Black Keys's Big Muff: dealing with mids frequencies

Another Big Muff clone! This time, I was inspired by the Black Keys (if you do not know this band, go check it out, it is awesome!). Dan Auerbach, the guitarist/singer, uses a lot of fuzz effects, and especially a green russian Big Muff, and a Earthquaker Devices Hoof (a Big Muff variant with some interesting modifications as we will see later).

So I decided to basically mix these two variants in one Big Muff! Here it is, a 4-knobbed Big Muff:

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

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.

Rub-a-Dub Reverb (1776 Effects)

Here is my latest build, a Rub A Dub reverb! It is a really simple yet useful "always on" reverb that can go from a minimal amount of reverb to long ambient delay washes, I love it! One knob, simplicity at its finest.

Inspired by Fender amps, I decided to cover an old messed enclosure (bad painting job) with tweed. Oldamp from guitariste.com was nice enough to give me a bit of tweed he had left from the amps he makes.

To apply the tweed, I used a pattern. Corners are especially difficult, because you need to bend the tweed (that is a rather stiff material finally) in a proper way. It was rather a long process because the glue takes quite sometimes to be perfectly dry.

The bottom plate was covered as well. I had to use longer screws in order to be able to close the enclosure! I used a plate that I found on the banzai music website, I thought it was really appropriated for the vintage fender style. There was 3 letters possible: "T", "B" or "V". "V" was the best for me, like "Verb".

It was quite a tight build finally. The enclosure I used was already drilled so I - stupidly - used the same layout. Unfortunately, positioning the power supply at the top of the pedal was not a good idea, I could not directly solder the pot to the PCB. I managed to fit everything. I used gold plated jack input, so gold plated connector could - maybe - finally be useful. I am quite happy with it, because in the end, there is even enough space for a 9V battery!

I got the PCB from 1776 effects, a builder based in the USA (1776 is the year of the independence of the USA). He sells a lot of different PCBs for different projects, from overdrives to delays (the multiplex echo machine for instance). This one is a simple, yet good sounding one knob reverb. There is also a "Deluxe" version of it with more tweakability, but for now I wanted a simple build! 

The PCB is really of good quality, double sided and have a nice layout. Components are spaced enough so it is not too hard to assemble, but it is compact enough to be relatively small and fit a 1590B enclosure.

Populating the board was really fun and easy, I begin to like these circuits with not too many components! For this Rub A Dub reverb, I used a Long Belton brick, Panasonic FC and SMF (except one that is a MKT standard because the value did not exist in SMF... )

How does it sound?

The "mix" potentiometer is really making a mix between the dry signal and the treated signal. So, the reverb is more or less present. It is really a nice feature, you can vary from a slight reverb in the background or a huge swampy reverb signal. I really like it at high settings. 

My only concern was that I used a Long brick, and as it is not possible to set the decay, the reverb is always very long, even when the mix potentiometer is low. If I remake one, I will use a medium or short brick. Or I will go for the "deluxe" version of the rub a dub reverb that has a potentiometer for decay using the 3rd new version of the belton brick.

I will compare with my Chasm reverb when it will be finished!
 

Circuit analysis

A stompbox reverb is most of the time a digital effect, simply because an analogue reverb requires a spring or plate system that is way too big for a standard 125B enclosure format. Fortunately for us, a manufacturer called Belton provides Digital Reverb modules that can be used for such projects. They are declined in 3 types corresponding to the possible duration of the reverb: short, medium and long.

This brick contains a rather complicated circuit with 3 PT2399 (numeric delay chip) that follow each other with really short delay time to act as a reverb.

Bam! I told you it was kinda complex...

They managed to make it smaller as possible with surface mounted components so we, small DIYers can use it at home to make our reverbs!

