Turn a BOSS DS-1 into a nice overdrive

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
DS-1 original schematic

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).

Modified DS-1 tone stack

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:

R922 Ohms100 Ohms
D51N4148Red LED

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 have 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.


BOSS OS-2 Frequency response in different settings

BOSS OS-2 is a good platform for illustrating the difference in frequency response between overdrive and distortion. I have tested my modified OS-2 (see BOSS OS-2 Overdrive/ Distortion MOD) with REW software and my laptop soundcard. It is very simple, you just have to connect the speakers jack to the pedal input and the mic jack to the output. And power the pedal, of course. To make it a little more complex and accurate, I made an adaptor that connects the left channel directly from speaker jack to mic jack and the right channel passes through the pedal. You can configure the software so that the left channel acts as a reference and discount the effect of the frequency response of the sound card itself. Output level has to be kept below some level, in order not to saturate the BOSS buffer, check it with an amplifier connected to the output of the pedal or with an oscilloscope or software tool (more on this in a future post).

REW stands for Room EQ Wizard, and is intended to measure the acoustics of a room and help in equalizing the sound in it. But one of its features is generating a sweep of sine signals and measure the frequency response of a system in front of those signals. While measuring you can see the harmonics, the average 2nd and 3rd harmonic level, and at the end you see the average frequency response for the whole sweep. The software is “donationware” and works in Windows, Linux and MacOS.

Below you can see the different settings I put under measurement:

Overdrive at max gain, blue

Distortion at max gain, purple

Distortion at medium gain, red

Blended overdrive and distortion, medium gain, green

And the frequency response of each of the settings:


As can be seen, distortion settings have a very characteristic “scoop” at middle frequencies (800Hz) that make them more flat sounding. A peak around 100Hz can be seen (and heard as I mentioned in a previous post).

In overdrive settings, on the opposite side, a prevalence of middle frequencies around 500Hz can be seen, what makes it more pleasant and cutting through the mix, as they say.

Combining both (green curve), you have less scoop and the prevalent frequency can be higher, giving more presence to the tone.


BOSS OS-2 Overdrive/ Distortion MOD

I have been playing for two weeks with a BOSS OS-2 overdrive/distortion pedal I found in ebay for repair. Repairing was easy, just needed some pots cleaning.

OS-2 with modified knobs and circuit
OS-2 with modified knobs and circuit

The concept is nice: there are two blended circuits:

  • Overdrive, by asymmetrical soft clipping in the feedback loop of an operational amplifier, like most overdrives out there.
  • Distortion by symmetrical hard clipping in the output of the other operational amplifier of the same chip.

You can blend the two by rotating a “color” control, from overdrive on the left to distortion on the right. After the blend section, there is a tone one. The “Drive” pot is a dual 270k potentiometer, each unit connected to the feedback loop of each of the amplifier units.

Out of the brown box (it didn’t come with the original BOSS box) I found some things I didn’t like:

  • Little bass response in the overdrive mode, too much treble for my liking. Usable for me, but could be improved (from my point of view, for my own needs).
  • I usually don’t like pure distortions and hard clipping, so I didn’t expect to like this side too much. I found it too flat and hissing, with little mids, but having a nice boost in the 100Hz (aprox) freq.

Original OS-2 circuit
Original OS-2 circuit

I think there are almost no “bad designs” or “bad pedals” out there, it is just a matter of taste. Many like pedals that I hate (don’t want to give names). Maybe the best overdrive or distortion pedal is no pedal at all, but for people like me that usually don’t play on stadiums, distortion pedals are a good tool to get close to the tone of your favorite player.

Moreover, when you modify a pedal, you are not improving the design, but improving your particular unit for your particular taste. Most components vary a lot in value from unit to unit, and manufacturers have to take many constraints into consideration in their designs. Therefore I think we have to be humble when “improving” a device.

