LPB1 mini PCB !

The LPB1 (Line Power Boost 1) is a boost pedal that was commercialized in 1968 by Electro Harmonix. It is the first boost pedal using a silicon transistor. Indeed, previous boosts like the Dallas Rangemaster Treble Booster were using germanium transistors. Another novelty with this pedal was that it was boosting all frequencies and not only trebles, making it the first clean boost available!

 

The circuit is really the most simple one you can imagine for a boost:

First, there is a 0.1uF coupling capacitor that prevents parasitic DC current from the guitar to go in the circuit. With the R2 resistance, it forms a high pass filter : by changing its value, you can modulate the amount of bass going through the circuit. If you increase C1 value, more bass will go through, and vice versa.

Then, there are two resistors forming a voltage divider (R2 and R1), to provide a certain voltage to the base of the transistor. Here it is : R2/(R2+R1)x9V = 43/(43+430)x9=0,81V at the transistor's base.

The silicon transistor is a 2n5088 (originally a 2n5133 - same transistors that were used in the Big Muff later), wired as a common emitter. R4 and R3 will define the amount of amplification. If you increase R4, amplification will be larger. If you increase R3, there will be less gain.
A second 0.1uF coupling capacitor that prevents DC current from the battery to go out of the circuit. Finally, a 100k potentiometer wired as a variable resistor defines the final volume.

If you look carefuly, you can see that the last stage of the Big Muff circuit is exactly the same! A LPB1 circuit is used to increase the final volume.

Indeed, this circuit can be used in a lot of different effects to boost the entry or final level. A fun thing to do is to add a LPB1 boost before a saturation effect to increase the gain. It is very fun on a Big Muff (like in the Musket Fuzz or Supercolider fuzz), or on a Tube screamer (like in the Fulltone Fulldrive)!

In order to be able to add this little circuit when I want, or to test it on several pedals, I decided to make a small PCB that allows you to add a "boost" knob on every pedal you want. There is a "in" and a "out" pad that allows you to place it anywhere in a circuit:

I will test it on a Big Muff or a Jan Ray! I could also try to see if it can fit a Hammond LB enclosure (super small squared enclosures like the one I used for my Strymon Favorite switch) Note that there is no voltage stabilization or polarity protection system on this board (too small, not enough space for it!), so do not hesitate to wire a 100uF capacitor between the two lugs of your power supply input.

I should receive all the parts to test it soon...

Klon Centaur Clone and mods (Aion Refractor)

The Klon Centaur is one of the DIYer's favorite pedal. Indeed, the original version of this mythical overdrive actually costs more than 1500 euros! This pedal is more like a legendary unicorn than a real pedal that you can test one day... Even the reissue, the KTR, which is not hand assembled costs more than 300 dollars... The amount of clones (aka "klones") of this pedal has grown insanely over the years: JHS, Rockett, ARC Effects, Electro Harmonix with the Soul Food, almost every pedal manufacturer has issued their clone, always closer to the original pedal.

Summary

• Klon centaur circuit analysis
• Klon centaur mods and tweaks
• Klon centaur history and versions

For those who do not know the Klon Centaur yet, it is an overdrive created in the 90s by Bill Finnegan and MIT engineers (yes!), that is probably the "hypest" pedals of all. It was used by many guitarists, including Jeff Beck for instance. It is a three potentiometers overdrive: gain, volume and trebles, famous for the light crunchy tones it provides to your sound. It has also an excellent reputation as a buffer or clean boost. In fact, I have already made Klon buffer PCBs to use it in a patch box.

Recently, different PCB makers made Klon Centaur PCBs available, so you can make your Klon Centaur yourself. I used a PCB from Aion Electronics, the Refractor overdrive. This PCB is amazing, a fantastic work has been done by Aion Electronics to make the circuit fit a 1590B enclosure ! If you want something a bit bigger (for instance if you want to make a Klon-like look), you can use the Madbean Sunking PCB. I also wanted to see if there were audible differences between my EHX Soul Food and a replica of an original Klon Centaur (without the goop ^^).

Here is the result!

I used a prepainted enclosure from Banzai Music, in a classic gold color to make it look like the first golden Klon centaur pedals. I used a stamp to "print" a centaur on the enclosure with China ink. Then, I varnish the enclosure. I used a french website, called tamporelle, to make a custom inkstamp. It was very quick, and the stamp is of really good quality. Plus, it is quite cheap. I really like the result, it is not really easy first, but after a few tries it looked good!


I also used my laser engraved plate on the front side of it.

To compact the Klon circuit in a 1590B enclosure, a fantastic work has been done by Aion Electronics. The PCB is very compact, yet quite easy to populate because it uses classic components, and you do not have to place resistors vertically. The PCB is of excellente quality, double sided of course. The guide made by Aion Electronics to help you to build it is really detailed and well done. If you want to make a small Klon clone, I really advise you to use this PCB.

However, it is not an easy build. The compaction of components is quite high, and you have to be quite precise when drilling the enclosure. Fortunately, the build document is really complete, and you have a drilling template included. Mine worked directly on the first attempt (This is a rare thing when building pedals...) !

I decided to build a replica of the original Klon Centaur circuit. I used tantalum, electrolytic and film capacitors like it was on the first Klon Centaur pedals. For that, I used the document realized by Martin Chittum from freestompboxes.org in 2009. Indeed, the original pedal is gooped. Goop is a kind of black resin that embed the circuit and components, and prevent you to trace the circuit. The freestompbox community decided to buy a Klon Centaur, and sent it to Martin Chittum who managed to "ungoop" the circuit and trace the schematic of the Klon Centaur, and report precisely the components used.

The only point that remained elusive after this work was the kind of diodes used. They were germanium diodes of unknown nature. Some tests determine that the diodes had a 0,35V voltage drop, which matches russian D9E diodes, that I used for my Soul Food mod. I used these diodes in this build:

However, Bill Finnegan himself recently anounced that he used 1N34A germanium diodes in the original Klon Centaur:

"The diode I have always used is a germanium diode with the part number 1N34A, but you should understand that this particular part has since the 1950s or so been manufactured by literally hundreds of different companies, and having listened to as many different ones as I have, I can say with confidence that they all sound somewhat different in my circuit, and often they sound VERY different." -Bill Finnegan, The Gear Page forum post-

These are really basic germanium diodes, quite surprising for these mythical diodes! The D9E are old soviet diodes, so it is true that managing to grab some of them in 1991 (when Bill started to develop the circuit), two years only after Berlin wall's fall, must have been difficult! Moreover, the marking is different between the soviet diodes (blue mark on the anode) and the diodes that you can find in the Klon Centaur (black mark on the cathode). For me, it is probable that diodes used were 1N34A, but maybe as he is saying these diodes were from a batch that sounded a bit different from more recent 1N34A... D9E sound good to me and have the right measured voltage drop anyway, so I'll keep using them.

