Klon Centaur mods and tweaks

Summary

• Klone centaur build (Aion refractor)
• Klon centaur circuit analysis
• Klon centaur mods and tweaks
• Klon centaur history and versions

 Here is the schematic of the Klon Centaur:
The Klon centaur circuit can be modded easily to modulate the gain, bass response or diode clipping. Lets see a few Klon Centaur mods together:

• Increase C14 value: it is a very common mod to set the tone response a bit better. It allows a bit more bass to go through and make the Klon less "thin sounding". I usually use a 6.8nF capacitor, treble pot is thus more useable.
• Separate the dual gain potentiometer in two pots: instead of using a double potentiometer, you can split each parameter with two 100k potentiometers. You can choose how much clean signal you will blend with the overdrive sound, or use only the overdrive part of the circuit. It is also useful for bass, if you want to let more bass goes through the circuit. I guess that the blend potentiometer on the bass soul food comes from this mod.
• Diode clipping switch: you can choose between two sets of diodes with a simple DPDT switch. This is what I did with my Soul Food mod. There are not much changes between germanium and silicon diodes, however LED gave some nice results. There is much more headroom, it feels a bit like a distorsion!
• Bass switch (madbean "fat" switch): this switch will affect the amount of bass that goes through the second OP amp. To do that, we can add a switch to choose between the 82nF stock capacitor and a 150nF one for C7.
  
  
• Bass contour mod: instead of using a switch, you can use a 50k potentiometer to blend the higher value capacitor in. You can also use a bigger value like 220 nF or even 1uF! However, you can see that this mod is only acting on the saturated part of the circuit, and not on the clean signal that is blend later. Thus, it is more hearable with high gain values... To have a proper bass knob, you can try to use a double potentiometer, and blend a higher value capacitor in parallel of C4 (68nF) too! I have never try that, maybe I will give it a shot at one point.
  
  
• Using different OP amps: instead of the TL072CP, you can use different less noisy OP amps, or more "classic" overdrive OP amps. The simplest solution is to use sockets for OP amps, and try any double OP amp IC that you would like. To test: LM1458, OPA2132, LF353, JRC4558D... Any double OP amp can give you interesting results!
• Increase gain: to have more gain on your unit, you can modify R10 resistor. It has a 2K resistor originally, and you can use a lower value to have more gain, or even a jumper!
• Using the Klon Centaur with a bass: if you cant to use the Klon Centaur with a bass, you can change a few values to let more bass going through  (from Madbean pedals): C1, C3, C4 : 220 nF, C5 : 100 nF, C6 : 1 uF, C8, C13 : 1 nF, C7 : 330 nF, C11 : 6,8 nF, C12 : 56 nF, C14 : 15 nF. I also strongly suggest to separate the gain and blend knob (second mod), so that you can dose how much bass will go through the circuit. The Bass Soul Food actually uses this mod. Here is the traced scheme for our bassist fellows:

• 1994's specs switch: in 1995, Bill Finnegan slightly modified the circuit (see below). Most of the changes were not affecting the sound, except the addition of R11 (15k resistor), that was supposed to boost a bit the bottom mids. If you want to hear what that does to your tone, you can put a switch there to choose between the 15K resistor or a jumper.

I will try to do some videos of some of these mods.

There it is, this is the end of this post! I know it is dense, so take your time, and do not hesitate to ask questions by posting a comment! I this point, I am not totally sure about how each part of the circuit works, so do not hesitate to correct mistakes I could have made in the circuit analysis.

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To go further

2009 document produced in 2009 by Martin Chittum from freestompboxes.org

Aion electronics building guide, well helpful and with a lot of informations about the Klon Centaur.
Refractor project page by Aion Electronics.
"Klown" Centaur page of the Revolution Deux website.
"Sunking" project from madbean pedals, another Klon clone with a bigger PCB for 1590BB enclosures.
Modded "Klown" centaur of the Revolution Deux website.
Website with pictures of Klon centaur, classified by serials
Manticore fx : lots of informations about the Klon
History of the Klon Centaur on Premier Guitar, with a Bill Finnegan interview.
Klone science on madbean pedal: frequency response plots of different klon clones