MXR Distortion +
Although labelled as distortion, this is a soft clipping device, using germanium diodes. It's a good example of how little you need for a good basic sound. You could easily swap (or switch) these diodes to silicon types for hard clipping.
Not necessarily the next pedal chronologically, but look at how similar this design is. It uses 2 silicon diodes for symmetrical hard clipping. I would also expect that at high gain settings, the IC also clips to the supply rails.
No discussion on overdrive pedals is complete without looking at the Ibanez Tube Screamer. There have been several minor variations of the pedal released by Ibanez, and a larger number of variations sold by boutique pedal manufacturers. As our guitar heroes die, it seems the equipment they used sometimes takes on a mythical status. In my opinion, this is the case with the genuinely legendary Stevie Ray Vaughan and the Tube Screamer. This results in some silly prices for original pedals, and a lively market to convert different pedals to Stevie's model.
Nevertheless, the green Ibanez box is a very smooth sounding pedal that retains the guitar timbre well, and for that reason works well with single coil guitars. There is not an enormous amount of drive available, and the tone control is subtle. Like many overdrive pedals, there is some middle boost, caused by the bass cut before overdrive, and treble cut afterwards.
Another common use for these pedals is as a middle booster to drive a valve amplifier harder. This is done by setting little or no drive, but with the level set high.
In the schematic, you can see two silicon diodes, back to back, in the negative feedback path of an op-amp. This arrangement gives symmetrical soft-clipping.
These were originally sold without the tone control. The design is nearly identical to the Ibanez Tube Screamer with 2 important changes. More boost is available, but is partly offset by using 2 diodes in one direction and only one in the other. This produces asymmetrical soft clipping, meaning that one side of the waveform is clipped more severely than the other. A more common implementation of asymmetrical clipping is to use 2 silicon diodes, with a germanium diode in series with one of them.
There is lively debate on the Internet about whether this sounds more natural, and whether it better emulates some asymmetric valve phase splitter designs. In any case, I think it does add a little character, and therefore suits humbucker guitars well.
These three pedals were released in the early 90's, and use different clipping and tone shaping techniques to deliver different sounds.
The Blues Breaker uses silicon diodes in series with a resistor, in the op-amp feedback path for very soft clipping. It's therefore a very subtle pedal, with warm sounds at low to medium overdrive, but can sound a little fuzzy at high gain. Retention of guitar timbre and dynamics is good, and intermodulation (read above) is acceptable.
This MXR Distortion + schematic is the one I am currently working on.
