JHS 77 DOD 250 MP3's

[John Lyons]

Here's my MXR distortion+ modified to a '77 DOD OD 250. The only deviation I used is a .068 cap bfore the distortion pot. This adds a bit more low end when the distortion is turned up. The stock .047 rolls off a good amount of low end (thanks Mark hammer!) I may go back to the lower value as it seems to flub out a little with a humbucker.
I also kept the 1meg distortion pot. It gets a little wild on 10 but you can always turn it down...

Clip: www.mrdwab.com/john/distplus77OD250mod.mp3
Bridge Humbucker, slightly distorted marshall type amp

Clip 2: www.mrdwab.com/john/distplus77DOD250twin.mp3
Clean Fender Twin Reverb , bridge Humbucker

John

[tungngruv]

Hey John, nice clips. They show the range of the Dist. + real well. With the 77 mods it does sound like a different animal. The shrillness is gone. Does it sound like it needs a bit more low end to you? It may be my comp. speakers. Great job and thanks for taking the time to record them. BTW, are the clipping diodes stock?

[John Lyons]

Yes, The tone is pretty agressive in the upper mids. The amp was set fairly mellow so all the buzz saw is coming from the pedal.
I used 1n2002 clipping diodes. The circuit is just as the '77 OD250 except for the .068 before the dist pot.

I made a sample of a section from the twin reverb clip above.
The first phrase is as above (no eq) , the second phrase is with an EQ notch around 2.5K.
www.mrdwab.com/john/Dist+wEQ.mp3

Sounds a lot better to me with the upper mid notched out, seems fuller and a lot less harsh.
Now it's just a matter of working out that curve with a few resistors and caps!

I haven't tried another chip but I doubt that the chip would vary that much and add so much harshness!! would it?

John

[tungngruv]

Yeah John, the second half of the clip sounds a lot better to me, not that that matters though
I have an extra couple of chips if you want to try them out, dual and single. Let me know.

[pi22seven]

Just finished this build last night and it sounds great! A LOT smoother than my other 250 builds and it's definitely replacing something on my board. Keeps my Tele sounding like a Tele.

My perfboard skills a improving!

[markm]

This circuit is finally getting the attention it deserves!! 

[g3rmanium]

Clip 2: www.mrdwab.com/john/distplus77DOD250twin.mp3
Clean Fender Twin Reverb , bridge Humbucker

Sounds cool, where are these subharmonics coming from? From the 250?

[John Lyons]

Things that make you go hmmm...

Here are the differences in the later DOD250 vs. The earlier '77 DOD250:

Larger cap across the diodes, .002 instead of .001 = less buzz and a smoother clipping, makes sense.

Larger cap on the  output 20uf instead of 1uf. Can't see why this would make much of a change with a guitar though...anyone know?

Larger cap to ground  at the input .0015 or .0022 instead of .001 = a slight high cut at the input (circuit amplifies the highs slightly less.
Some had no cap to ground, some had a small input cap which would make a sort of treble booster distoertion. (malmsteen unit)

No cap 10uf at the voltage divider. I do not know what NOT having a cap here does. Most every FX circuit has one Anyone know?
Smaller bias reisistors in the voltage divider 22k/22k and 470K as oppossed to 1M/1M and 1M.

There are a couple other differences from the MXR Dist+ and variants but the ones above are the big differences.

Can anyone give an explanation of the questions above?

Thanks!

P.S. g3rmanium The twin is a pretty mid scooped and full sounding amp. But as you can hear from the The clip, the clean phrase at the begining is a little weak. The clip sequence is an Agile Les Paul, the "77 DOD250 circuit and straight into the Fender Twin Reveb, Electrovoice RE mic, Royer tube mic pre then into a $30 Behringer U-control UCA202 USB interface and into Cool Edit Pro V2. No eq added.

John

[John Lyons]

Bump!

Anyone?

[tungngruv]

Hey John, this is quoted directly from Joe Hart's website (www.rabbathrecordings.com). It pertains to the DOD 250 and what each component does and possible mods. I'm not the brightest person in the world so I can't guarantee his accuracy about the parts, etc.... but he posts here occasionally and I'm positive he got a lot of his info about this pedal right from this forum. Here's the link and what he said:

http://rabbathrecordings.com/DOD250.htm

And now, onto what the parts of the circuit do and what modifications you can make to the circuit. Just remember that changing one thing will likely change more than one aspect of the resulting tone because of the interaction of the different elements in the circuit. So, feel free to play around, but always listen to what the circuit sounds like after each change.

C1 is the Input Cap. This sets the tone going into the circuit. A smaller value cuts more bass out of your signal and a larger value lets more bass in. So, a .001uF capacitor would create a more high-endy pedal because it's one-tenth the value of the original capacitor. Likewise, a .022uF capacitor would create a bass-ier sounding pedal. A lot of people like to socket this component and then plug in different capacitors and see what they like for their individual guitar and amp setup.

R1 help set the Input Impedance and filters out radio frequencies (RF is bad). This is best left alone. Things will work better using the 10K.

C2 is a Filter Cap for the DC current (battery). Not much happens playing around with 10uF value, but I've seen values ranging from 1uF to 100uF - so, whatever.

