LIMBO overdrive offers "Variable voltage clipping". But what is happening actually? In this post, I will show how LIMBO and other overdrive pedals distort the sound.
Before we start the article, this is written for the people who understands how overdrive pedal works. If you don't know the basics, please refer other resources first by searching like "how overdrive pedal works".
Since real guitar sound is inconsistent, I use pure sine signal for this analysis (See the picture). Signal frequency is 1kHz. I will demonstrate how Positive and Negative dial work. In order to see accurate result, I set Tone dial to fully clockwise (This pedal doesn't have any treble boost). By doing this, there is no high cut filter rather than clipping.
Let's see what happens when I turn to counter-clockwise for Negative and Positive dial. In this case, Signal is clipped by Op-amp. Soft clipping function is completely disabled. As you see, the edge is very sharp.
When I turn Positive dial to fully clockwise, then shape becomes like the picture. Now positive side is clipped by transistor, not by Op-amp. It is much softer clipping compare with Op-amp clipping. Also total output level goes down because it cuts at lower voltage, but you can normalize output level by changing Volume dial.
This picture shows when I turn Negative dial to fully clockwise. Now negative side of signal has softer clip. You can create your own original clipping shape by combining Positive and Negative dial. Some may notice the effect of Positive and Negative is not symmetry, and yes, it is not symmetry due to transistor Hfe difference and other circuit characteristics.
This picture shows when both dials go to the end. Signal clipped by positive and negative side as expected.
So how other pedals are clipping guitar signal? I show some examples here. First one is EHX's Soul Food. That is one of very famous pedal and known as one of Klones. Very good sound pedal anyways. As you can see, wave shape is like triangle, but please note wave shape becomes more complex at lower frequency and maybe that's why the pedal has unique sound character. Limbo can get similar result just by turning Tone knob down and keep counter-clockwise side for Negative and Positive dial, but result will not be same because Soul Food has much more gain than Limbo. Limbo may need another boost or overdrive pedal in front to get closer sound.
Second example is MXR's Super Badass. As you can see, it has more steep angle when signal goes other side, that means this pedal's cut-off frequency is higher than Soul Food and Limbo. That defines wild character of the pedal. This wave shape is consistent across wider frequency range. I guess that is one of the reason why the result tone is similar regardless what guitar I use for the pedal, when comparing with other pedals.