It’s been more than a quarter century since the Klon Centaur overdrive debuted. So why are we still talking about it?

The Klon became a cult item soon after it appeared, celebrated for its transparent overdrive. “Transparent” isn’t exactly a scientific term, but it makes sense when comparing the Klon to the popular overdrives that preceded it—notably the Ibanez Tube Screamer, which to this day provides the template for a large percentage of overdrive pedals.

A Screamer trims highs and adds a prominent midrange bump at around 750 Hz. A Klon’s core sound is less highly colored, with clearer highs and greater clean headroom. A Screamer’s compression smoothes out note attack, while a Klon delivers faster, crisper transients. Screamer tone never cleans up completely, even with the gain knob at minimum. But a Klon with its gain knob fully counter-clockwise delivers a pure clean boost. (Though it might not sound clean, depending on how hard the Klon’s output is slamming your amp’s input.) To many ears, the Klon was a sonic upgrade over previous overdrive pedals. When the guitar magazine I worked for in the ’90s covered the initial version, I bought the review model. I used that pedal (serial number 309) for reference while exploring our five klones.

Secrets of the Klon Kult
How does the Klon work its magic? Like most overdrives released since the 1970s, it owes its tones to an overdriven op amp, with extra color and compression from a pair of clipping diodes. Klon fanciers tend to focus on those two parts: the TL072, a low-noise, JFET-based op amp, and two NOS germanium 1N34A diodes.

Generally, germanium diode distortion is slightly softer and “spongier” relative to brighter, crisper-sounding silicon diodes and LEDs. But you only hear the diode’s character at the circuit’s highest gain settings—most of the distortion comes from the op amp. Meanwhile, the TL072 is a common and inexpensive part. But even if it weren’t, any number of op amps would yield similar results.

My tests suggest that the Klon’s character comes from its ingenious circuit topology, and not from “magic” parts. There’s a charge pump that generates 18 volts from a standard 9-volt power supply, and you hear the difference—tones are sparklier and more dynamic, with more clean headroom. The gain architecture is even more innovative. Incoming audio is split into three paths, one of which passes through the distorting op amp and diodes. Another path sends undistorted lows to the output, anchoring the bottom end. Meanwhile, the third path is a relatively hi-fi clean boost. The pedal’s gain knob simultaneously controls the levels of both the distorted and clean boost paths. Turning the pedal’s gain knob clockwise emphasizes the distortion while lowering the clean signal, and vice versa.

Other factors include an especially nice-sounding input buffer that prevents guitar pickups from loading down the circuit, a handsomely voiced treble-cut tone control, and memorable cosmetics. Original Klons have expensive, custom-cast enclosures with that striking bronze-and-oxblood color scheme. And, of course, there’s the sword-wielding centaur icon, usually referred to as “the horsie.” (Horsie Klons have the highest resale value.)

Klon vs. Klone
Original Klons tried to maintain circuit secrecy with a coat of black epoxy (see Image 1).

Image 1 — Original Klons obscured their circuitry with black epoxy. That trick never works! The Klon schematic is readily available online.

It didn’t work. The schematic is widely available. Copyright law doesn’t protect circuits, so the schematic is free for the cloning. On the other hand, copyright law says you can’t mimic a product’s “trade dress”—the visual appearance that identifies it to consumers. But that hasn’t prevented klones from paying “tribute” to the original’s gold/bronze enclosure, dark red knobs, and centaur sketch.

The original Klon employed traditional through-hole parts. Our contender klones have modern surface-mount components, which facilitate automated production and keep list prices low. Two of them employ NOS through-hole germanium diodes, while the other three substitute modern silicon ones. All are scaled-down relative to the original, with enclosure sizes ranging from a standard B-sized box down to tiny AAs. There’s also a dramatic price spread: from $69 to $199. All pedals run on standard 9V power supplies.

Testing Procedures
To keep things as objective as possible, I recorded the demo clips straight into my DAW with no processing, and then re-amped them through each of the klones. That way, you hear the identical performance through each pedal. Aside from switching klones, nothing in the signal chain changes. For the audio-only clips (see the page at the end of this article) I used a Fender Telecaster Deluxe with Lollar Regal wide-range humbuckers, a clean-toned Carr Telstar amp, and a Royer R-121 ribbon mic, recorded into Logic Pro via a Universal Audio Apollo interface. For the video I used a "parts" S-style guitar with Lollar Firebird pickups and Fender-style Carr Skylark.

In the audio-only clips you’ll hear four audio comparisons arranged into four playlists. First, there’s a 1:23-long passage with all pedal knobs set to noon. Next comes a gain test: You hear each pedal, first with the gain control at minimum (that is, in full clean-boost mode) and then at maximum. The third comparison displays the full range of the tone controls, first at minimum, then at maximum, and then back to the noon position. The final test compares the output controls. First, you hear the passage with a low output setting, with the knob around nine o’clock, and then at maximum. The low output setting is too quiet to overdrive the amp, isolating the distortion color produced by the pedal, as opposed to how that sound interacts with an overdriven amp.

In each case, you hear the original Klon first, followed by the five modern pedals in alphabetical order, from Electro-Harmonix Soul Food through Way Huge Conspiracy Theory.

Spoiler alert: These pedals sounds remarkably similar, and that’s not just a subjective impression. Consider Image 2, which compares the spectrum (EQ curve) of a passage recorded through the original Klon and through the EHX Soul Food.

Image 2 — A spectral comparison of a passage played through the original Klon and through the EHX Soul Food reveals that their EQ curves are nearly identical. Other comparisons yielded similar results.

The orange portion depicts the Klon, the blue portion the EHX. Their EQ profiles are superimposed in the main window. The orange and blue lines track so closely that it’s sometimes hard to discern two separate measurements. Comparisons between the Klon and the other contenders yield similar results. In other words, prepare to listen for very subtle distinctions.

Diodes to Die For?
Finally, a word about those germanium diodes: Judging by the online forums, this is the crucial feature separating the Klon from the klones. Even Klon founder Bill Finnegan cites particular germanium diodes (from his stash of NOS 1N34As) as the most important ingredient in the Klon casserole.

I think the audio suggests the opposite.

(Visit this page to hear all the pedals head to head with matched settings.)