It has 6 pins:

• Pin 1 should receive  a regulated +5V voltage
• Pin 2 should be connected to power ground
• Pin 3 is the input of the circuit
• Pin 4 is the signal ground
• Pin 5 is the output 1
• Pin 6 is the output 2 (for stereo applications)

So it is like having another circuit in your circuit (circuitception!)
The Rub a Dub reverb is a circuit surrounding this belton brick in order to get the most of it with a simple layout. Here is the schematic:

 

Let's divide it in three sections:

The input buffer is a simple OP amp boosting the signal before it is split between dry (analogue signal) and wet (numeric conversion and treatment by the Belton brick). First, there is a 1M pulldown resistor that prevents "pop" noises when a charge accumulates at the entry of the circuit. The excess of current is going to the ground when the effect is activated. Then, there is a coupling capacitor (22nF), preventing any parasitic direct current to go in the circuit. The signal then goes through a resistor and the first OP-amp of the TL072. The TL072 is an IC containing 2 OP amps, which is a low noise JFET amplifier.  It is wired as an inverting amplifier. Thus, we can calculate the gain of it, which is simply R3/R2 = 360/180 = 2. The OP amp multiply the signal by 2, so when it is split between the dry and wet section, it is like having 2 signals like the original one going on each side of the circuit.

The treated signal section (wet section) simply is the Belton brick circuit. The signal enters through pin 3 and goes out through pin 5 and 6. It is then mixed with two 4,7k resistors accordingly to Belton mono circuit example. The signal that goes out the brick is the reverb signal ("wet" signal).  The signal then goes back to the amplification loop of the second OP amp. First, the "mix" potentiometer sets how much wet signal is going back in the circuit to be mixed with the dry signal. So you define how much signal there will be in the final sound! A coupling capacitor prevents any parasitic DC current to go in the circuit, and a 20k resistors defines how much signal can go through at the minimum mix level. A fun mod to do is to put a switch before the 22k resistor in order to make the entire signal going through the wet section: all the signal will be treated, and you will not have a dry signal anymore, creating a "wet" reverb.

The mixing section mixes the dry signal with the wet signal using the second OP amp of the TL072. The amplification gain is around 0,5, so that the mixed signal will be of the same amplitude that the original input signal. Thus, the volume will stay constant. By modulating the resistors values, for instance the 12k resistor, volume can be increased or decreased. It is also possible to put a potentiometer there in order to have a volume / boost knob. Then the signal goes through a 1uF coupling capacitor, and there is another pulldown resistor to prevent noises, and it finally gets out!

The power supply section stabilizes the current and offers a 4,5V voltage, and a regulated +5V voltage. This is important to supply the voltage needed by the TL072 and the Belton Brick. The current enters via +9V(T), goes through a 33R resistor. A 1n4001 diode protects the circuit against polarity inversion, and a 100 uF capacitor eliminates all the residual oscillations that may come from the power supply. More details about the stabilization of the power supply in my dedicated article. On the left side of the power supply section, there are 2 resistors that act as a voltage divider. Half of the current goes through the 10k resistor linked to the ground, and half of it goes to VB. We have a 4,5V VB voltage, perfect for the TL072! On the right of the circuit, there is a 7805 REG regulator. Even if it looks like a transistor (with 3 pins), it is an integrated circuit with a simple role: a voltage between 7 and 36V can enter through pin1, and a stable 5V voltage goes through pin3. Pin2 is connected to ground. It is really convenient when you need a stable particular voltage. There are many possible versions for voltage regulators, depending on which voltage you need:

• 78xx: positive voltage
• 79xx: negative voltage
• "xx": voltage value of the regulated voltage output.

There it is! I hope that everything is clear for you!
Any questions? Post a comment!

To go further:

Belton brick BTDR2H official webpage and datasheet.

Rub a Dub reverb bill of material and informations

About 7805 voltage regulator: more infos here.

What is guitar signal?

We hear about it all the time: it should not be degraded, you have to respect it, we can boost it, clip it or digitalize it... What are we talking about? Electric guitar signal of course! As you may know, the vibration of the strings is converted in an electric signal by your pickups, to restitute guitar sound under an electric form. How does it work? How can an electric signal encode pitches or volume? How can it be modified by guitar pedals?  

Where it all began: your guitar pickups!

What is electric guitar signal?

As you know, there are 2 types of currents: direct current (DC) and alternate current (AC). Direct current has a fixed value (for example, the +9V that you get from a battery), whereas alternative current oscillate between a positive and a negative value, at a given frequency. For instance, the current oscillate with a 50 Hertz (Hz) frequency on a classic house electrical outlet. 

