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Distortion, overdrive, and fuzz - the original family of guitar effects. So metal.

Bazz Fuss Edit

This incredibly simple circuit is my starting point (check out the sound samples linked from that page). No I didn't come up with that name, yes it is lame. Let's just do some analysis.

Biasing Edit

The transistor I'll be using is a BC548. Ignore everything on the other side of the coupling capacitors.

V_c = 0.7 + 0.7 = 1.4

I_c + I_b \approx I_c = (9 - 1.4)/100K = 7.6 \mu A

g_m = \frac{I_c}{V_T} = \frac{7.6 \mu A}{25mV} = 3mA/V

r_{\pi} = \beta ( \frac{V_T}{I_C} ) \approx 100K for a beta value of a bit over 300, not unreasonable given the data sheet

Small-signal Edit

Try to imagine the hybrid pi model cos I'm not gunna schematic capture it.

Ignoring the feedback/clipping diode, this would be an inverting amplifier with a gain of about -\frac{\beta \cdot 50K}{r_{\pi}} \approx 150. However,

When the input is negative, the output is positive, the diode is forward biased and limits the output to 0.7V above the bias point. Clipping and fuzz result.

When the input is positive, the output is negative, the diode is reverse biased and an open circuit. The output is presumably clipped when the transistor hits saturation, at about 0.7V below bias point. Wow, that's symmetric and shit, the designer did his ELEC2133 homework.

Frequency Response Edit

With default cap values of 4.7uF and 0.1uF. TODO

Possible improvements Edit

Put a small resistor in parallel with the feedback diode to provide some actual feedback. I think this should 'soften' the fuzz as the output will be more rounded. Even better, make it a pot - hey presto, a 'drive' control.

Check out the Deluxe and Buzz versions on the link up the top. Using a darlington would presumably give insane clipping.

Changing components would also be cool. Try stuff like germanium diodes, LEDs (that would look way cool..), different transistors. Cap values.

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