How does it sound?

As you may know if you know me or follow my blog, I am really sceptical about the Klon "mythical" reputation. My Soul Food seemed like a good reproduction of the sounds delivered by the Klon, and I did not really get the buzz around this pedal. It is a nice clean boost, and good for really light saturations, but was a little too trebly for my ears. With more gain, the pedal becomes really "transistor" sounding, and quite boring... Finally it became more my boost pedal than a true overdrive pedal.

So I started to test the pedal without great expectations. I have to admit that I was surprized! If the pedal really sounds like the Soul Food in low gain settings (really nice for light crunchy riffs), the high gain settings sound really nice with the Klone!

It is very dynamic and powerful, yet quite transparent, with some added mediums and trebles that makes you want to play big crunchy solos! I understand better the setting that Jeff Beck uses on his Klon Centaur:

(gain pot is on the left)

I am really surprised... I will do a blind test to be sure that my brain is not playing with me, but it seems like there are differences with high gain settings.

The pedal was compared to a Klon KTR, and sounds are really close, no audible difference between them. Proof that everything is about circuit and components, and not mojo and magic!

 

You understood, it is really close to the KTR. If you want to have an idea of how it sounds, listen to demo video of the KTR. I will try to record samples as soon as I get gear for recording audio samples.

I am currently making a small serie of 5 pedals to buy some gear to record proper video and audio samples... Send me an email if you are interested.

Circuit analysis

The Klon is a rather complex circuit. It is quite weird because there are lots of informations and tests about this pedal online, however there are few informations about how the circuit works precisely. I have to remind you that the circuit was mainly conceived by electronics engineers from the MIT (and not by Bill Finnegan, who tweaked it more than conceive it), and thus, the circuit is quite "non-standard" compared to classic overdrive circuits (Tube Screamer, Rat Distorsion...etc). So brace yourself, winter is co... lets analyze this!

If you are not very familiar with the different components, and the theory around guitar pedals, I suggest that you read a few articles about theory and other circuit analysis before reading this one!

Here is the famous circuit:

You can already see that, contrary to many overpriced "bouteeek" pedals, the circuit is not a tubescreamer, and is entirely original!

So we can already see 4 operational amplifiers (OP amps), the famous germanium diodes between the second and third OP amp. If we try to see where the signal goes, we can see that there are several pathes that the signal follows, we can see that the signal is divided between clean and saturated, and then mixed again. When the pedal is "off", the signal still goes through the top part of the circuit.

I traced the path followed by the signal:

We can see that the bypass signal (blue) is splitted in two: clean and saturated signal (green and pink, respectively). The amount of each splitted signal is dosed by a double potentiometer, the gain potentiometer. So basically, drive and clean signal are mixed, which allows really light and low saturation mimicking an amplifier just at the break up. If the Klon is so good with light drives, it is surely thanks to this mechanism that allow to have very slight saturations mixed with your clean signal.

You can also see that when the pedal is off, your signal can still go through an OP amp (it is the blue loop at the top of the schematic)... It is the famous buffer!

If we divide the circuit in different sections like we are used to, it can be this:

Finally, in the bottom right corner, you can see a power supply section using a MAX1044, that allows to have different tensions in the circuit. If you remember, the MAX1044 integrated circuits can be use to double a tension, or to invert it. Here, both of these functions are used!
Lets see this part of the circuit first!

1. Klon Centaur power supply

As you can see, there are no more than 4 different tensions delivered by the Klon Centaur power supply! We have V+ (9V), VB+ (4,5V), V- (-9V) and V2+ (18V)! Simple circuits are for the newbies lel!

V+ (9V) is stabilized using a 47uF capacitor, and a diode (D4) prevents polarity inversion. It is the classical power supply scheme.

V+ is then divided by two using a voltage divider with R29 and R30, to have VB+ (4.5V). If you do not know what a voltage divider is, read my post about resistors and their role in effect pedals. This tension is also stabilized with a 47uF capacitor (C18).

Finally, the MAX1044 is used to have an inverted tension (V-, -9V) and doubled (+V2, +18V). The MAX1044 is an integrated circuit sold by Maxim, that allows to transform tensions. Here, it is used as indicated on the datasheet (as simple as that!). For more infos about the MAX1044, read my article avec voltage doublers.

2. Klon buffer analysis

The blue loop on the top of the circuit that goes towards the output of the circuit is only active when the pedal is "off". It is the famous buffer! A buffer has a high input impedance, and a low output impedance, so that high frequencies are maintained all along your signal chain. Here is the scheme of the klon buffer:

(remembre, I designed a PCB to make it)

The first part, until C2, is simply the circuit input. There is a resistor at the input of the circuit (R1), and a pulldown resistor to avoir "popping" noises when the circuit is turned on. For more infos about pulldown resistor, read my article about resistors in guitar pedals.

The C1 capacitor is a coupling capacitor: it prevents parasitic DC currents from the pickups to go in the circuit. With R2 resistor, it also forms a high pass filter. If the value of the capacitor is bigger, you will allow more bass to go though the circuit. Here, with a value of 0.1uF, most of the bass of the guitar will go through the buffer, so the buffer will be transparent!

Then, the signal enters the OP amp, first one of the TL072CP. The TL072 is a double OP amp, very transparent. It is wired in non inverter (signal enters the OP amp through the + input). Usually, they are some resistors to define the gain of the amplifier, here, there are none. Thus, the gain is around 1, which allows you to have the same volume at the input, and the output. The OP amp has a high impedance input, and a low impedance output: the buffer diminishes the signal impendance, and maintains the volume!

Finally, there is a 4.7uF coupling cap, combined with a 100k resistor connected to the ground. This is a high pass filter (again), that will let almost all bass frequencies go through it (again). A last 560R resistor will adjust the final volume, and the signal gets out of the buffer.

3. Signal splitter

When the circuit is "on", the signal does not goes through the buffer, that is disconnected from the circuit by the footswitch, but through a "splitter". This part of the circuit splits the signal in two: clean and saturated signal.