That said, in this case, I considered these objectives:

  • Distortion side:
    • Raising mids and cutting that hiss
    • Trying asymmetrical clipping in this kind of circuits, just for fun
  • Overdrive:
    • Raising mids too, and add some more bass response
    • Trying leds clipping for a supposedly more natural overdrive, and also for fun

After some tweaking and some regrets, I performed to the following changes:

component old value new value why
U2 (opamp) JCR1458D JCR4558D JCR4558D has better characteristics:
Input Resistance = 1M
Slew Rate = 0,5V/uS4558:
Input Resistance = 5M
Slew Rate = 1V/uSSlew rate affects the circuit bandwidth higher limit, in this case from 8KHZ to 16KHz. Maybe it is overkill, but I think this gives more freedom at adjusting the frequency response of the circuit.
C6 1nF remove More bass in the output of the tone stack
C27 47nF 220nF Lowers freq. in high pass filter at the input of the overdrive section -> more bass
R39 100 150 Lowers gain in overdrive. See C23
C23 4.7uF 2.2uF In combination with R39, it forms a high pass filter, attenuating frequencies below the cut-off frequency. The modification changes the pass freq from 338Hz to 482Hz. In combination with the C27 change, it results in more mids
D7,D8,D9 Junction diodes D7,D8=Red LED – D9=BAT46 (Schottky) It changes the form of the clipped signal. Red LEDs have Vf=1.8V (instead of 0,7) and different I/V curve. I put a schottky just to experiment, another kind of diode can be used, or just a cable for symmetrical clipping. Another LED would be too much Vf and can result in no clipping
R2 20K 68K This resistor is part of the circuit that balances overdrive and distortion. Since LEDs are used for clipping, the output voltage of the operational is too much when compared to distortion output. Raising the value of this resistor lowers the output of the overdrive section. R13 at the end of the distortion circuit can be lowered too, but that raises the cut-off frequency of the low pass filter formed by R13 and C8, not contributing to eliminate the hiss
C16 18nF 22nF Lowers the cut-off frequency of the high pass filter after the hard clipping section, raising mids in the overall circuit
D3,D4 Junction diodes D3=1N4148 – D4=BAT85 (Schottky) + 10 Ohms resistor It changes the hard clipping section from symmetrical to asymmetrical. The schottky + resistor gives a smoother I/V curve than the diode alone. Just an experiment (successful for my ears), as in the soft clipping section
C8 820pF 4.7nF Lowers the cut-off frequency of the high pass filter after the hard clipping circuit. This is key to cut the hiss

BOSS OS-2 modified circuit
BOSS OS-2 modified circuit

Lessons learned

At first I tried LEDs also in the hard clipping section, getting a not so nice result. Probing the circuit with the oscilloscope, I discovered that it was not clipping at all, you could remove the diodes and get the same output. Forward voltage is so high, even for red LEDs (different color LEDs have different Vf), that it didn’t clip at all. The distortion came from the saturated transistor and was not very pleasant.

Then I tried different combinations of Schottky and junction diodes (I like Schottky diodes lately…) until I got to the above blend.

I tried green and blue LEDs and combined LEDs with junction and Schottky in the soft clipping section, but I didn’t like the results. If you have read my other post about SD-1, green and schottky was my final combination in the BOSS SD-1, but it does not seem to work in the OS-2. The final combination was the nicer for my ears, just that.

I put a trimpot instead of R13, in order to adjust the output of the distortion circuit, but the result was catastrophic: more hiss and even oscillation when the trimpot got near zero ohms. So I changed R2, getting much better results.

As you can imagine, the values of the capacitors and resistance are not casual, I have tried many combinations and calculated some filter frequencies to get to those values. Some starting points came from forum posts and some other pages, and I changed some components and at the end returned to the original values (C26 for instance) . The lesson here is: calculate values for the filters involved and act with a purpose. I took some ideas from this post: https://www.roboticbeast.com/modification-de-la-boss-os-2/ but some didn’t wotk for me or with my unit. Another lesson (I already knew, of course) is that every modification affects the whole circuit in some measure, so you shouldn’t change a single component and see if you like it.

Also found a very useful tools for analyzing frequency responses by generating signals and capturing the output of the pedal with a computer and its sound card, more on this in some future post.


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, tsd-1he 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.

BOSS FW-3 sweep range switch (mod)

In a previous post (BOSS FW-3 Mods) I told how I did some basic mods to a BOSS FW-3 Wah pedal, which is basically a Cry Baby design with a potentiometer for varying the Q.

For the capacitor that controls the sweep range (C12), I chose a value of 0,022uF, to give the effect a very personal lower sweep range. I found it problematic in some situations, because the effectiveness of the variable band pass filter as a musical device is very dependent on the color of the signal that it encounters at its input. For instance, if I am using the brigde pickup in an already bright sounding guitar (tele or strat), playing high pitch notes, a low frequency sweep will let the sound almost without volume at the low end of the range. The same happens in the opposite situation – high sweep range with dark sounding pickups, or lower notes.