Your electric guitar signal is alternate current! The vibration of the string is captured by your guitar pickups coils, which generate an alternate current. The values between it oscillates depends on many parameters, especially your pickups output. The more the output level of your pickups is important, the bigger the amplitude of your initial signal will be.

How is volume defined by a voltage?

The signal amplitude (difference between the maximal and minimal voltage value) is directly linked to the volume. The greater the amplitude, the greater the volume! (and vice versa) It is as simple as that.

Most boost pedals just transform a low volume signal into a high volume one, by amplifying the signal. We can understand the origin of the word "amplifier"! It just amplifies the initial signal loud enough so that it can be heard through a speaker. It is also what happens when you strum the strings in a stronger way: the strings vibrate more and the amplitude of the signal is larger.

Historically, amplification could be done only with tubes in the beginning (that you can find in old amplifiers or radios). When transistor was invented, it was then possible to create stompboxes capable of amplifying signal! A common collector circuit is a classic layout that you will find in many circuit, for instance in the big muff circuit. Today, with integrated circuits, it exists smaller op-amps that can amplify the signal with one simple chip! Some of these chips / recent transistors even use almost the same technology as tubes to amplify an audio signal (FET/MOSFET burr brown for example).

100 years of evolution of signal amplification in one picture: a 12AX7 tube, an AC188 germanium transistor, a 2n5089 silicon transistor and a TL072 integrated circuit OP amp

To reduce the signal amplitude (and thus the volume), a simple resistor is enough! That is why there is generally a potentiometer wired as a variable resistor at the end of a circuit to set the output volume. You can find this in many many circuits, like in the Jan Ray.

How alternate current can define a note or pitch?

As we have seen above, alternate current is characterized by a period, which defines a frequency (frequency = 1/period), written in Hertz (Hz). For example, alternate current in an electrical outlet has a frequency of 50 Hz.
Each note has also a frequency! For instance, you may have heard of the 440 A that is the standard pitch used as a reference for many instruments. It is named like this because of its 440 Hertz frequency!

The higher the frequency of oscillation of the current, the higher the pitch is.

Thus, each note is defined by its frequency. By modifying the frequencies, the sound can be modified. The characteristic sound of a wah wah is due to the fact that it will only let some frequencies go through it. The "ducky" sound that you get in the middle position of a wah is due to the fact that it lets pass only mediums frequencies. Tone potentiometers also affect frequencies by eliminating high frequencies (diminishing trebles) or low frequencies (less bass).

Human ear can perceive frequencies between 20 and 20 000 Hz. Practically speaking, it also depends on individuals. With aging, higher frequencies are less and less heard. It is interesting to note that medium frequencies between 1000 and 4000 Hz are perceived louder for a same volume (in decibel). Which is nice for us, guitarists, because electric guitar has a lot of output in these frequencies! And this is how guitarists became famous and not the bassists! (duh!)

Of course, it is a bit more complicated than that. When you play a note, there are many frequencies that are played at the same time, and not only one (harmonics for instance). When you play chords too! When several frequencies are played at the same time, voltages are simply added.

What are effects doing to your signal?

Lets see a few examples of how effects modify your electric guitar signal.

1. Compressor
Compressors delimit a maximal amplitude for your signal. By diminishing the amplitude, you "compress" the sound! (all names make sense now, right?)

You can see here that the original signal has a lot of variations of volume: there is a lot of dynamic. However, when a compressor is used, signals are closer to each other: a bit of dynamic is lost. A compressed guitar signal actually sounds very good, that is why many guitarists use a compressor, with a price to pay: you loose a bit of dynamic.



2. Saturation: overdrives, fuzz, distortions...
Saturation happens when the top and bottom of the signal are clipped. This can be done with different ways: saturation of tubes, transistors, using diodes that will let some voltage go to the ground...etc.

We can clearly see here that the top of the signal curve has been cut: the signal was clipped. To better understand what happen here, read my article about the Vemuram Jan Ray circuit. Diode clipping is explained in it.
We can also see that the more the signal is saturated, the more it is compressed! This is something you can clearly hear when you use too much saturation for your guitar, or for instance with a big muff.


