid drive (Justin's) idea and put in a socket to try different IC's.  A LM741 although a bit noisey has a rolled off top and a  midrange squishyness.. Not really RAT like but cool...  Small Bear has 741's if you can't find any...

[Joe Hart]

Check my website for a whole page explaining mods to the DOD 250.

www.RabbathRecordings.com

-Joe Hart

[Joe Hart]

It's at the bottom of the "stompboxes" page.
-Joe Hart

[Mark Hammer]

Please note that the rather extensive description I gave of the Rat-ified 250 ignores some fairly significant differences with the actual Proco Rat:

1) The Rat uses *two* paths to ground with different cap choices to yield more low end chunk.

2) The Rat has a FET on the output stage, which, th9ugh not exactly "pummeled" is still pushed hard enough to contribute its own little stamp on the sound.

3) The Rat uses an LM308 with an external compensating capacitor to shape tonal response.  The Rat-ified 250 goes a different route.

4) The Rat has a variable lowpass filter as its tone control.  The Rat-ified 250 has no tone control, opting only for the influence of the cap in the feedback loop and the other cap in parallel with the clipping diodes.

My intention was to suggest mods that come as close as one can, using the same PCB, to mimicking the manner in which gain and tone are related in a Rat vs a 250/Dist+.  Obviously, if one truly wanted to nail the sound of a Proco Rat, you should either buy one, new or used, or follow the actual design a little more closely than I did.  Whether it "nails" the tone or not, I expect Dan will be a little more pleased with its behaviour.

[imguitardan]

Thanks a lot, guys!  You've given me a lot to think about and try.  I realize that a 250 is not a Rat, nor will it be.  I just want to lose some of that high end sizzle for more mids.  I generally use this pedal as a boost anyway (Gain @ 9:00/ Vol @ 3:00), so I'm not looking for a bunch of gain.  More of an "AC/DC" kind of sound (for lack of a better description).  Thanks again!  You've all been so helpful!