At the input of the splitter, there is the beginning of the buffer circuit. Instead of going to the top partof the circuit, the signal can now take several pathes: down through the 5.1k resistor and the C4 capacitor, go through the second OP amp, or can go through the R17 resistor... What is this mess?

These different splitters selects some specifics frequencies. Unfortunately, I am not good enough with theory to tell you which ones... It involves rather complicated calculations with Fourier transforms and all... If any talented personn could help me with that, it would be awesome!

Lets focus on the bottom part. The signal goes through different components (R5, C4, R6, C6 and R9), then goes through the GAIN2 potentiometer, that set the maximum . You can see that the signal does not goes through any diode clipping system, so this part of the signal stays clean. In the end of the loop, it is mixed with the saturated signal.

The clean signal can also go through the top part of circuit, with the 1,5 and 15k resistor. It adds a bit of clean signal in the end that is mixed with the GAIN2 clean signal and the saturated signal, in order to always have a bit of clean mixed with the saturated signal, even when GAIN2 is maxed.

Finally, the remaining signal goes to the input of the second OP amp. A GAIN1 potentiometer set the amount of signal going into the amplification loop of the OP amp. GAIN1 and GAIN2 are in fact one double potentiometer: when the value of one increase, the other one increase! Thus, it doses the amount of clean and saturated signal to mix together: the more saturated signal, the less clean signal! It is an unique feature that is not present in any other pedal, and this is one of the novelty brung by the Klon Centaur: a part of the signal is kept as is, and the other part is saturated. Thus, the overdrive is really good for low drive.

The saturation comes mainly from the OP amp saturation. Another thing that is not usual with classic overdrives circuits! It has quit a high gain. We can calculate it. It is wired in non-inverter, so the gain will be 1 + loop resistor / resistor to ground = 1 + 422 / (15 + 2  + value of the 100k potentiometer  = 1 + 422 / (117) = 4,6 with the lowest gain, and 1 + 422 /  (17) = 25,8 when gain is maxed.

Then, there is a coupling capacitor (C9), and signal arrives towards the famous germanium diodes D2/D3. Most of the times, with moderate gain, these diodes are useless! I could test that with my soul food mod. They only are important when the gain is set up pretty high.

Finally, at the end of the splitter, all the signals are mixed again together.

4. Output signal mixer

Here we are, this is the last part of the circuit, the "signal mixer"!

The two signals that were mixed at the end of the splitter are going to be amplified and filtered a bit.
A first OP amp wired in inverter amplifies the signal. This OP amp has quite a high gain, that insures a high volume gain, useful if you want to use the Klon as a boost. This is where the incredible amount of volume of the Centaur comes from.

A second OP amp with a gain of 2 (look at the values of R22 and R24) will allow to set the final amount of trebles in the output signal. It is a classical high pass filter: you let the signal go through until a certain frequency, defined by the C14 capacitor. Decreasing the value of this capacitor will let more treble go through. A common mod is to increase the value of this cap to have less treble (we will see it later). The amount of trebles that can go through this cap is set by the treble potentiometer.

Finally, a 4.7uF coupling capacitor prevents parasit DC current from going into the last section. A 560R resistor the volume potentiometer will define the final volume of the output! There are also two resistors, R27 and R28, but I have to admit that I have no clue about their role... if anyone knows?

There are still things that I do not really get with this circuit... I guess it is a little bit overcomplicated. I would love to understand the splitter in details, but I am still lacking some precious filters knowledge...

I hope it still gave you an idea about how the circuit works. If you have any question, do not hesitate to post a comment. Lets have fun now that we know the circuit: what can we mod?

Klon centaur mods and tweaks

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.

Klon centaur versions

Aestheticaly, the enclosure design changed a bit around the production time. Lets do a bit of "klonology" (chronology, get it ?! OK, I'm out...)

Gold and silver klons were produced at the same time, however the silver Klon was introduced in the early 2000s, and the graphics changed a bit with time. Three graphics can be considered: no centaur, big centaur with "open tail", and small centaur with closed tail. There were 5 different colors: 4 different gold colors (that you can see on the picture above), and the bare polished aluminium color (aka "silver" centaur). Around 8000 Klon centaurs were made between 1994 and 2009 according to Bill Finnegan (1.5 Klon centaur a day!). All those Klon worth today more than 12.000.000 euros!

First, there are no differences between gold and silver centaurs. The circuit and component values are exactly the same. Hearable differences should be really low and due to component's tolerances (Bill Finnegan used carbon film resistors with a 5% tolerance,  capacitors with 20% tolerance, so you can expect some slight variations from units to units)

Concerning the electronics, contrary to many other guitar pedal lines (Big Muff...), no big changes happened over time. The first version produced in 1994 lacked the resistor at the beginning of the circuit (R1), had no ground plane, and missed the R11 resistor. All these changes were processed in 1995. The 15k R11 resistor was added to have a bit more low-mids response. However, if you try to remove, the changes are incredibely subtle...

“The fact is, under the hood they’re all basically the same. In 1995 I made three small changes: I added a resistor to give the circuit some protection against a static charge delivered to its input—a change that has no sonic effect. I also had the circuit board redesigned with a ground plane for better grounding—again, no sonic effect except the potential for a little less hum. And I added a resistor to give the circuit a very small amount of additional low-mid response—I wanted it to have a little more roundness when used with, say, a Strat into a Super Reverb. I made no other changes.”   - Bill Finnegan, Premier Guitar interview -

Another change noticed by Manticore FX is that another resistor was added at some point at the end of the circuit. It is R28, a 100k resistor that is present just before the switch. I do not really know its role, if you have any ideas...

The KTR version was issued in 2012. It basically has the exact same circuit as the Klon centaur, and the same diodes for clipping, but it was intented for mass production. It uses surface mounted devices (SMD), so the production could be automatized (the KTR is not handmade, but made by robots), so Bill Finnegan could focus on control quality. The price tag is still high though, especially for a mass produced device. The four years during which the Klon was discontinued has been the Klones golden age, and a lot of klon clones were issued during these years. Even today, as the KTR is quite expensive and big, there is still a lot of room for klones, and some builders find their way there (Rockett for instance), making Bill Finnegan a bit angry. Indeed, it is a bit smaller than previous versions, but uses 1590BB enclosure. Bill spent a lot of time testing different SMD components to make it sound exactly like the original Centaurs units. He also avoided to use electrolytic capacitors, and kept the tolerances of the components low, in order to have similar sounding units.

