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Hopefully without starting a holy war I would like to discuss the differences, electronically and sonically, of silicon and Germanium fuzz. This is inspired by my recent birthday present (my precious gollum). As I usually like to do with my questions, please keep the answer objective but feel free to comment on which you prefer.

How do silicon and germanium transistor based fuzz pedals compare and contrast sonically? Are there any interesting physical properties of each that lend to their tone?

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A fuzz pedal, when silicon v. germanium is brought up, almost always refers to a "Fuzz Face" circuit. This is an extremely simple circuit, with no input buffer, two transistors, a few capacitors and, I think, 5 resistors. There's also to potentiometers.

Silicon v. germanium in this refers to the type of transistors used. Germanium transistors pre-date silicon transistors, and in most respects are completely inferior to silicon. They vary widely in electrical properties, change properties as the temperature changes, leak (meaning current flows when there's no input signal), they've got low gain and wretched slew rates (how well they change directions to follow the input wave). They're totally crappy for computing and high fidelity applications, which is why they are no longer manufactured and have been replaced by silicon.

But in fuzz face circuits, they are magic. Germanium FF's have a really nice soft fuzziness to them that's really musical. By contrast, silicon FF's tend to be harsh and buzzy - kind of an "ice pick in the forehead" type of sound. There are a few models of silicon transistor that have a sufficently low gain to be a better fit in a fuzz face, but still don't sound as good as germanium transistors.

However, as germanium transistors of the same model vary like crazy, a FF circuit has to be custom tailored to each pair of transistors used. In addition, the transistors have to be selected to be a good match for each other by measuring the gain of each transistor and pairing them up accordingly. Once the transistors are tested and matched up, the resistor values in the FF circuit need to be adjusted to match the specific gains of the transistors in order to get the best sound out of the stompbox.

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By germanium vs silicon I assume we are taking current in production pedals which use germanium or silicon clipping diodes (the parts that typically makes it distort)? Some vintage gear used germanium transistors which I won't touch on.

Typically, a designer would use germanium clipping diodes, with 0.3V bias, to emulate a more subtle, soft, smooth, warm, vintage sounding drive.

Whereas, a designer would most likely use silicon clipping diodes, with 0.7V bias, to make it more aggressive, edgy, bright, has more bite.

DISCLAIMER: But, I would say each pedal, whether silicon or germanium, would be different depending on how the designer wanted it to sound. You could have two pedals with both germanium or silicon and they can sound completely different. Or you could possibly design pedals that sound the same (difficult) but one uses germanium and one silicon. The sound of a pedal comes down to a lot more than the type of material. Things like synchronous vs asynchronous diodes, hard vs soft clipping, different opamps (albeit subtle) and different capacitors/resistors. All could change the sound.

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I like the extended caveat. Both the material used to make the clipping diodes is but one small part of a complex system that is a fuzz pedal. –  Ian C. Jan 21 '11 at 15:39
    
It's not so much the forward bias difference that makes germanium diodes sound different from silicon diodes as it is the difference in the efficiency with which they transition between conducting and not conducting. Silicon diodes are way better at it, which is why nobody uses germanium diodes in digital circuits any more. Germanium diodes seem to wobble a little bit as they start to conduct, which leads to a softer clipping and sucks up some of the very high frequencies. Silicon diodes result in a slightly harsher clipping. –  Dave May 26 '11 at 3:18
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