Skip to content

Refractor Professional Overdrive

Based On
Klon® Centaur / KTR
Effect Type
Build Difficulty
Project Summary
A part-for-part replica of a mythical overdrive effect noted for its high-end tone and price. The Klon Centaur and its successor, the KTR, remain highly original designs in an industry full of clones and tweaks to existing circuits.

Complete Kit

What's included?
PCB, finished enclosure, hardware,
and all electronic components.


In stock

Refractor Professional Overdrive printed circuit board

PCB Only

What's included?
Just the PCB. Build instructions and parts list can be viewed or downloaded from this page.


In stock

Project overview

The Refractor Professional Overdrive is a clone of the Klon® Centaur Professional Overdrive and its successor, the KTR.

While the original Centaur was buffered bypass, the newer KTR added the option to switch between buffered bypass (called “Almost Always Better” by the designer) and true bypass (called “Almost Always Worse”). There were an overwhelming number of requests for this to be added to the Refractor, so this updated version implements the switch.

As of its release, the Refractor is the only DIY version of the Centaur or KTR that uses the correct true/buffer switch wiring from the original unit. Several other projects use a simpler wiring method that does not preserve the original circuit in buffered mode, which was one of the inventor’s stipulations.

Aside from the addition of the true bypass / buffer switch, the KTR is identical to the original Centaur circuit except for two components:

  1. A pulldown resistor, RPD, has been added to counteract switch pop in the true-bypass mode.
  2. R2 has been changed from 1M to 2M so that the input impedance stays the same with the addition of RPD (since two 2M resistors in parallel make 1M, which was the original value).

Demo Video

Check out RJ Ronquillo’s demo video of the Refractor Professional Overdrive.
Watch on YouTube →

Klon Centaur technical details

Circuit revisions

We’ll bust a few myths throughout this article, but let’s start off with a big one: There is no difference between a Gold and Silver Centaur. There were a couple of very small revisions that were made throughout the Centaur’s production run, but these were not made in conjunction with the Gold-Silver transition.[1]

The only change that impacted the tone at all was the addition of R11 (labeled R14B in the KTR), a 15k resistor, in 1995 at around serial number 300. The other changes were R1 (R1B in the KTR), a 10k series resistor that prevents static on the input, and R28 (R1C in the original), a 100k resistor on the output that changes the impedance slightly. Even R11 is incredibly subtle. 7,700 out of 8,000 Centaurs have it in place, so it should be considered the accepted value.

You may hear someone swear that a “Gold Horsie” Centaur sounds different than the later Silver ones—but the truth is that if a person were to A/B a Gold with a Silver, any tonal differences they heard would be due to component tolerances and nothing more. They would be just as likely to hear the same difference between two Silvers or two Golds side by side.

Most of the resistors in the Centaur have a +/-5% tolerance with a select few at +/-1%. CTS potentiometers are usually +/-20%, but can get down to +/-10% if you special-order them. The film caps are +/-5%, and the electrolytic capacitors are +/-20%. While any tonal changes will be incredibly subtle, it’s definitely possible that someone might hear differences. Just understand that it’s extremely anecdotal evidence if one person A/B tests two pedals that happen to be different colors, and it doesn’t mean the tonal differences are representative of all Centaurs.

Silver Pony specs

The 2009 "Chittum schematic" of Klon Centaur #S698.

The 2009 schematic of Klon Centaur #S698.

We’ll say more on this later in the history section, but briefly, to clear up another misconception: the April 2009 schematic by Martin Chittum is 100% correct. This is the schematic that the Refractor is based on. The original Silver Pony from Build Your Own Clone introduced the tone control “resistor swap” where it was claimed that the 2009 schematic had R21 and R23 reversed. This reversal would not change the sound of the unit at all, but it shifts the tone control’s range slightly downward, reducing the maximum available treble. However, it’s been confirmed that the KTR has the resistors in the Martin Chittum’s original positions, so there’s no reason to believe the original 2009 trace was incorrect.

The “magic diodes”

Gut shot of the Klon KTR. Note the diodes right above the ribbon cable.

Gut shot of the Klon KTR. Note the diodes right above the ribbon cable.

The diodes used in the Centaur have been a source of a great deal of technical controversy. Bill has always claimed that he has a huge stock of a certain type of germanium diode that are long out of production and that can no longer be found anywhere because he bought them all up in the early 1990s. He maintains that these exact diodes are essential to the sound and that no true Centaur clone can ever be built because he’s the only one who has them.