Bill Finnegan was aked whether he would try to make another pedal, but obviously he has some issue with kloners all around (especially with Rockett):

"If any new product I come out with will be ripped off immediately after its release, and if unscrupulous people will again be making money off of my work, and if on top of that Klon’s reputation and my own personal reputation will be at risk every time someone decides to put out his own version of one of my designs, then where is my incentive to release anything new at all? Over the past few years, I’ve talked with a number of other pedal designers about this stuff—good people who design their own circuits, and whose circuits have also been ripped off—and we all agree there is now an enormous disincentive for any of us to create and release new products." - Bill Finnegan, Premier Guitar interview -

Maybe next Klon product will be a numerical SMD pedal (not klonable!). Obviously, the Klon case introduced the ethical problem about cloning pedal in the DIY and guitar pedal afficionados community. As there is almost no legal protection to clone circuits, the only barrier is ethics. I think it is an interesting debate to have. I am currently writing an article about that, including some pedal patents and reflexions about cloning.

Fun facts about the Klon

• The name "Klon" is a shortener for "Klondike", a region of the USA famous for its gold rush during the 19th century (among other facts).
• To order a Klon Centaur when it was produced, you had to call Mr Finnegan to discuss the pedal and it could fits your needs, and give him a professional adress (it could not be shipped to your home!)
• Hitler is not satisfied with his Klon Centaur (Bill Finnegan actually found it hilarious)
• There is a 20-pages thread on the Gear Page to discuss whether the KTR design was real or not when it was released
• Some people are ready to spend more than 2000 dollars for the original version of the Klon Centaur!

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.

Did you like this article ? Thank me by liking the Coda Effects facebook page!

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

Boss Hyper Fuzz (Boss FZ2) : wall of fuzz from 1993

1993 is the year when Jurassic Park came out. I think it is no coincidence that another monster appeared this year: the Boss Hyper Fuzz! This pedal, produce from April 1993 to June 1997, is a Univox Super Fuzz clone, with some added features. Mine is from August 1993!

The Univox Super Fuzz is a pedal produce in the late 60s, famous for its apocalyptical sound! It was used by Pete Townshend during live performances of the Who, and then used by many bands with a heavy sound like Kyuss, Fu Manchu or other stoner rock bands. In particular, the Hyper Fuzz is used by Electric Wizard, a band famous for its subtle and thin sound (or not!)... In short, as you understood, this fuzz is a war machine that will crush your ears once activated! It is so evil that legend says: each time you step on it, someone dies...

The FZ-2 is really different from a FZ-3, which is a fuzz with more classic sounds. The Boss FZ3 is close to a silicon fuzz face, whereas the Boss fz2 is really peculiar, really close to a Super Fuzz.

The pedal has 5 controls (the second potentiometer is a double one, and allows you to set bass and trebles). Level sets the volume, second potentiometer sets bass and treble amounts and the gain potentiometer sets the fuzz amount. The last knob lets you choose between three modes: Fuzz I, Fuzz II and Boost that a simple clean boost.

Once activated, the pedal delivers all the loudness and heaviness of the Super Fuzz. There is an octave down that is added to your guitar tone, which provides you a huge bassy sound, and a slight octave up, more hearable when playing near the 12th fret. The octave up sounds a bit like an octavia, but with less octave. The saturation is extreme, especially in "Fuzz II" mode that is a scooped mids setting.  I really like it, it is a riff machine that makes you wants to tune down your guitar. It is easy to play powerful and heavy riffs. It is also really interesting on bass guitars.

Here is a nice example on a bass:

The boost mode is a clean boost with quite a lot of gain (+25 Db!), and you can set the bass / treble amount with the second potentiometer. Nice to boost an overdrive pedal, or to make your amp crunch, but nothing really new.

After opening the pedal, we can see a monolayer PCB, with shapes that look a bit like the Boss DD2:

Nice conception job! Boss engineers are monolayer PCB experts!
Here is the most interesting side:

Vintage style! Old film capacitors, lots of 10 uF electrolytic capacitors, lots of transistors, 2 diodes (D4 / D5) to create saturation, this is definitely a Super Fuzz! In the middle, the two black bars are in fact ICs, operational amplifiers, old school version! They are used to set bass and treble content of the circuit. We can also notice 1/8W resistor to make the circuit fit in a Boss enclosure, and the high number of components, it goes minimum to R62 and C35!

If we take a look at the transistors:

We can see that they are 2SC3378, old transistors that are not produced anymore, which have a really small amplification factor (hfe) of 50. For instance, MPSA18 that are modern transistors have a hfe of nearly 1500!! Indeed, the Super Fuzz has a lot of transistors, and if the gain is too important, the sound will be "mushy" with no sustain. Whereas with low gain transistors, the sound is smooth, heavy and saturated. When the circuit was conceived in the 60s, there were no such things such as high gain silicon transistors, this is why low gain transistors are required to make it work correctly.

Lets take a quick look at the circuit schematic:

First, we can recognize the famous buffer from Boss ("flip flop" circuit in the bottom left corner) that allows the signal to be transformed into low impedance signal. The Super Fuzz circuit is clearly recognizable (from the "gain" inscription to the right side of the image), with the two transistors Q13 and Q14 that are facing each other with a common emitter: they will create the octave sound. Then, there is the bass and treble control circuit with 4 OP amps, and the switch that allows you to choose between Fuzz I, Fuzz II and the boost.

Other added circuits (before and after the Super Fuzz circuit) reduce the signal/noise ratio (evenif it has a lot of gain, this fuzz is not that noisy), and increase the final signal. Indeed, in vintage Super Fuzz, the output signal was really low, and generally lower than the bypassed signal...

It is not a simple circuit, maybe it is even overcomplicated! However, it is true that the vintage Super Fuzz already have a lot of components...

Anyway, I am really happy to have it, it is quite complicated to grab one now, and it is a nearly indispensable tool if you want to play stoner / doom music...


To go further:
Boss Fz-2 manual (pdf)
Boss Area Hyper Fuzz page

All you need to know about resistors in guitar pedals

In this article (that will be part of a series of articles about the different components you can find in a guitar pedal), we will try to decipher the role of a component you surely already know: the resistor! The resistor is a simple 2-legged component. It is the basic brick of every electronic circuit, like the simplest cubic Lego brick. You will find resistors everywhere, it has many, many, many possible uses. Here, we will be interested in its role in guitar pedals.

