The Plasma is a clone PCB for the Colorsound Overdriver and Power Boost, two nearly-identical circuits that serve different functions. The Power Boost came first, powered by two 9-volt batteries for 18v operation. It had a transistor-based Baxandall tonestack along with a gain control. While it would distort at incredibly high volumes, its primary purpose was to overdrive the amp. The Overdriver came later, having a couple of minor tweaks along with dropping the voltage down to 9 volts. The end result was that the Overdriver had less headroom and distorted at lower volumes.
The Plasma project allows you to build either an Overdriver or a Power Boost, with the incredibly useful mod of adding an output volume control. This way you can set the gain according to the tone you want without being tied to the volume that is produced along with it. It also has trimmers for each of the three transistors so that you can dial in the optimal bias.
The gain of the Overdriver can be “splatty” at medium gain levels as the notes decay. It works best at low gain as a clean boost with tone-shaping capabilities, or at high gain where it’s not unlike a Big Muff. If you experience this splattiness, know that it’s a byproduct of transistor-based distortion and you didn’t do anything wrong!
The Power Boost was first released in 1970 by Sola Sound / Colorsound, the house brand of the Macari’s music store on Charing Cross Road in London, who also created and marketed the Tone Bender variants (check out my Deimos and Phobos projects). It wasn’t long before it found its way into the rigs of such legends as David Gilmour and Jeff Beck.
In 1971 the Power Boost was revised slightly, changing a few values and dropping the battery requirements down to a single 9v, and released as the Colorsound Overdriver. According to Macari’s, this was done after taking the Power Boost to a trade show in the United States and getting a great deal of interest. They say that the Power Boost was just renamed and that it’s otherwise the same circuit but for the power supply.
Since the schematics we have are obviously different, given this information from Macari’s I might be tempted to conclude that the differences between the versions were just routine tweaks to what was basically the same circuit—in other words, the Overdriver is just the new name given to the final version of the Power Boost, and they did not see it as a new pedal with distinct schematics. This is supported by the fact that the Power Boost has at least two known variants, an early one and a later one, with a few different parts between them. Either way, I do give parts lists in my documentation for the three known variants: an early Power Boost, a later Power Boost and a standard Overdriver.
Back to the history… the one thing that was sorely lacking in all of the original Power Boosts and the Overdriver was a true volume control. People would use the gain knob to create the one they were looking for (the amount of grit) but usually the output volume would be enormous by that point. It took more than twenty years, but in 1996 a reissue Overdriver was created that incorporated a volume pot on the output. The trick with this one, though, is that for some unknown reason they incorporated a ‘minimum volume’ resistor which prevented it from going to zero. (I’ve only ever seen this on one other pedal, the Ibanez SD-9 Sonic Distortion.) Because of this, at higher gain levels the circuit can’t actually even get down to unity gain. As far as I know this minimum volume resistor is still in current reissues, but a common modification is to jumper this resistor so the volume can turn all the way down like a normal pedal. My project doesn’t even have a space for this resistor because it serves absolutely no purpose.
- Kit Rae’s Fuzz and Muff Timeline. The release date is sometimes cited as 1968 or 1969, but according to Kit’s research, “the Colorsound brand was not created and used by Sola Sound until 1970, the first ads appeared in 1970, and the preamp core of the circuit (likely where the design originated) appeared in a Mullard application book in April 1970.” Contrast this with the Macari’s website that says it was produced in 1968—probably the only way of determining this was to look at the pot codes of an original unit, but this doesn’t mean the pedals were in stores that year, just that the pots were manufactured that year. Call me crazy, but I trust Kit Rae’s date more for the reasons given. ↩
- Macari’s product page for the Colorsound Overdriver ↩
- Kit Rae’s page on the Overdriver ↩