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10 best electronics analyses of 2015

23 Dec 2015  | Stephen Padilla

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Exploring tube tones sans tubes

In my previous article, I tackled guitar tone and the almost magical qualities attributed to vacuum tube amplifiers. As we saw, mixed in with the science there's a healthy sprinkling of musician opinion, lore and superstition. And some stratospheric prices for legendary amplifiers.

Given some of the issues surrounding vacuum tubes – reliability, lethal voltages, size, weight—for decades designers have been trying to replace them by more "modern" components as semiconductor technology has improved; since the distortion characteristics of classic amplifiers from the 50's still represent the "gold standard" of guitar tone for many, subsequent designs have concentrated on duplicating the characteristics of tube distortion.

In the late 1960s, the widespread availability of germanium diodes and transistors led to the first "fuzz" boxes. Later designs made use of discrete FETs and then op amps. As more powerful microcontrollers came on the market, DSP-based digital modelling came to the forefront, although most of the earlier approaches are still viable.

In this article, we'll take a quick look at some of the classic designs, tracing their evolution from germanium technology to modern digital techniques. Along with a dose of mojo and voodoo, of course.

Early designs: The Dallas Arbiter Fuzz Face

This design was one of the first "legendary" pedals, introduced in 1966 and made famous by Jimi Hendrix. The earliest versions used NKT275 germanium transistors, but their parameters varied widely, both part-to-part and over temperature, and they were later replaced by BC183, BC130, BC108 and similar silicon devices. These were more stable, but had a harsher sound to many as well as an unfortunate tendency to pick up AM radio broadcasts.

The Fuzz Face circuit is extremely simple, and based on the shunt-series-feedback amplifier topology with R4 providing feedback. The two transistors are direct-coupled; C1 blocks the DC component of the input signal.

The original designs were discontinued around 1976, but later reissued and are still being manufactured in both silicon and germanium versions and boasting a long list of famous users. As can be seen, both the design and fabrication are extremely simple compared to later designs, yet an original Fuzz Face can fetch over $1000.

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