What does it looks like?

The common resistor looks like a brown / blue cylinder, with rings of different colours and 2 legs.

 

Everything is about color with resistors (being color blind is quite painful with it... If you are, there is an app to help you). The overall color of the resistor indicates its composition: carbon comp are dark-brown, carbon film are beige and metal film resistor are blue / cyan like the picture above. Which ones are the best for guitar pedals? Check my post: how to choose resistors for guitar pedals.

The coloured rings allow you to know what is the value of the resistor, written in Ohms. Each colour correponds to a number:

 

 

Here for instance, we can read on the 3 first rings: "1", "5" and "0" = 150

The 4th number is 100, so we multiply the first value by 100 : 150 x 100 = 15000 = 15k

The last ring indicates the tolerance of the component. Tolerance is the possible maximal difference between the theoritical value indicated on the component (15k for instance), and its real value (14,99k for example). Practicaly speaking, metal film resistors that are used most of the times in guitar pedals have a 1% tolerance value (that is precise enough!), but some "precision" resistor can have tolerances down to 0.01%! They are often use in measurement devices like multimeters. In our case, 1% is really a sufficient value, there will be no audible difference if you swap a 100 ohms resistor by a 99 ohms one...

Knowing this code is not essential, you learn it gradually by practicing! If you forget, Google is here for you anyway ;)

Inside the resistor, below this coloured capsule, there is a resistive film arranged as a spiral, composed of metal or carbon (carbon film or metal film!). The longer this film is, the higher the value of the resistor will be.

Here for instance, the upper resistor is 27 ohms, the middle one 330 ohms and the bottom one 3.3 Mega Ohm (3 300 000 Ohms !)

The resistor has 2 symbols:

On the left is the european symbol (R5), and on the right is the american symbol (R6). It is not the only component with two different symbols. Capacitors also have different symbols between Europe and America. For resistors, I actually prefer the american symbol that evoques maybe more the role played by the resistor and the resistive strip. It is also more common to find it on guitar effects schematics.

If you want to buy cheap (but nice quality) resistors, I have this great deal.

What is its role?

A resistor, as its name suggests, resits against the flow of current. It will "absorb" a bit a current and transform it into heat. Thus, current will diminish when going through a resistor. This will diminish the voltage of the signal. The tension of the resistor is characterized this by the relationship:

U=R x I
with U = tension of the resistor (Volts), R = resistance (ohms)
and I= current intensity (amperes)

So basically, a part of the guitar signal will be absorbed to generate this tension. Thus, we can modulate the signal amplitude by modifying the resistor's value. This is really useful in a lot cases. Indeed, yout guitar signal is an alternative tension! If you want to learn more about it, read my article about electric guitar signal.

When the signal goes through a resistor, its amplitude decreases. A lower amplitude signal is simply a lower volume signal!

With a resistor, it is thus possible to reduce guitar volume! However, a resistor can only diminish the voltage amplitude. To amplify a signal, you need semiconductor devices like transistors or an OP amp.
The resistor has a lot of other applications of course! Lets see of few of them together.

How are resistors used in guitar pedals?

A resistor can have many, many applications... Here are some resistors standard applications that you can find in guitar pedals.

1. Adjusting the input or output level of an effect

Here is an example from the Big Muff circuit. An input resistor adjusts the amplitude of the input signal that will be amplified by a transistor. By changing this resistor, you can change the amount of signal entering the circuit, and thus you can change the overall gain of the pedal. Inversely, if its value is increased, the signal will be less amplified and you will have a smoother, less gainy Big Muff!

You can do the same thing at the end of the circuit to set the output volume! Usually, we use a potentiometer wired as a variable resistor, so you can adjust the final volume.

2. Pulldown resistors

When a circuit is off, some voltage can stay at the beginning the disconnected input of the circuit or in capacitors. When the pedal is turned on, the voltage goes through the circuit and causes these annoying "popping noises". To avoid that, we can add a pulldown resistor at the entry or / and output of the circuit: 

Generally, a resistor with a high value (like 1M) is connected to the ground on the signal path. Thus, the excess of current that goes through the circuit when the pedal is turned on is absorbed by this resistor, and you avoid these loud noises.

3. Adjusting the gain of a transistor or an Operational Amplifier

Generally, transistors or OP-amps are used in a really simple schematic that allows you to amplify the signal: common emitter / collector for the transistor, inverter or non-inverter for the OP-amp. Each of these simple schematics usually use resistors. These resistors will define the gain of the circuit.

For an OP-amp, the gain value is easy to calculate: for an inverter circuit, it will be R2/R1 and for a non-inverter circuit, it will be 1 + R4/R3. Thus, you can adjust the gain in a very precise manner! It is used in the Rub-A-Dub reverb to adjust the entry and output level of the effect.

For a transistor wired in common emitter like here, you can adjust the value of the resistor connected to the ground (bigger value : less gain), or to the +9V bus (bigger value: more gain). You can already use this tip in the Big Muff circuit!

4. Voltage divider

This really simple scheme allows you to adjust the voltage that you provide to some parts of the circuit.

The delivered tension V1 is simply : 9V x R2 / (R1 + R2).
If you put the same value to R1 and R2, you simply divide your input tension (here, 9V) by two.
This can be really useful for some components. For instance, a lot of operational amplifiers (AOP) need a 4.5V tension to work properly. It can also be really useful to bias the base of a transistor. You can see such a scheme in the volume / output section of the Big Muff circuit.

What about potentiometers?

A potentiometer is simply a variable resistor! Any resistor of a circuit can be replaced by a potentiometer, so that the user can modify manually a value. We will see how it works in detail in another article.

Just an example: if you put a potentiometer in the loop of an operational amplifier, you can make the gain vary just by turning the potentiometer! Ideal for a volume boost, or to set the maximal amount of gain with clipping diodes. You can find such thing in the Jan Ray / Timmy circuit.

You can also put a potentiometer wired as a variable resistor to the ground at the end of the circuit, just before the output, so you can set the final volume! It is the simplest possible volume control, which you can find in many circuits, including the Fuzz Face.

An example

Lets see an example together. Here is a simple circuit, the Linear Power Booster 1 (LPB1), from Electro Harmonix. It is the first boost using a silicon transistor.