Bill has claimed the diodes are 1N34A, but that they were from a specific manufacturer and that other 1N34As do not sound the same. As a result, the closest we can get is measuring certain characteristics (primarily the forward voltage, or voltage drop), measuring other diodes using the same test conditions, and then trying them in the circuit. The original diodes tested at around 0.35v forward voltage, at least in #S698. These tests were done using a Peak Atlas DCA55[2], which has a 5mA test current.

Keith from Build Your Own Clone tried a number of different germanium diodes during the development of the Silver Pony and was the first to note that the Russian D9E diode was an exact tonal match in his testing. (The D9B, D9F, D9G, D9J, and D9K are alternates which should sound identical and may be found much cheaper; the differences between the suffixes do not matter for what we’re doing.) Others have concurred in A/B tests, so this is the closest the DIY community has come to matching the sound of the original 1N34A’s.

But the quest for “closest diode” is only part of the story. People have tried all sorts of diodes in the circuit, not necessarily aiming for the most authentic results, and have come up with some really good alternates that are worth trying. The BAT41 Schottky diode is one that gets really high marks for having a great sound—it has a different character than germaniums but works well in the circuit. But don’t be afraid to experiment!

Klon Centaur circuit history

Very few pedals can match the levels of mystery, hype and drama that surround the Klon Centaur Professional Overdrive, but then, very few pedals can match its tone, either.

First released for sale in 1994, the Centaur was the product of four years’ research and development time. The visionary behind the Centaur was Bill Finnegan, a guitarist out of Boston, MA who wanted an overdrive that would allow him to coax cranked sounds from his amp at low volumes. He was dissatisfied by the Tube Screamer and other similar options, and decided to create his own with the help of a friend from MIT.[3]

With a custom-designed enclosure, special-ordered pots and knobs, and the highest quality components available, the Centaur definitely earned its $225 price tag, though this was incredibly expensive in 1994. Nonetheless, the Centaur made its way onto the pedalboards of a few well-known musicians like Joe Bonamassa and gained traction.

Bill hand-made each of the Centaurs in his apartment for the entire 15-year run of the pedal. His sales tactics were a little unconventional: the only way to buy a Centaur was to call and talk to him on the phone, and he’d ask you about your rig and your expectations in order to make sure that the Centaur was a good fit for you. He took an immense amount of pride in his circuit and felt that it was worth his time in order to make sure that nobody purchased one who would end up dissatisfied with it.

Silver enclosure

Starting in the early 2000’s, Bill began transitioning to a silver enclosure design. They were built alongside each other for awhile—probably Bill offered the silver enclosure as an option for a period of time if people wanted it, while the gold enclosure remained the standard. At some point during this time, the screenprint of the centaur was removed from the artwork as well, from both colors. As a result, there are a number of gold enclosures without the centaur and a number of silvers with it.

Before too long, the gold enclosure was retired completely in favor of the silver enclosure. The serial numbering for the silvers was reset at 1 and prefixed with “S”, so S698 is the 698th silver Centaur produced and would have been built long after #698 without the “S”. The price also jumped up to $329 around this time.

In 2009, after fifteen years of production and nearly 8,000 units sold, Bill unexpectedly discontinued the Centaur. It had been selling well and had never been more popular, but he later said that the stress was too much since he was building them all himself and that he had to come up with a more sustainable strategy.


Bill’s solution to the problem was to redesign the circuit so that it could be built by a contract manufacturing firm. This way, he could just do quality control and final assembly without having to solder anything. Instead of building it himself and selling them one at a time over the phone, Bill would outsource the building and sell them through distributors.

Sounds easy, right? Well, all told, it was nearly four years between the time the Centaur was discontinued and its successor was first released, the Klon KTR. (No one’s sure what “KTR” stands for and Bill won’t tell.[4]) The first units shipped in October 2012, but the initial run was extremely limited and had some reliability issues. These were eventually worked out, and throughout 2014 a few more batches were distributed to a variety of online music equipment stores. Today they’re much easier to find and the lack of scarcity has driven resale prices back down to normal levels.

The Klon KTR enclosure with screenprinted text.

The Klon KTR enclosure with screenprinted text.

One head-scratching characteristic of the KTR is the artwork: the pedal name is nowhere to be found, just the text “Kindly remember: the ridiculous hype that offends so many is not of my making.” Bill clearly wanted to distance himself from the myth and drama that surrounded the Centaur and focus on the quality. As an editorial aside, this becomes a bit of a paradox: more than anything, Bill wants you to know that he didn’t create the hype, and yet he’s the kind of person who feels so strongly about this that he would print a statement on the front of the pedal saying so. Wouldn’t it be reasonable to say that he actually did create the hype—even if it was unintended—and that he continues to create it with all the production delays and bizarre artwork decisions that seem to insult the owner?[5]

FSB degooping and Attack of the Klones

A history of the Centaur would be incomplete without mentioning the Centaur’s relationship with the DIY scene. Since the Centaur had a thick layer of epoxy covering the circuit, it was always kind of mythical among DIY pedalbuilders, with no shortage of speculation on what it might be based on and people claiming to have a schematic that they would trade for another equally valuable schematic. But nothing ever really materialized and it remained elusive.