R2 and R3 are Voltage Dividers. This means that they take the 9V and feed 4.5V to the Op-Amp. Again, nothing much happens playing with these values, except that the values need to be the same. You can use values from 10K to 1M. Lower values use up more battery power, then higher values. But I like sticking with what the schematic calls for.

R4 sets the bias for the Op-Amp. This is best left as is.

R5 is the Feedback Resistor. This works with R6 and R7 to set the gain level of the Op-Amp. If we look at R6 and R7 as one resistor, and we know that R7 is variable from 0 to 500K, then the two resistors together are variable from 4.7K to 504.7K. Now the way the gain is calculated, you divide R5 by the sum of R6 and R7. So, if R7 is all the way to 0, it would be 1,000,000/4,700 which gives us a gain of 212.77. So, the guitar signal will be amplified over 200 times. If R7 is turned to 500K, the gain would be 1,000,000/504,700. This is a gain of 1.98 - just under 2 times. I like to play with the value of R6 to adjust gain. A lower value will give more gain, and a higher value will give less gain. You can also play with the value of the pot (or even the feedback resistor), just keep in mind the calculations for gain.

C3 also affects the tone of the circuit. A larger value here will cut highs, and a lower value will cut bass. Mess around with this component.

IC1 can be any number of Op-Amps. And they all sound slightly different. And when I say slightly, I mean SLIGHTLY. There is a lot of debate over the differences (and even if there are any differences). I can hear differences if I do a side-by-side comparison, but it is very subtle. And playing with caps and resistors has a much, much bigger effect on the sound of the pedal. So, don't get too wrapped up in the Op-Amp. But on the plus side, you can use tons of different ones and still end up with a cool pedal.

C4 is the Coupling Capacitor. It blocks DC current from continuing onto the output. I have seen values from 2.2uF to 22uF used.

R8 - I think this sets the output impedance so the pedal will play well with others. If there is something funky with the impedances, pedals start to act strangely.

C5 helps cut some high end out of the signal because creating distortion also creates harsh higher frequencies. This helps "smooth out" the distortion. Feel free to play with this capacitor.

R9 is the Volume Pot. Not much happens here with different values (you could use anything from 20K to 1M). A higher value pot will allow a little more volume, but this pedal should have more than enough volume for you anyway.

And the diodes. These are fun to fool around with. What is happening here (in the circuit), is that the Op-Amp boosts the signal, then the diodes clip the tops of the waveform off, and this creates distortion. Again, there is a wealth of information on the internet about this, so feel free to research this. There are a few common types that are used and they all sound a little different. Different diodes clip at different voltages, so a lower clipping threshold will create more distortion but a lower overall volume, and diodes with a higher clipping threshold will give less distortion, but more volume. The types of diodes generally used are (in order from lower clipping threshold to higher): germanium diodes, silicon diodes, 3mm LED's, 5mm LED's. Feel free to experiment. Socketing these diodes may be a good idea. Also, the diodes are set up in the original circuit to produce symmetrical clipping. This means that the signal is clipped the same amount on both sides of the waveform. Many people like asymmetrical clipping better. This can be achieved by mismatching the diodes. Either use two diodes with different clipping thresholds or add another diodes in series with one of the original diodes. This creates one side with twice the clipping threshold (this side will clip less). I have become quite fond of using a 6 position rotary switch. This way you can switch between 6 different sets of diodes. I can get a LOT of different distortion tones with this setup.

You can also toss stuff into the feedback loop of the op-amp. If you add two back-to-back diodes (one going one way and the other going the other way) between the output of the op-amp and the inverting input of the op-amp, you will get even more fuzziness. You can put diodes in the feedback loop, and take the original diodes out, and you will get a softer distortion (more of an overdrive like a Tube Screamer pedal).

Also, you can put a capacitor in the feedback loop. This can help smooth out some of the high end in the distortion. Try capacitors in the 20pF to 100pF range. This is a subtle change, but you may like it.

[Michael]

The DOD250 is a great pedal, i moddet mine with these things:

opamp: MC1458

cap removed (25pf),

More Low-End Response: To get a more low-end response out of the circuit, I increased the value of the 0.047µF capacitor that's attached to pin 2 of the IC to 0.1 uf.

True bypass with 3pdt switch and red LED.

Thats it!
Michael

[tungngruv]

Here's the schem that the above component descriptions apply to:

[John Lyons]

Thanks for the link Tony

My conclusions so far:

The Voltage divider and cap. Not sure what the cap is going other than supplying a resovoir of current for the bias voltage.
There is no cap there in the '77 OD250 version so maybe the bias shifts a little more or is a little unstable. Good, bad???
Larger resistors has a lower current draw, makes sence. Effect on sound? not sure.

The ouput cap is 22uf in the '77 version verses 1uf in the later units. I can't see how this would make any difference.

Everything else makes scence and I know what it does...and it's outlined in the linked article above.
SO it would seem that the main difference in the tone of the '77 version of the OD250 is the extra capacitance at the input and output caps to ground which = a smoother sound as more high end it dumped to ground.
Obviously capacitor material type, diode choice and chip # makes a difference but the caps seem to be the main tone difference between the 250 variants.

What' next?

John