I have tested them using a Linux computer, its sound card and qloud software. I know the measurement is not very precise, but the real thing is not going to vary very much, I think. All controls were at noon during the measurement.
Original BOSS DS-1
As expected from a pure distortion effect pedal, frequency response is mid scooped, with a bump at 100Hz.
Modified BOSS DS-1
As I expected after the modifications in the tone stack, frequency response has much more mids. The maximum is not at 800Hz, just because the circuit is not only the tone stack. By touching the tone control, the curve can be modified (I leave it for a next article).
As you can see also, at aproximately the same “level” value, the overall volume is higher (over -60dB for a wider range in the test), without changing any measurement parameter, even if the perceived distortion is higher in the original unit. The reason, I guess, is the red led in the clipping section instead of a common diode, see the article https://electric-safari.com/2019/03/17/turn-a-boss-ds-1-into-a-nice-overdrive/. Since the Vf of the led is higher than that of the diode, it clips at a higher voltage, giving the circuit more headroom. This is very common when doing this kind of modifications. I think the tone stack change alone does not explain that higher volume.
The fence that separates distortion and overdrive pedals is very thin. Usually pedals based on hard clipping, high amount of distortion and mid scooped response curve are called distortion and those ones based on soft clipping and mid boosted are called overdrive. But some of the more famous overdrive pedals, known for being “transparent”, “clean” or “dynamic” are based on hard clipping (clipping diodes at the output of an operational amplifier or transistor). Think of Klon Centaur, for example (mid boosted, by the way).
Hard clipping seems to have usually a more dynamic overall response to the guitar touch than soft clipping, although it depends on the circuit design. Some soft clipping pedals are very dynamic while others are too compressed, but most of them seem to be compressing and distorting all the time.
Hard clipping, theoretically limits drastically the signal when it reaches a certain level, so it creates those high harmonics that give it its character. Soft clipping limits the signal more gradually, giving that “soft”, characteristic valve-like distortion sound.
Having taken all these considerations into account, let’s review my last pedal modification. I wanted to experiment those concepts with a BOSS DS-1 pedal I purchased broken and then fixed. I liked as it was, but that kind of distortion is not for me, frankly.
BOSS DS-1 is based on classical symmetrical hard clipping, two diodes at the output of an operational, and the tone stack tends to mid scoop the response curve, so it should be considered a distortion pedal. It is orange and is called “Distortion”, so in this case it was easy.
My experiment had these objectives:
Reducing distortion (amount of clipping)
Give it an asymmetrical clipping character, more pleasant to my ears and more valve-like distortion
Mid boost the response curve
For reducing distortion, I have made these two changes:
R9 at the emmiter of Q2 controls the gain of the first stage, previous to the opamp stage. With a value of 22 Ohms, the output is so high that the signal at the output of the operational is almost always clipped. I tried with different values and for me 100 Ohms gave the sweet spot for having the right amount of distortion with the gain control at 12 o’clock, for both single coil and humbucker pickups.
LED diodes have a Vf higher than common bipolar diodes. Bipolar diodes have Vf=0.7V and LED have Vf > 1.5V, depending on the color. Vf is what causes the clipping, and the higher Vf is, the higher the signal can be before it starts to clip. Replacing only one of the clipping diodes (D5 for instance) by a red LED yields more headroom and at the same time serves my second purpose of having asymmetrical clipping. By the way, this is one of the changes that the famous Keeley Ultra Mod makes to the DS-1.
For the mid boost thing, I changed several components of the tone stack. DS-1 tone stack is based on the Big Muff tone stack. It is passive and tends to mid scoop the tone. By simulating the circuit with LTSpice, I found the spot where the signal is mid boosted without loosing the ability to control the tone (although it is somewhat reduced compared to the original DS-1 design).
The bump happens at 800Hz, which for me is the best point to get a nice tone that is capable of cutting through the mix in a band context. It is a simplification because the response curve of the pedal depends on the rest of the circuit as well, but I can tell you that the result is quite acceptable.
The components replaced in the tone stack are C11, C12, R15 and R16. As you can se, only six components have been replaced. I have prefered to keep it as simple as posible, but the change in tone and behavior is drastical. Now the pedal is a very dynamic, mid boosted overdrive, with a wide range of distortion. With gain at max level, it reveals it true distortion spirit. Put a compressor before and an equalizer that boosts the 100Hz frequency after it, and you’ll get the real sound of rock and roll.
As a summary, these are the replacements:
For C12, 82nF is not a very common value, but you can put 47nF and 33nF in parallel to get 80nF, enough given the usual tolerances of capacitors. Use a capacitor tester if possible to get a close value. The board has plenty of space to put the two capacitors in that position. For C11 and C12, any kind of capacitor will do, but the values are very important (test if possible).