The S698 PCB degooped by Martin Chittum (soulsonic).

The S698 PCB degooped by Martin Chittum (soulsonic).

In 2008, the community decided that needed to change, so they pooled their money and purchased one from eBay, a silver Centaur with the serial number S698. This unit was sent to Martin Chittum (soulsonic) to do the degooping honors, and in May 2008 the initial schematic materialized. It was later revised in April 2009 with a few very minor corrections, and this is the final schematic that pretty much every clone was based off of.

Bill’s discontinuing of the Centaur was very poor timing, because the four-year delay at the height of the Centaur’s popularity left a huge void that was very quickly filled by Centaur clones—which were naturally given the nickname “Klones”—that were far cheaper than the original unit, and usually in smaller form factors. (Check out the video below comparing a DIY Refractor to a real Centaur unit to see the size difference.) A few notable ones are the RimRock Mythical Overdrive, the Ceriatone Centura, and interestingly the Electro-Harmonix Soul Food—the latter of which retails for around $70, though it has some odd circuit tweaks and can’t be considered a faithful recreation.

BYOC Silver Pony V1

In 2014, Keith from Build Your Own Clone degooped a silver Centaur (serial S2207) and released a kit called the Silver Pony. Keith found a number of discrepancies from the Soulsonic schematic, which were attributed to Gold vs. Silver differences. This was latched onto by the DIY community, and these “Silver Specs” changes were widely distributed within a few days of his posting.

However, it was later pointed out that of course S698 was a Silver as well, so we were definitely not looking at the “Silver Specs”, at least not in a manner that contrasted with S698’s “Gold Specs” as was commonly claimed for awhile. In retrospect, no one’s quite sure what to make of the Silver Pony situation, whether the trace was inaccurate or whether it was an experimental unit built by Bill—which was not unheard of.

So the “Silver Pony specs” are as a curiosity, maybe worth socketing and testing out if you’re interested. Some people definitely like it, but just know that it’s not representative of a silver Klon Centaur.

The original BYOC Silver Pony kit was discontinued in early 2015, supposedly by direct request from Bill. In 2018, the Silver Pony 2 was released, an updated version with a diode clipping switch and optional clean boost on the output, and without buffered bypass.

A bit of trivia

The name “Klon” derives from Klondike, a region in the Yukon Territory of Canada bordering Alaska, best known as the site of a gold rush in the 1890s. The clue is in the domain name, This leads to three possible inspirations for the name:

  1. Klondike and Siberia were the names of adjacent mining towns along Route 66 in California’s Mojave Desert, each optimistically named after a region known for its gold. Both towns have long since been abandoned and no trace of them remains, but at one time they were located about three miles from each other. You can see both locations (still labeled) on Google Maps.
  2. There’s a book called In Search of a Siberian Klondike that describes efforts to find gold in Kamchatka, Siberia. It was published in 1903, only a few years after the Klondike gold rush—so even though Klondike isn’t historically considered one of the more successful gold rushes, at that time it was the most recent one.
  3. A reference to the Bering land bridge that at one time connected Siberia with the Klondike (by way of Alaska, anyway).

Regardless of where the name originally came from, the “Klondike gold” connection is obvious. Beyond that, Bill Finnegan is the only one who could say whether there’s another layer of meaning.

Notes & references

  1. We are indebted to ManticoreFX‘s institution-quality Klon research for much of what follows, but for this section, most directly this article detailing the changes made in the Centaur throughout its run.
  2. Confirmation from soulsonic on
  3. Premier Guitar interview with Bill Finnegan
  4. There is one strong contender, though. When developing the KTR, Bill initially wanted to outsource production to an established pedal company who handled their own manufacturing. It’s well-known that he had a deal with J. Rockett Audio Designs that fell through in a dramatic fashion and resulted in them making their own version called the Archer. However, before Rockett, it’s rumored that Bill was in talks with T-Rex Engineering and that that they even produced prototypes. If “Klon T-Rex” was an early project codename, and the initials “KTR” had stuck even after the T-Rex deal fell through, it explains why he’s been so guarded about what it stands for.
  5. To illustrate the point, there’s a 20-page thread on The Gear Page that discusses just the artwork. Most people thought it was fake at first.