Colorsound Power Boost

The Colorsound Power Boost is a transistor-based boost and drive pedal with a two-band tonestack, built around a simple three-transistor preamp and a two-band Baxandall EQ¹². It was created around 1968 by Gary Hurst¹, released by Sola Sound, the London company behind the Tone Bender³, and first sold in 1970 through the Macaris music store in London². Tru-Fi dates the release to 1969³, and Macaris' own website says 1968, so the exact year is disputed⁴.

The design goal was not distortion of its own but push: Tru-Fi's history describes it as a clean, high-headroom booster created to drive the front end of guitar amplifiers into overdrive⁵. At 18 volts it had enormous output, and before a master volume mod was added later, the pedal was, in Tru-Fi's words, "TOO LOUD"⁶. David Gilmour used it extensively during the 1970s⁵, and Jeff Beck is also named as a user³, which is why the circuit is still copied today.

How the circuit works

The Power Boost is a simple three-transistor preamp feeding a two-band Baxandall EQ¹. A Baxandall tone stage is the classic cut-and-boost shelving arrangement found in hi-fi equipment: instead of a single passive tone control that only rolls off treble, it gives independent bass and treble controls that can either boost or cut each band from a flat center position. In the Power Boost that two-band tonestack sits in the signal path of a high-gain silicon preamp, so the pedal can shape the pushed signal as well as amplify it.

The three controls on the pedal were Volume, Tone, and Sustain, with Sustain acting as the gain control⁵. One quirk matters to anyone playing an original: the knob labeled Volume is really a gain control rather than a true volume control, so turning it up raises both drive and output level together⁷. The original units shipped in a distinctive orange enclosure, though later versions came in different colors⁵.

The transistors are ordinary NPN silicon parts. The original came with BC184L transistors, but as Tagboardeffects' Zach notes from his own builds, any NPN silicon transistor can work in the circuit⁷. Builders have substituted BC169C and 2N5088 successfully; Zach reports that "2n5088s, which match well with the hfe of bc184l and they sound great"⁷. One detail matters for anyone building the circuit: BC184L parts can arrive in ECB pinout order, in which case the legs need to be crossed⁷.

Power: 18 volts from two batteries

The Power Boost ran at 18V using two batteries in series², which is the heart of both its sound and its reputation for being loud. The higher headroom is what lets the preamp stay clean and then deliver a very large signal to the amp. Tru-Fi notes that before the master volume mod, the pedal was simply too loud at 18V, which is why the 9V Overdriver was released to tame it⁶.

For DIY builders, the 18V supply has practical consequences. Electrolytic capacitors must be rated above the supply: Zach advises that "25v caps would be fine. As long as the max voltage of the caps is higher than the power going in"⁷. Interestingly, not every builder prefers the higher voltage. Zach tried the circuit at 18 volts and concluded, "I actually think it sounds much better at 9 volts"⁷.

Early and later versions, and the Overdriver

The Power Boost exists in at least two known variants, an early one and a later one, with a few different parts between them²⁴. Later versions also came in different enclosure colors, and some variations featured slight modifications to the circuitry or controls⁵.

The bigger story is the Colorsound Overdriver. Aion FX describes it as an updated version of the circuit, a third revision from 1972², while Tru-Fi dates the change to 1971, when Colorsound changed the Power Boost to the 9V Overdriver⁸. Macari's account is that the Power Boost was taken to a trade show in the United States⁴. The circuit topology is the same as the Overdriver's, but the Power Boost ran at 18V⁷, and the Overdriver dropped the second battery and ran on a normal 9V supply⁴.

The missing volume control is the shared flaw of both originals. Aion FX notes that "the one thing that was sorely lacking in all of the original Power Boosts and the Overdriver was a true volume control"⁴. Colorsound later added a master volume mod⁸, and a 1996 reissue Overdriver incorporated an output volume pot, though it included a minimum-volume resistor that prevented the output from going to zero⁴.

