DIY Guitar Pedals 7 Min Fuzz Manual - Página 13

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Though positioned schematically as a soft-clipping diode, D1 is not soft clipping in the same way as the diodes in a Big

Muff Pi or other soft-clipping transistor fuzz pedals. What it does is bias the base of Q1 to one voltage drop below the

collector voltage. Example diode voltage drops.

Diode

Silicon Diodes (1N914, 1N4148, etc)

Schottky Diodes (BAT41, 1N5817, etc)

Red LEDs

Blue LEDs

Germanium Diodes (1N34A, 1N270, etc)

Now if there is an input signal, the diode tries to keep the base at the same voltage but can only do so to a limited

degree because R1 limits how much current is available for that.

The diode feeds the base, and clips at the same time. The more current through the diode, the more the deviation from

Vc=Vb. Darlington transistors need a tiny amount of current to work, so the voltage across the diode is almost zero.

Single transistors need more current, so you'll notice larger resistors (R1) at the collector to force the base to draw less

current. This way the diode just clips, and the base resistor feeds the base.

So the value of R1 does matter quite a bit. It serves mostly as the drain resistor, which also influences the bias. Installing

a potentiometer (250ka or so) with pins 1 and 2 replacing the R1 pads and playing around with it should be interesting

and possibly enlightening as this could be used to fine tune the Vc = Vb. This trick works fine with germanium transistors

and diodes, but you have to adjust constantly because temperature and leakage are a nightmare. Not a must with silicon

transistors and diodes.

Let's say the input swings to the negative side. What happens? As the input becomes more negative, more current flows

through D1, effectively killing the negative swing. However, this only works to the degree that R1 allows, because as a

resistor, it limits the current flow. At the same time the current sucked through D1 is also upsetting the collector bias.

The result is a lot of distortion.

When the swing goes to the positive side, D1 can no longer funnel current to keep the base where it is, which results in a

lot of amplification, limited only by the reverse leakage of D1. The amplification is so great that it exceeds the abilities of

Q1, meaning the transistor itself is driven into clipping.

This makes the clipping of the signal asymmetric. One half of the signal clips via the diode and the other half has the

natural transistor clipping. On top of that, the biasing is different for both halves of the signal, making a proper analysis

not quite as simple and straight-forward as some other fuzz pedals.

Voltage Drop

0.7 V

0.3 – 0.4 V

1.2 – 1.4 V

2.1 V

0.2 – 0.5 V