3. Numeric versus analogue signal
The biggest difference between a numeric and an analogue signal is that an analogue signal can have an infinity of values between 0 and 1, whereas numeric signal is limited because of its binary nature (only 1 and 0...). An analogue signal with an infinite value of 3.3333333...etc. can exist, however a digital signal will stop at some point depending on the quality of it. This quality of the digital signal is generally expressed in bits / kbits per second. A mp3 has a quality between 64 and 256 kbits per second. A CD is 1411 kbits/sec.

So you have a fixed number of points / seconds. The possible values of these points depend on the encoding quality.

The sampling rate also defines the numeric signal quality. The higher it is, the most precise your numeric signal will be (and will not generate noise).

Thus, we can understand why people did not really liked numeric effects at some point. The "coldness" of the sound that you can feel with a numeric pedal comes from this encoding process.

However, today, technology have evolved so much that encoding can be done without an audible quality loss.

For instance, the first digital delay, the Boss DD2, had a 12 bits converter, with a sampling rate of 32kHz (just near radio quality...). Today, Strymon uses 24 bits converter with a 96kHz sampling rate (DVD audio quality!)

Here it is, I hope this article has enlightened you a bit about this mysterious "signal"... If you liked this article, thank me by liking the Coda Effects Facebook page!

Any questions? Post a comment!

To go further:
Analogue versus numeric signal: detailed analysis about differences between analogue and numeric guitar signal by Screaming FX
A bit of theory about compression, clipping...etc.

Big Muff Ram's Head clone (73 version)

Here is the Big muff Ram's Head clone I have made recently. The Ram's Head is one of the most iconic Big muff models. Produced in 1973, it was the second issued version of the Big Muff, just after the Triangle model. Multiple versions of the Ram's Head existed until 1977, including the "violet era" Ram's Head that sells around 1000 dollars on ebay! The Ram's Head v2 is well known as it was used by Pink Floyd's guitarist, David Gilmour. He used the Big Muff combined with a boost in order to have a nearly infinite sustain during some solos, like the famous Comfortably Numb's one.

I decided to make this version, with a PCB from musikding. I am currently prototyping a double sided PCB to use it in 1590B enclosures. Here, the PCB is single layer, so I had to use a 1590BB enclosure. I polished the enclosure with sandpaper 140, then 400 and 800 to have a thin polish, following the same direction to make it look like brushed aluminium. I washed it with acetone, and add 2 layers of satin varnish to avoid corrosion.

I followed the classic scheme of a 73' Ram's Head Big Muff (Gilmour!). I used Wima MKP2 capacitors and one electrolytic to stabilize the power supply. I added two switches to control the amount of mids (switch 1) and bass/trebles (switch 2). The first switch allows you to change the 4,7nF capacitor of the tone stack for a 10nF, thus increasing mids. The other switch plays on the resistor and capacitor at the entry of the circuit, giving us 2 options: a very bassy / smooth one which is the classical ram's head, perfect for doom or stoner rock, and another more trebly, gritty and scratchy!

With the PCB, it was quite easy to organize the wires inside the enclosure. With a BB sized enclosure, there is a lot of room!

How does it sound?

I did not have time to records samples of this pedal. You will have to believe me then! However, no surprise with this one, it sounds like... a Big Muff! Heavy, oversaturated, compressed, ideal to generate walls of saturation. Ideal for stoner, I played it during almost one hour, playing some Kyuss, Fu Manchu, Queen of the Stone Age whereas I just wanted to test it quickly! The tone potentiometer is really efficient and allows you to go from a bassy heavy sound to something more gritty and aggressive. The bass switch is also really efficient, on one side you have a classic ram's head with a lot of bottom end, quite smooth in the highs, and something more gritty on the trebles on the other side of the switch. The mid switch is also quite efficient, we can hear that sound is more "full" when switched on, and it really works well in a band situation. I am really happy with the result, it is exactly what I was expecting!

How does it work?

The Big muff is one the favourite circuit of guitar pedal builders (with the Tubescreamer). Indeed, its circuit is really well known, there is a lot of information available all around the internet, and it is really easy and fun to modify! Each component plays a particular role, and changing it for a different value will change the overall sound of the pedal without damaging the circuit most of the time. It is one of the most cloned guitar pedal, by DIYers or by boutique companies: Mojo Hand FX, Earthquaker Devices, Pete Cornish... Almost every brand has its version of the big muff!

