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Re: MXR Distortion+ Mods?

1/31/2000 4:39 AM
Mark Hammer	Re: MXR Distortion+ Mods? I was not pleased with the sonic results of the one I made, but I suppose there is always room for rehabilitation. In order, there are the following things about this box you'll want to address: 1) NOISE: Given the resistance values involved, you'll probably want to replace the 741 with a BiFet op-amp like a TL071. A TL061 will work but it has more noise than the 71. 2) VOLUME: Looking at the two schematics at JD's site, you will see many similarities, but a major difference in volume controls. The MXR should have a larger value for the volume pot than it does (10k). I'd recommend a 50K or perhaps a 100K pot to replace the value listed for the Dist+. This should not affect tone much, but should give you noticeably more output to work with. This is important for being able to get a decent working volume when the drive is turned down. If you replace the Ge diodes with silicon ones (so, stick 1N914's instead of 1N34's or 270's), you'll get a big volume boost since it sets the clipping threshold and maximum level higher. 3) DISTORTION: The quality of the distortion can be tinkered with in a variety of ways. The 2+1 diode configuration is one way. You can also try other diode configurations and combos. John Greene's schematic of the Fulltone TS-808 clone shows 3 different types of diodes in the clipping path, and Aron Nelson's Shaka Brada has 2 different types as well. Another thing to try is to stick a small value resistor (e.g., 68-100R) in series with the diodes to soft clip. Whatever you do, make an effort to measure the clipping threshold with a meter. You'll find that diodes of the same type vary in threshold. 4) TONE SHAPING: There are many possibilities. If you read RG's tech paper on the Tube Screamer, you'll see that complementary tailoring of the low-end rolloff at entry, and high-end rolloff at exit are partly responsible for the tone of the Tube Screamer. So, you can tinker with the input cap values, and pre-volume control cap values to achieve different tones. One suggestion is to use a 3-position toggle (centre off) that can add in two addition cap values in parallel with whatever the original is to yield different rolloffs. Note that the cap on the output was not on the original unit, but was a suggestion made that kind of attained formal status as a part of the unit. Aron's schematic also shows a dotted-line path for a cap in the op-amp feedback loop. This is a good spot to alter tone. Using the freq=1/(2piCR), you can try different cap values there and in parallel with the clipping diodes. I should point out that 2-pole lowpass filtering usually sounds more pleasing to the ear than 1-pole. Take this to mean that tone shaping at both sites (loop and diode) will provide more pleasing results than either one alone.

1/31/2000 4:39 AM

Mark Hammer

Re: MXR Distortion+ Mods?

I was not pleased with the sonic results of the one I made, but I suppose there is always room for rehabilitation.

In order, there are the following things about this box you'll want to address:

1) NOISE: Given the resistance values involved, you'll probably want to replace the 741 with a BiFet op-amp like a TL071. A TL061 will work but it has more noise than the 71.

2) VOLUME: Looking at the two schematics at JD's site, you will see *many* similarities, but a major difference in volume controls. The MXR should have a larger value for the volume pot than it does (10k). I'd recommend a 50K or perhaps a 100K pot to replace the value listed for the Dist+. This should not affect tone much, but should give you noticeably more output to work with. This is important for being able to get a decent working volume when the drive is turned down. If you replace the Ge diodes with silicon ones (so, stick 1N914's instead of 1N34's or 270's), you'll get a big volume boost since it sets the clipping threshold and maximum level higher.

3) DISTORTION: The quality of the distortion can be tinkered with in a variety of ways. The 2+1 diode configuration is one way. You can also try other diode configurations and combos. John Greene's schematic of the Fulltone TS-808 clone shows 3 different types of diodes in the clipping path, and Aron Nelson's Shaka Brada has 2 different types as well. Another thing to try is to stick a small value resistor (e.g., 68-100R) in series with the diodes to soft clip. Whatever you do, make an effort to measure the clipping threshold with a meter. You'll find that diodes of the same type vary in threshold.

4) TONE SHAPING: There are many possibilities. If you read RG's tech paper on the Tube Screamer, you'll see that complementary tailoring of the low-end rolloff at entry, and high-end rolloff at exit are partly responsible for the tone of the Tube Screamer. So, you can tinker with the input cap values, and pre-volume control cap values to achieve different tones. One suggestion is to use a 3-position toggle (centre off) that can add in two addition cap values in parallel with whatever the original is to yield different rolloffs. Note that the cap on the output was not on the original unit, but was a suggestion made that kind of attained formal status as a part of the unit.

Aron's schematic also shows a dotted-line path for a cap in the op-amp feedback loop. This is a good spot to alter tone. Using the freq=1/(2pi*C*R), you can try different cap values there and in parallel with the clipping diodes. I should point out that 2-pole lowpass filtering usually sounds more pleasing to the ear than 1-pole. Take this to mean that tone shaping at both sites (loop and diode) will provide more pleasing results than either one alone.

Replies:

JD Sleep I've built the stock dist+ and I re... -- 1/31/2000 10:50 AM
Bob Mark,Thanks for your sugges... -- 1/31/2000 1:11 PM