As you can see, there are five resistors (6 if we include the boost potentiometer) in this circuit. With all the things we saw before, you should be able to determine what is each resistor's role in this circuit.

The smarter guys (girls?) among us should even know which resistors to change to have more or less gain with this boost!

Some help with reading the schematic: first, try to see what path the signal is going through. For that, link the input to the output of the circuit, and you can see where the signal is going. Here, it goes through C1, Q2, C2, and then the "Boost" potentiometer. Now, you should be able to find the role of the resistors.
If you do not, do not worry! Here is some help: http://www.coda-effects.com/2015/09/lpb1-mini-pcb.html


Was this article useful? Thank me by liking the Coda Effect facebook page!
Any question? Post a comment!

Pour aller plus loin :
Nice guide from guitarPCB, about all guitar fx components.
Pulldown resistors: AMZ FX blog
Sparkfun post about resistors, easy to read and didactic. 
www.resistorguide.com: a whole website only about resistors! Great reading, highly recommended.

The different types of Fuzz

Fuzz is one of the most emblematic guitar effects. Satisfaction (The Rolling Stones), Purple Haze (Jimi Hendrix), Think For Yourself (The Beatles)... The number of great songs incorporating this effect are countless, and have progressively transformed the Fuzz effect into a reference in the electric guitar world!

Today, we can find a lot of different fuzz, with multiple variations around the same circuit... In order not to be lost in this mess, I wrote a small overview summarizing the different types of fuzz that you can find. Of course, it is not an exhaustive listing, I tried to recapitulate the main fuzz, and modern and "boutique" variations that can be found today... Make your choice!

1. Gibson Maestro FZ-1 (1962)

This is the first fuzz ever invented! Originally, it was intended to imitate brass! To give you an idea of how it sound, it the fuzz used on "Satisfaction" of the Rolling Stones.

It has a nasty, sax-like sound, quite aggressive in the treble range. It is a harsh, dirty sound that can feel almost like a ring modulator in last strings! It is a very peculiar sound, very "vintage-sounding" (well, of course, it is from 62!). To be honest, I do not really like it, but I think it can be used a bit in some styles like garage rock or psychedelic rock.

Of course, the original pedal is extremely rare and expensive... (I had one to repair recently!) The Boss FZ5 emulate with a numeric algorithm the Maestro fuzz, and clones are pretty rare (you can find clones in small boutique companies), because of the weird sound that makes it a bit special... Gibson did a reissue at some point, but it was also quite expensive, and had the downsides of the original pedal (size, no power supply input...etc).

2. Sola Sound Tonebender Mk1, Mk1.5 and Mk2 (1965 et 66)

Created in England in 1966 to compete the Gibson Maestro (which was too expensive to import...), this fuzz quickly became a reference. The MK2 version was used by many english rock musicians in the 60s, like Jeff Beck or Jimmy Page for instance. It has been also been copied by many brands: Marshall Supafuzz, Vox Tonebender are examples of fuzz "heavily inspired" by the Tonebender.
 

It has a more classic fuzz sound, loud and powerful, with quite a lot of saturation. The saturation can be modified with the volume knob of the guitar, like with Fuzz Faces. The sound is warmer, softer than the aggressive Maestro sound, with quite a lot of mids. Basically, it sounds a bit like a Fuzz Face, but with more saturation and a bit more aggressive, close sometimes to a distortion.

It has been clone, modified and reproduced by many builders: D*A*M (who makes incredible replicas), Earthquaker Devices Tone Reaper, Fulltone Soul Bender, Basic Audio Scarab Deluxe, JHS Bun Runner, Ramble FX Twin Bender...etc.

3. Fuzz Face (1966)

This THE Fuzz, the most emblematic fuzz effects with its famous round and red face (well, gray at the beginning). Used by Jimi Hendrix, it became rapidly famous! Conceived in 1966 by Dallas Arbiter in England, it used germanium transistors at first, which were later replaced with silicon transistors, more stable. Read my article about the fuzz face circuit here.

The sound of this fuzz is characteristic: very soft, warm and round, it has a very "creamy overdrive" feeling that evokes almost always (especially if you have a Stratocaster-type guitar) the sound of Hendrix. It also responds very well to the guitar volume knob: by turning down this knob, you can go back to an almost clean sound! Versions with germanium transistors are ideal for this, but are less stable (sensitive to temperature, demanding about the transistors characteristics). It is less saturated than a Tonebender. This is really a classic fuzz, easy to use and very good sounding

The Fuzz Face is still produced by Dunlop (with a lot of versions: mini versions, germanium and silicon versions...etc.), but many builders took the opportunity to build their own Fuzz Face version and make original versions: Analogman Sunface (a great replica of the vintage Fuzz Faces), that I cloned here and here, Fulltone 69, JHS Pollinator, Boss FZ3, Chase Tone Red Stardust... are all modified Fuzz Faces.

4. Roger Mayer Octavia (1967)

With its strange spaceship shape, the Roger Mayer's Octavia will not leave you indifferent. Roger Mayer was Jimi Hendrix's guitar tech (...yep!), and has created and modify pedals for the master... The Octavia is one of them.

One of the things that make this pedal unique (beyond its weird look), is the fact that it produces an upper octave sound above what you play! This octave effect is more pronounced around the 12th fret, so it is more hear-able during solos. It has quite an aggressive sound, which can be slightly dissonant (because the octave is never perfet in terms of pitch). Here again, some Hendrix's song are representative of the sound of this pedal, as the solo Purple Haze or Fire.

Roger Mayer is still producing the pedal, but it also has been cloned and modified: Fulltone Octafuzz, Catalinbread Octapussy (I love this name), Electro Harmonix Octavix...etc. Which seems to please Roger Mayer a lot, as he put on his website this beautiful quote: "Those who can invent do, those who can't invent copy" (Angry Roger is angry, but well, it is understandable)

5. Univox Super Fuzz (1968)

Well, this is a personal favourite, for me this fuzz is the evillest fuzz of all, a monster in a pedal enclosure, a true Pandora box! This fuzz, invented in 1968 by the Japanese company Shin Ei (which became Univox later), has not only a killer look (well, from the second version in 1970), but is also the most violent an loudest fuzz ever! It has been used by Pete Townshend during the Who's live performances, and has been re-discovered in the 90s by many stoner rock bands, some of which used it almost on every song like Fu Manchu!

