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! Read a precise analysis of this part of the circuit here! 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!
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