Attack of the clones: all the pedals in this picture are Big Muffs 
or "heavily inspired" by the Big Muff pedal

Technically the Big Muff is not a fuzz but a distortion with most of the saturation coming from diodes. However, it has 2 distortion stages, causing the heavily compressed and saturated sound of the Big Muff. The sound is finally close to a fuzz. Lets have a look on the circuit schematic:

I know, it seems horrible and over complicated... Too many components! But lets divide it in 5 sections:

The first section (input section) is simply a buffer to adapt the impedance of the guitar to the first distortion stage, and which amplify also the signal. As OP amp did not exist at that time (1973, remember?), this is made with a transistor wired in common emitter. Today, a simple OP-amp would be enough! First, there is an input resistor which will diminish a bit the amplitude of the signal. Then, a coupling capacitor will remove any parasitic DC current that mights come from the guitar. Changing the values of the coupling capacitor and input resistor can changes the amount of bass and trebles of the big muff. That is how I made the switch on the pedal. A bigger value resistor and a higher capacitance for the coupling capacitor will increase the bass response of the unit. Then, a transistor wired in common emitter will amplify the signal. Depending on the amplification of this stage, the saturation will be more or less important. The amplification is set by the different resistors on this circuit. Changing resistor's values can increase or decrease the overall gain of the unit. Next, there is a coupling capacitor (again), and a potentiometer wired as a variable resistor that will allow the user to set the amplification on this stage = gain of the Big muff! 

The second and third part (first and second clipping second) are in fact exactly the same circuit. It is used to distort the signal, through the clipping induced by diodes. There is first a coupling capacitor (if you look carefully there is one at each section's start except the tonestack), followed by a resistor. Then, there is a transistor wired in common emitter like in the first part of the circuit with resistors to define its gain. A 470pf capacitor will allow more or less trebles to go through the section. Changing the three 470pF capacitors for a larger value (like 560pF) will diminish the trebles of the unit. This is the case in many clones, like in the Iron Bell from Mojo Hand FX for instance. The diodes will clip the signal and generate the saturation. In classical Big Muff, silicon diodes are used, but in some "boutique" clones, germanium diodes clip the signal (pharaoh fuzz for instance). With germanium diodes, there is less volume, and the sound is even more compressed and saturated! For more infos about diode clipping, read my article about the Jan Ray circuit.

The third part of the circuit is for me the most important part, the tonestack! This little circuit is what gives the characteristic sound of the Big Muff, and the medium loss, the enemy of guitarist playing in a band. Indeed, when you loose mediums, you litteraly disappear from the mix, as if the guitar was muted! A lot of mods do exist to avoid that: a switch which modify the values of the resistor linked to the ground or the 4,7nF capacitor (which I did on this Big Muff), or to use a potentiometer instead of a resistor, and modify the values of the components to have flat mids, to have a proper medium knob (AMZ tone stack). It is also possible to replace the tone stack by another one, like on the Pete Cornish G2. You will loose the Big Muff style distorsion for a much classical overdriven sound. You can even remove it! You will then have an "in your face" sound with a lot of mediums. You can find this system on the Dwarcraft Eau Claire Thunder for instance. Experiment by yourself, the tone stack is a simple circuit and changes a lot the sound of the Big Muff. A lot of fun! 

Finally, in the output / volume section, the signal is amplified again (to prevent volume loss due to the diodes clipping). It uses a transistor wired in common emitter biased to have a strong amplification. A coupling capacitor is present, followed by a potentiometer wired as a variable resistor to modulate the output signal amplitude. We can recognize (again !) the volume control of a fuzz face!

I really advise you to make a muff once. It is really fun to make and to mod. After reading the Big Muff page, you will know exactly what to change to make it sound like you always wanted!

To go further:
Big muff Page: THE website about big muffs! All the versions, schematics and more! A must read for muff lovers.
Big muff pi circuit analysis (electrosmash): precise analyse of the circuit
AMZ tone stack: study about the big muff tonestack, alternative tone stacks
Big muff mods and tweaks: Some easy mods for your big muff. They are easy but funny mods!