I hope you enjoy this mod, please let me know of any improvements or mistakes you may find. If you are a good guitar player, please record a demo and upload it to youtube. If you are rich, please send me a Klon Centaur to compare with my DS-1 modified pedal.
PD: if you make this mod, please comment below, I would like to keep track of people enjoying (or not) my mods.
Out of curiosity, I bought a second hand SD-1, just to try it as is, and try some of the mods people have been doing to this pedal for the last decades. Some mods try to transform it into a TS9 or even a TS808, so you can turn this relatively cheap pedal into a more expensive one. But this box has its own personality, the design is very similar to TS9 but with some remarkable differences:
Clipping is asymmetrical, two diodes forward and one backwards (in the direction of the operational). Asymmetrical clipping sounds different than symmetrical one, some people describe it as harsh or hairy of fuzzy compared to the latter.
There is no capacitor in the feedback loop of the clipping amplifier. This capacitor smooths the clipping a little, giving the TubeScreamer part of its particular tone.
Tone control is very similar but with different component values and with a capacitor (C6) in the feedback loop of the tone control operational amplifier. This capacitor is the first thing I have seen every mod removes because it sucks a significant amount of bass frequencies to the signal.
After trying the pedal for a while, I quickly noticed what most people complain about, this makes the guitar sound thinner. This has not to be bad in every ocasion, especially in live gigs situations where you want to sound in a different spectrum space than the rest of the instruments. But ok, I want more bass too. In this clip I recorded the original sound of the pedal, playing with a RS420 (humbuckers) and a Fender Blues Deluxe amp (please don’t pay attention to the music, just the sound 😉 ):
To correct this defect or feature, there are multiple mods out there, usually people remove capacitor C6 and change values of R6 and C3, which connect the feedback loop and the negative operational input to a voltage divider, in a similar way to he TS9 connects those to ground. lowering R6 and raising C3 values results in more bass response from the clipping circuit. See below the schematic:
I chose to make this changes in order to get a nice tone from the pedal:
C6: instead of removing, changed it with a 100pF capacitor from its original value of 10nF.
C3: raised from 47nF to 100nF.
R6: lowered to 3K3
The other section where people make changes is the clipping circuit. Many mods aim in the direction of getting symmetrical clipping, but I didn’t want another TubeScreamer. Instead of that, I wanted to enhance the personality of the effect by exaggerating the asymmetry of the clipping section. Asymmetry comes from using a different quantity of diodes in each direction, or from using diodes with different specifications, especially with different forward voltages (Vf). Vf is different for the following types of diodes:
Ge diodes: low Vf, around 0,3V
Si diodes: usually around 0,6V
LED diodes: depends on the color, 2V for green ones, 1,6 for red ones.
So using a Ge diode in one sense and a LED in the other, we get asymmetrical clipping, right? Right, but Ge diodes are expensive, hard to find and unstable. I read about simulating Ge diodes with schottky diodes in this article:
Ge diodes are used in some mods also because of the smooth I-V curve the exhibit. Schottky diodes have a low Vf too, but the I-V curve is more abrupt than the Ge one. By adding a resistor in series, you can get a similar response, at a lower cost and with more stability.
In my mod, I have replaced D5 and D6 (in series in the original circuit) with a resistor (10 Ohms) and a schottky diode (BAT46) with a Vf of 365mV. And D4 has been replaced with a green LED with VF=2V. You can see the measures below:
The components used:
And the changes to make:
For the clipping section, that I suppose will be subject of further modifications, I have replaced the original components with sockets. In the picture below you can see the other components replaced too:
Here you can see a detail of the changes in the clipping section:
And the result in the following record. I was not trying to play a song, just to get all the possible tones from the pedal. Please forgive me if I get sloppy:
I can perceive these differences after playing for a while:
More volume at the same “level”.
Much more bass frequencies, without being too much for a band situation, I think (has to be tested in a band situation).
More of the asymmetrical character: “harsher” when playing harder, keeping dynamics (I think more than the original, maybe I am somewhat subjective).
I think the sound changes more than the original when moving the drive control: from a smooth, almost TS9-esque overdrive at low gain levels to almost fuzzy and even octave-like at maximum gain.
If you are interested in this mod, please try it, test it and give me your opinion in the comments section. If you improve it, tell me how. If you like it, give me a “like”, and if it’s possible for you, mention my blog to yout friends 🙂 Mods have no copyright, but we like some recognition, don’t we?
Edited on March 16 2019:
Some days ago I put a red led instead of the green one. Now the overdrive is more usable and versatile, although the octave effect is less pronunciated.
PD: if you make this mod, please comment below, I would like to keep track of people enjoying (or not) my mods.