In 1994, one Bill Finnegan (and two engineers from MIT) sat down and created the Klon Centaur, one of the most talked-about circuits of all time. While it was at its core an overdrive circuit, it boasted several innovations that put it ahead of the rest of the stuff out there. For one, it included a robust power supply that provided a theoretical 27 volts of headroom from a nine-volt supply. Perhaps more notably, the Centaur thrust the idea of dual-gang potentiometers into the groupthink of pedal development. While these weren’t cutting-edge tech in 1994, very few pedals utilized them. In fact, the Shin-Ei Uni-Vibe is one of the only popular pedals I can remember using one (C100K in fact).
For the uninitiated, a “dual-gang” potentiometer means that rotating the shaft controls two different parameters simultaneously. Typically, these are found on stereo devices, where one dual-gang potentiometer would control, say, the master volume for two separate circuits; one for the left channel and one for the right. In the case of the Centaur, the dual-gang potentiometer in question controls a clean blend circuit and the op-amp gain. By doing this, the circuit is completely clean on one end of the control and fully saturated on the other; the resulting signal is summed by another op-amp gate. The Centaur is what one might call the first “transparent overdrive.” Yes, it’s a tad overengineered, but the proof is in the pudding. The thing sounds excellent. The arrangement is perfect.
It was so perfect in fact, that a handful of companies caught wind of the Centaur’s convolution with its dual-gang pots and juiced power supply. Soon, a handful of circuits emerged that didn’t explicitly copy the secret sauce, but set out to whip up a painstakingly designed effect with the goal of extreme nuance. The three that come to mind are the Rocktron Austin Gold, Zoom Power Drive and the best of the bunch, the Maxon OD-820 Overdrive Pro, a not-so-subtle nod to what Klon called the “professional overdrive.” It also might be better—you decide.
The Centaur utilizes a buffer circuit up front that’s always on even when the effect is turned off; the Maxon OD-820 is no different, featuring a JFET buffer instead of an op-amp based affair. Gain is derived in a different way from the OG but the idea is similar; one knob controls the clean blend and op-amp feedback gain in one stroke. The dry signal careens through a network of filters, preserving the integrity of the signal.
Nowadays, “charge pumps” are somewhat commonplace but in 1994, virtually no pedals used them. Through a series of diodes and capacitors, this IC “pumps up” the voltage in nine-volt intervals while decreasing the available current. A separate function of these chips is generating negative voltages. In the case of the Centaur, positive 18 volts meets negative nine volts in an op-amp and 27 volts of headroom is the result. The OD-820 goes there, but instead generates the negative voltage while keeping the positive untouched.
The tone circuit of the OD-820 is a near copy of the Tube Screamer tone knob but with two crucial differences. One of them is that the circuit uses an op-amp gate and so the headroom is a bit higher than a Tube Screamer, but the second is the omission of a 220nf capacitor to ground on the input. In a Tube Screamer, this forms a low-pass filter that attenuates frequencies above 723Hz, giving this stage a bit of a mids push before the Tone knob. As for the Maxon, this simple omission turns the gears in the direction of transparency by allowing the full frequency range to hit the tone circuit. When the Drive control is rolled back, a clean, buffered signal hits a tone control and gets amplified by a post-volume knob recovery circuit. Clean as a whistle. And that’s showbusiness, baby.
With all things considered, the Maxon OD-820 has a lot going for it. If price is no object, comparing the two may lead one down the Centaur path, it’s true. Considering price, it’s hard to pass up the OD-820; you can get one used for about $150 on a good day. Compared to the Centaur, time hasn’t been exceptionally kind to the OD-820; they’re rarely mentioned in the category of perfect drive units while the Centaur has legions of devoted fans. Maybe that should change.