Is it a fuzz or an overdrive?

It is neither a fuzz in the Fuzz Face sense nor a modern overdrive in the Tube Screamer sense. It was designed as a clean, high-headroom booster to push amplifiers⁵, and PastFx calls it "one of the first overdrive effects pedals made available to the masses"⁹. Tru-Fi describes it as known for its powerful overdrive and incredible boost factor⁸.

At full gain, though, it can get aggressive enough that players reach for the f-word. Zach of Tagboardeffects writes that the sound "to my ears boarders on a fuzz"⁷. Builders also report that the drive control bunched up at the end of its rotation; one fixed this by swapping the 10k volume pot for a 1k, giving full gain clockwise with a controllable response, and another found a 5k pot made the fuzz side kick in earlier, though still at massive volume⁷.

Pedals based on the Power Boost

Because of the original's popularity, there have been reissues and clones from various manufacturers⁵, and vintage units are sought after by collectors⁵. The circuit has become a small family of its own:

• Tru-Fi Colordriver: a handmade recreation of the vintage Power Booster / Overdriver circuit¹⁰, built in both an 18-volt version done, per Tru-Fi, "to a true Power Boost replica done to the correct BC184 Transistors"⁸, and a 9V version¹⁰⁸. Tru-Fi also makes a compact version in the original orange color⁵.
• PastFx 18V Power Booster: described by its maker as inspired by the legendary Colorsound Power Booster circuit, and as a pedalboard-friendly recreation and expansion of the Buffalo FX Power Booster¹¹.
• ThroBak Overdrive Boost: ThroBak has reproduced and improved the circuit of the original, which it identifies as also known as the Colorsound Overdriver¹².
• BLAMMO! Voltage Booster: "recreates that classic Power Boost circuit"¹.
• IdiotBox Power Drive: built around the old Colorsound Power Boost¹³.
• GUP Tech Lab Series #23 Power Booster: based on the early 70's Colorsound power booster¹⁴.
• PedalPCB Powersound Overdrive: a DIY PCB to compare to the Colorsound Overdriver¹⁵.
• Aion FX Nucleus and Plasma: DIY projects for the Power Booster and the Overdriver respectively²⁴.

For DIY builders, Tagboardeffects published four stripboard layouts in September 2017: two identical to the original early and late versions, and two with an added master volume pot, so gain can be maxed while the output level is controlled⁷. The layouts went through corrections from the builder community, including a short link that prevented power reaching Q3 and a reversed 22uF filter cap, before being marked verified⁷.

Where the circuit sits in history

The Power Boost sits between the British fuzz tradition Sola Sound helped establish with the Tone Bender and the later overdrive pedals that followed it. PastFx credits it with being popularized in the 1970s and imprinted on the recordings of various classic rock guitar legends of the era⁹. RockBoard's PedalPedia entry for the ThroBak Overdrive Boost sums up the lineage: "The original was used by David Gilmour, Jeff Beck and other pros to become an integral part of 70's signature tones"⁸¹⁶.

Its influence also shows up in how often the Baxandall EQ it used reappears in later designs, and in the number of makers, from boutique builders to PCB suppliers, who still offer the circuit in some form. The Overdriver itself has seen different variations and reissues over the years, some with slight modifications to the circuit or enclosure design¹⁷.

Pedals on this page

Colorsound Power Boost

Colorsound · Boost

Tru-Fi Colordriver 18 Volt Version

Tru-Fi · Overdrive

$209.95 list price

Colorsound Overdriver

Colorsound · Fuzz

$259 list price

Throbak Overdrive Boost

ThroBak Electronics · Overdrive

$389 list price

IdiotBox Power Drive

IdiotBox Effects · Overdrive

$139 list price

PastFx 18V Power Booster

PastFx · Boost

Sola Sound Tone Bender MKI

Sola Sound · Fuzz

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