Fuzz Face

Posted on Jun 24, 2012

This is simple almost beyond belief for distortion devices. Two transistors, four resistors, three capacitors, and two controls make all that tone. The first transistor is set up with the simplest of arrangements - input through a DC blocking capacitor directly to the base, emitter grounded, and a single collector resistor; the driven load is a second transistor base, directly coupled. For DC purposes, the second transistor acts as an emitter follower. The voltage at it's emitter must follow the first transistor's collector, although it is lower by the amount of the base emitter drop of the second transistor. To the first transistor, that means that the 100K resistor from the emitter of the second transistor to the first transistor's base is effectively a feedback resistor, passing a current proportional to the first transistor's collector voltage to the first transistor's base.

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This arrangement is called the "voltage feedback biasing" circuit, and some of the Fuzz Face's unique properties stem from this. The Voltage Feedback (VF) circuit has the properties that it offers the potential for the highest possible gain from any given transistor - good for a distortion device. It also has a very low input impedance, which means that it heavily loads anything trying to drive it. This will be an important point later. Finally, it can't really hard saturate like many other transistor hookups. When driven with a large signal on the base, the collector voltage moves toward the emitter. This lowers the bias voltage through the 100K biasing resistor, and steals some of the input signal. Saturation is mushy - again, very good for a musical sounding device. Was this intentional on the part of the designer? Probably not. It's likely it was a happy accident, as the VF circuit was common in the early days of circuit design to wring more gain from poor transistors. The voltage swing in the other direction, when the input is trying to turn the first transistor off, is not as constrained as the swing towards saturation. Fuzz Faces naturally tend to bias with only about half a volt on the collector of the first transistor, so there is a lot of room upwards. The input stage will first hit mushy saturation on one polarity of signal and then if driven hard enough, hit cutoff on the other polarity. The Fuzz Face...

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