Like I said, this Fuzz has an over saturated heavy sound. There is a lower octave that is added to your tone and make it heavier, and an upper octave, quieter than on the Octavia, that is hear-able around the 12th fret. To give you an idea of the apocalyptical sound of this pedal, listen to any song of Fu Manchu. (check Cyclone Launch for instance)

Unfortunately, the production of this wonderful pedal was stopped. Boss produced a clone in 1993, the Boss Hyper Fuzz FZ2, which had some success in the stoner rock / doom circle, but is not produced any more. Today, Behringer produces a cheap clone, and some boutique builders make clones (Wattson Super Fuzz (replica of the 68 version), Solid Gold FX Formula 76). Producing a fuzz respecting the spirit of the original Super Fuzz (sound and look) is one of my current projects. If you're interested, please email me for details.

6. Electro Harmonix Big Muff Pi (1969)

This is an absolute classic created in 1969 by Electro Harmonix: the Big Muff Pi. This pedal is still produced, an had many different versions (Deluxe, Russian, Triangle, Ram's Head...). It has been used by a lot of different artists because of the sound it has, and especially the almost infinite sustain it gives to your tone. David Gilmour (Pink Floyd) is a well known afficionado of the Big Muff.

Comfortably Numb's solo is a good example of what kind of sound can be expected from a Big Muff. It is a heavy, saturated sound, but still compressed and smooth, that gives a nearly infinite sustain and crazy harmonics. The tone potentiometer allows to go from a trebly aggressive fuzz to something warmer and bassy. One of the characteristics (and weaknesses) of this fuzz is the lack of mediums that the EQ create. You can disappear in a mix because of that. To avoid this, David Gilmour was using it with an overdrive that was bringing more mids, and today, there are versions of the Big Muff with a different tonestack that have been created to avoid this. If you are interested, I wrote an article about the Big Muff circuit.

This pedal is the clone's queen, probably the most cloned pedal ever! There are hundreds of versions of the Big Muff, as it is a very well documented circuit, and very tolerant to modifications. Pete Cornish G2, P1 and P2, Skreddy Mayo, Way Huge Swollen Pickle, Mojo Hand FX Colossus and Iron Bell, Blackout Effectors Musket, Black Arts Tonework Pharaoh...Etc Each "boutique" builder has its own version of the Big Muff, and I already made a few Ram's Head Big Muff myself!

7. Shin Ei companion Fuzz (1970)

Lets finish this list with an original rarity, the Companion Fuzz, created in 1970 by Shin Ei (again). If I put this pedal in this overview, it is that despite its rarity, it really has a very special, unique sound!

It produces a "chainsaw", buzzy, nasty sound! It has a very harsh and raspy sound that is really recognizable. This pedal has been used by some psychedelic rock bands, and more recently by Dan Auerbach from the Black Keys for the dirty sound that it makes... It is really an unique sound, either you like it or you hate it!

Due to this weird sound, demand is quite low for this kind of effect, and re-editions or clones are quite rare. The original is of course not produced any more (and reaches crazy prices on ebay), but Earthquaker Devices recently issued a clone: the Terminal Fuzz. Some small boutique builder still make clones of it. 

8. 1970-2010: what happened?

Where did all the innovations of the fuzz pedal go during this period? The 70s marked the beginning of overdrives, that gradually replaced fuzz, both in music and guitarist's gear. Fuzz was replaced with amp and overdrive saturations. Indeed, since the beginning of the 70s, amplifiers (finally!) have a master volume, and fuzz are no longer necessary to have a distorted sound. New musical genres that emerged at this time, like hard rock, are more into heavy saturation from the amplifier than the "weird" saturation of fuzz.

However, in some genres played irreducible musicians, fuzz still holds out against overdrive and distortion invaders. Stoner rock guitarists rehabilitate fuzz effect to produce heavy saturated sounds (Kyuss, Queens of the Stone Age, Fu Manchu...). Some indie bands make fuzz become their sound trademark (Smashing Pumpkins, shoegaze players...). During the 2000s, with the arrival of bands inspired by the 60s (like the White Stripes and others garage rock bands), fuzz resurrects and becomes a classic effect again. Other artists use it in a new way to create new musical landscapes, and make it popular again (like Matthew Bellamy and his Fuzz Factory)

Traditional and boutique manufacturers modify and enhance vintage fuzz by making them easier to use (9V power supply input, smaller size), less noisy and more stable.
Some builders innovate and create entirely new circuits, most of the times completely crazy: Death by Audio (with the Fuzz War for instance), Devi Ever (Hyperion, Soda Meiser) or Zvex with the Fuzz Factory. Others invent new designs with more classical sounds: Wampler Velvet Fuzz or D*A*M Meathead for instance!

Personally, I am a huge fan of fuzz. The Super Fuzz, the Big Muff and the Fuzz Face are must-try pedals for me. Moreover, fuzz are simple pedals that are easy and fun to make, and that can easily be modified and customized! I really advise you to try to make one... The Big Muff is the perfect pedal to begin with (with a PCB if possible), even for a beginner, and can be modded really simply (the circuit is very tolerant to modifications).

Here it is ! I hope that this article gave you a good overview of the different fuzz that exists, so you can choose the one you like the most! Do not forget that this is not an exhaustive list, and that many other fuzz exist... The quest for the ultimate fuzz is nearly infinite (gotta catch them all!)

Do you have any question? Post a comment!
You like this article? Thank me by liking the Coda Effects Facebook Page.


To go further:
"Fuzz timeline" from the Big Muff page
Another Fuzz timeline by Hewitt's garage
Fuzz Face history : a nice video about Fuzz Face history.
Maestro Fuzztone history: great article from vintageguitar.com
History of Tonebenders by D*A*M

Another Jan Ray clone !

Here is a pedal I made, a Vemuram Jan Ray clone!

This time, contrary to my previous Jan Ray clone, I decided to make it in a smaller 125B enclosure. It is not particulary a big deal with this pedal, you just have to be precise, and use a guide. I generally use a plastic card or a piece of cardboard with 4 holes disposed as the holes for the potentiometers on the enclosure, and put the potentiometers inside. Then, I cut and solder an excessive length of wire to the veroboard. I use a wire that you can bend so it can maintain the veroboard. Then, I place the veroboard above the potentiometers, and start to cut the wires at the minimal length, and solder it to the pots.

I used a prepainted enclosure from Banzai music (Vintage Orange sparkle), and classic small MXR style 17mm knobs. I also added a switch similar to the one on the Timmy that allows you to choose between a compressed (2 diodes) and an uncompressed (4 diodes) setting.

