Mojomojo simple mod

Not long ago I acquired a TC Electronic Mojomojo on ebay in perfect conditions. I love the brand and wanted to try their analog overdrive pedal. I liked it and kept it, some people complain it is bland, or dark, or anything, but I think in the right ecosystem it can perform very well. I have to confess that it has only be used a few times, just because lately I am using my modified DS-1, BOSS OD-3 or Tim Pierce Overdrive. Maybe I have too many overdrives?

Yesterday I put it on my home pedalboard for some recording, and it sounded really really bad, with very low volume and an unbearable noise. Ok, let’s fix it. Oh, well, it is SMD! Do you know what SMD is? Those pretty boards with tiny little minuscule components. They are not easy to troubleshoot or fix, components are too close together and are not mounted through holes but soldered right on the surface (hence the name: Surface Mounted Devices).

This was not the first time I fix a SMD circuit, I have replaced some USB connectors, and not long ago I fixed a black MXR 6 band equalizer bought broken on ebay. Not impossible, but as I mentioned, components are very small, the board is usually multilayer, and soldering and desoldering is not easy. To make things a little harder, I couldn’t find Mojomojo schematic. When I bought it, I immediately opened it to see what was inside, and although didn’t have the time to trace the complete circuit, I could see it is based on soft symmetrical clipping with four bipolar diodes, a là Vemuram Jan Ray, Tim Pierce, Blue Note, to mention some.

After tracing the clipping operational surroundings and testing the implied components, could see that one of the diodes was behaving abnormally. The oscilloscope gave different lectures for the connection between diodes of the two clipping sections (positive and negative), and it could be explained by a faulty diode. Some additional tests and the diode was located. I was lucky because if the problem had been on some capacitor or in the tone stack it would have been much harder to fix.

Now the problem was that I didn’t have SMD diodes, I am in quarantine because of coronavirus and wanted to fix it soon. So, why not making a virtue of necessity and try some modification of the pedal?

First I soldered a conventional through hole bipolar diode similar to the faulty one to confirm that it was the only problem around, with success. The pedal was alive again.

Now let’s decide about what to drop instead of the faulty diode. I didn’t want to replace any other components because of the lack of schematic and the intrinsic difficulty of replacing SMD components. Since I have experimented before with symmetrical clipping against asymmetrical clipping, it is not hard to guess that the replacing component had to be a different kind of diode. To make the clipping more open and asymmetrical at the same time, a LED was the perfect option. Common bipolar diodes have Vf = 0.6 V, while red LEDs have Vf = 1.2 V. Vf means forward voltage, that can be simplified as the minimum voltage that needs the diode to conduct a significant amount of current. That current is what makes clipping possible because the moment the audio signal reaches Vf, signal voltage starts to be limited. The higher the Vf for the clipping, the higher the signal can get before being clipped. In fact the clipping is not a all or nothing thing, but responds to a curve that is characteristic of each kind of diode.

In our original Mojomojo circuit, we had two diodes for the positive part of the signal and two diodes for the negative part. Total Voltage for clipping was 0.6 + 0.6 = 1.2 V. With a red LED replacing one of the four diodes, we will have 1.2 V in one direction and 0.6 + 1.2 = 1.8 V in the other. Why a red LED and not green, blue or yellow? Red ones have the lower Vf of all visible colors.

LED forward voltage and current (IV) curves for IR, red, orange, green, yellow, blue, white and UV LEDs.

Red is a good choice for making it assymetrical and more open, but not too much. Sometimes, especially for mild overdrives, the total Vf is so high that you have no clipping at all. I think 1.2 V versus 1.8 V is a good balance.

The LED needs some shaping in order to be soldered on the surface of the board.

Shaping the mini LED for SMD soldering
Red LED replacing an SMD diode

We have to be very careful and not let it contact any other component. As usual, but a little harder. Finally some hot glue to make it more resistant mecanically:

How does it sound? I didn’t have a recording of the original sound of my particular Mojomojo, and I think it is not very useful to upload a demo because the difference is subtle, to my ears it gives a little more mojo to the Mojomojo, or at least a different mojo. Some more high end and dynamics and less compression, given the higher Vf. Another LED colors can be tried, of course. Maybe I upload several demos of this and other mods in another post. But I don’t believe very much in online demos, they depend too much on the rest of the gear, and the player itself, of course.

DS-1 vs modded DS-1: frequency response

Let’s compare the original BOSS DS-1 with my modified DS-1, so we check out the difference in tonal character between a distortion pedal and an overdrive pedal (or whatever DS-1 has become after being modified in

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

Original DS-1 frequency response

As expected from a pure distortion effect pedal, frequency response is mid scooped, with a bump at 100Hz.

Modified BOSS DS-1

Modified DS-1 frequency response

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 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.

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: 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.

PD: if you make this mod, please comment below, I would like to keep track of people enjoying (or not) my mods.

Asymmetrical Octaver Effect

In a previous post, I told how I made an EQD Tentacle Clon in a 1590 Enclosure. I took the layout from and made some minor changes:

  • I used NPN transistors with a high beta (>400), I think they will give higher input gain and more output gain in the final stage.
  • I had a very anoying noise in this circuit from a cheap power supply. Added a 56 Ohms resistor between supply 9V and circuit and the noise disappeared completely. It seems that the 100uF capacitor was not enough and needed a little resistor to absorb the noise.
  • Ommited the two bottom rows with no function.

Yesterday a made a new, more drastical change: I swapped one of the two rectifier diodes (1N4148, Vf = 0.7V) with a BAT46 Schottky diode, with Vf = 0.3V, just to experiment with this component.

The electrical result is an asymmetrical rectification of the input signal, as can be seen in the oscilloscope:

input signal below, output signal above

The audible result is a more natural sounding octave effect. Usually octavers have to be combined with overdrives or fuzzes in order to be bearable. Otherwise they sound too robotic and are unusable, at least for me. With this change, even if visually does not seem to be very different from its symmetrical counterpart, at least for my ears it sounds like a very dynamic distortion, and of course gets improved with some kind of light distortion after it.