So, in order to have an all terrain wah effect, I have put a switch for alternating between three different values:

  • 5nF – higher frequencies sweep range
  • 10nF – the original one
  • 20nF – lower frequencies sweep range

Given the behavior of a DPDT on-on-on switch, you can get those three values and their corresponding sweep range values (see https://www.electrosmash.com/crybaby-gcb-95) using three 10nF capacitors, this way:


Other values can be chosen, of course, to obtain more radical or subtle changes. For instance, with 10nF in the center, 56nF at right and 4.7nF on top you get:

  • 3.2nF – very high range, good for a very bright guitar and higher notes
  • 10nF – original sweep range
  • 66nF – very low sweep, suitable for a bass guitar

The real life device, as built for this project:


As I mentioned in the previous post (BOSS FW-3 Mods), this pedal, apart from being a tank, has space enough inside for storing all the money I have. In this picture you can see how the DPDT switch fits inside the case:


In the main PCB, the cables from the switch are soldered instead of the original capacitor:


And the final result from the outside (the scratches and dents were already there, they are natural relic as this pedal has more than 20 years):


If you have any advice about the execution of the project, or simply have tried it, or in case you had any problem, please let me know in the comments below.


BOSS FW-3 Mods

I have a BOSS FW-3 pedal bought in the 90’s when I started to play guitar, looking for a Jimi Hendrix or SRV sound. Why this pedal instead of a Cry Baby or VOX? Because back then you bought what they had in the shop.


Some months ago I decided to sell it because I didn’t like its sound and behavior any more. Too subtle when placed before dirt and too uneven in volume when placed after. But nobody seemed to be attracted to this squared tank in spite of the low price I asked for it, so I kept it.

It is not a bad product compared to other wah pedals, it is just you get sick of the same tone and problems, and want to get a different tone (and problems). In fact I like its size and robustness, it is solid like a tank and fits very well under my feet.

So I decided to tweak it a bit in order to find a different sound, and started to look for possible mods in the web: nothing at all.

After studying its circuit, I found that the BOSS FW-3 wah pedal is essentially a modded Cry Baby. A very clear and in depth analysis of the Dunlop Cry Baby can be found here: https://www.electrosmash.com/crybaby-gcb-95

There is plenty of information about mods for Cry Baby. In many sites they talk about these particular mods, in Cry Baby terminology, for a GCB-95 PCB:

  • True bypass (more details in the referenced sites)
  • Mid range response: R1 resistor change from  1.5K to 2K-2.7K
  • Q: R5, from 33K to a bigger value, up to 100K. This mod is already done in a BOSS FW3, there is a pot in the pedal panel for adjusting this value.
  • Sweep Range, change C5 from 0.01uF to a bigger value for lower frequencies and a lower value for higher frequencies range. Typically 0.068uF for a bass wah.
  • Gain and bass response: changing R9 from 390 to a lower value will raise gain and add bass content, while a higher value will reduce the gain

In order to apply this valuable information to modify a FW-3 and find our personal tone, we need to map these components to those found in the BOSS PCB. I didn’t fight with the true bypass mod because I like the BOSS buffer, and it is useful for me in most situations.

From the schematics:


Mod Cry Baby (GCB-95) FW-3
Gain R9 R6
Sweep range C5 C12
Mids response R1 R4

See below the pictures for FW-3 pedal PCB. In the top picture I have marked these components in the board.


Which values to choose depends a lot on your personal preferences. I have experimented with some values and ended choosing these ones:

  • R4 (Mids): 2.2K, gives some more mids content.
  • C12 (Sweep range): .022uF, lowers the sweep range in the frequency spectrum, giving a very pronounced WOOOO effect in the lower frequencies end.
  • R6 (Gain): 390. It lowers the gain a bit, making it more even with the effect not engaged

These values give a very personal tone to the FW-3 wah. Even placed before dirt, the effect is very pronounced and significantly different from the original, specially in the lower end of the sweep range.

In a next post I will try to record the sound of the modified pedal.

I’m planning to add some pots in the future to regulate the mids and gain, and even a selector to change among different values for C12. The unit has space in the panel to place the pots and enough space inside to place another two PCBs, so I think it is a perfect platform to play around a bit.