 As usual, I use the laser engraved logo:

I had many problem with the enclosure this time, Banzai music apparently have trouble with suppliers, and after 2 months I did not have the enclosure... I finally contact them and change the color of the enclosure. They were really helpful.

If you want more informations about the Jan Ray circuit, read my article here.

JHS Superbolt clone / ROG Supreaux

This is a JHS Superbolt clone! JHS is not one of the most honest builders on the planet, and he is known for his ability to steal circuit designs from other builders. The Superbolt is no exception, and is in fact a really slightly modded Run Off Groove Supreaux. Runoffgroove is a community creating circuits for DIY people, and making it public through Creative Common license. There are many nice circuits there, take a look! JHS did not bother with the license and just copy it and started to sell it.

The circuit is inspired by an old class A amplifier, the Supro 16T. It is a small 5W amplifier from the late 60s. It became quite famous, because like many other class A amplifier, if you push the volume really loud, the amp begins to distort in a really nice way. It was used in studio by many guitarists, especially by Jimmy Page who recorded several Led Zeppelin songs with it.

I decided to make a clone, mainly because some videos on the web of the Superbolt were really good sounding. I used a prepainted grey enclosure from Banzai Music, and the "Supro logo" was laser cut by HPM Laser. 

The pedal has 3 knobs: gain, which acts like the volume of a class A amp: the higher it is, the higher the volume and the saturation. The volume knob acts as a master volume; the tone knob add or remove bass and treble (slightly). There is a two positions switch that allow you to switch between a gainy bassy position, and a slightly less gainy with more treble position.

I also used the laser printed logo I used before:

The build is quite simple. However, I went through many problems doing it. At first the pedal was not working at all, then only a huge buzz came out of this box... I checked everything several times and was quite despaired about it. So I let it aside on my bench for a few weeks... And finally decided to make it work! The audio probe was really a life-saver on this build.

It is a bit messy inside, but not that much for a veroboard with 3 pots layout. At first I was planning to have space for a battery, but the input jacks were too big for that... So 9V power supply only! Adjusting the 3 trimpots was quite easy. I started with 9V on the emitter of each transistor, but this was too gainy in my opinion, so I diminished it (except for the last transistor). The pedal is thus less gainy.

Circuit analysis

Here is the schematic of the ROG Supreaux:

And here is the JHS Superbolt schematic:

You can see that it is basically the same circuit, with an added voltage doubler (top part of the circuit), and a switch adding a 120k resistor connected to the ground before Q3.

Lets divide it in several sections as usual:

You can see that it looks a lot like an amplifier! It is really inspired by an amplifier schematic, except that tubes are replaced with JFET transistors. Which is really appropriate because JFET transistors (Junction Field Effect Transistors) use a technology that is really close to tubes, and they sound quite "tube-like" when saturating: warm, creamy overdrive compared to the aggressive gritty saturation of silicon transistors.
All the saturation of this circuit is based on the saturation of these transistors: you can see that there are no diodes in the circuit. It is different from what we are used to with the Big Muff or the Jan Ray (Timmy).

To sum up, it works like this: the first preamp section amplifies the signal, and will drive the second transistor. The saturated signal will be slightly modified in tone by the tone stack section, and a third transistor will set up the final volume. Like I said, it really works like an amplifier!

Let's see every part of the circuit in detail.

Power supply section

First, there is a protection against polarity inversion with a 1N4001 diode, and voltage is stabilized with two capacitors. More information about voltage stabilization and polarity protection here.
A MAX1044 IC is then used to double the voltage, in order to have an 18 volts bus that will be provided to the JFET transistors. Having 18 volts instead of 9 gives a bit more headroom and clarity to the circuit, and this had been used in the Supreaux Deux version of the circuit. It uses the example circuit that you can find in the datasheet:

It is a very classical layout that you will find in most pedals using a voltage doubler (Klon centaur for instance).


Preamp section 1: gain

First, there are two 1M pulldown resistors. Pulldown resistors prevent loud popping noises when you turn the effect on. There is a 22nF coupling cap that prevents any parasitic DC current from your guitar to go through the circuit.

The signal is amplified by the first transistor (Q1). You can see that the overall gain of the transistor can be adjusted with a 100k trimpot that will adjust the DC current on the drain of the transistor. That way, you can set what the maximum gain of the pedal will be. The maximum possible gain is set also by the resistor R3 (1k5).

Then, the signal is filtered a bit by different means: a 470 pF capacitor connected to ground let high frequencies going to ground, thus eliminating a bit of high frequencies. To eliminate more high frequencies, value of this capacitor can be changed to a bigger value like 560 pF for instance. A 470K resistor connected to ground will set the bass response of the unit, and will eliminate a bit of bass. A potentiometer wired as a variable resistor will then set how much signal will go to ground. Thus, it defines how much the signal is amplified, and how much signal will drive the second transistor: it is the gain potentiometer!


Preamp section 2: tonestack

The amplified signal of the first section will then make the second transistor (Q2) saturate. The amplification of Q2 is also set by a 100k trimpot and R5 (2k7 resistor), and can be modulate to have more or less saturation.

There is a 10 nF coupling cap to eliminate parasitic DC current remaining from the 18V bus, and then the signal is modified by the tone potentiometer. It is wired as a variable resistor connected to ground, and will eliminate more or less bass frequencies. An optional switch also allows you to connect a 120k resistor to ground, which will diminish overall gain (a part of the signal will go to ground), and bass response (more bass will go to ground than high frequencies with a 120k resistor). This allows you to simulate the two High and Low inputs of the Supro amplifiers.

Again, a 470K set the maximum gain of this part of the circuit.


Amplifier: master volume section

This part of the circuit simply amplifies the final signal. The amplification is set by the 220R resistor and a trimpot (again). Residual DC current is then filtered by a 10nF capacitor.
Final volume is set by a potentiometer wired as a variable resistor connected to ground, like we have already seen in many many circuit, like the Fuzz Face for instance. The higher the value of this resistor will be, the less signal will go to ground and the bigger the output signal will be: more volume!

Here it is!
I hope that everything is clear, do not hesitate to ask question if some stuff is still unclear by posting a comment!
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To go further:
Runoffgroove Supreaux webpage
Runoffgroove Supreaux Deux webpage
MAX1044 datasheet.