Bijous: pretty BJT overdrives

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Bijous: pretty BJT overdrives

Dec 27, 2023 9 min read

Updated: Dec 28, 2023

Also on FSB! https://www.freestompboxes.org/viewtopic.php?p=297316

Introduction

Electric guitarists have been long enamored with distortions made from discrete transistor stages. This relationship isn't going away anytime soon, given the recent rise in popularity of this kind of circuit. I'm not talking about the many fuzzes which fall into this category, but of "discrete transistor overdrives".

Tracing an history from current pedals through the most memorable transistor pre-amps from Roland and Peavey, we can say that this similar arrangement have accompanied electric guitar from the beginnings, being a direct transposition from circuits using vacuum tubes. Whether the purpose was to have distortion or to have enough amplitude to drive what comes next, usually two or more amplification stages are present, with an interstage attenuator controlling signal gain and provisions for equalization.

I haven't mentioned so far JFETs, which are usually the preferred device for these circuits. Usually, a reason for this is that they are depletion devices like vacuum triodes, but as "The Art of Electronics" made me realize, this just means that the linear range of operation is shifted from 0V down, whereas it's up from 0 for enhancement devices. This is hardly important as long as biasing is done correctly, and enhancement mode actually makes biasing easier on single supply. There's no denying the importance of the Fetzer valve concept in motivating the first adaptations of historical guitar preamplifiers and successive derivations; I'm not contesting the conclusions reached, which follow naturally from JFET theory. I'm claiming that they missed the point in focusing on the linear transfer function of common source amplifiers, when saturation behavior is what matters most. It's easy to claim that for a guitarist, a circuit analyzed this way, that doesn't clip even a tiny bit, will seem useless, and the few % of THD, or more exactly the different distribution of the harmonic contribution to this quantity, will go almost fully unnoticed. Once clipping is reached, they do go fully unnoticed, because the waveform is affected (distorted) on a much larger scale; we then are concerned with things like symmetry of the clipping, the range of the waveform that isn't affected (with the hard clipping threshold approximation), the shape of the clipping and other behaviors like rectification and blocking distortion.

By starting with the trivial assumption that the goal is some amount of clipping, from subtle to blatant, we can reframe the objective of matching the input sensitivity of one stage with the amplitude it receives. The choice between JFET and BJT is then mostly between arbitrarily high input impedance and more transconductance. I'm leaving noise outside of the comparison because, depending on the bias current, the source impedance and the specific part, either category could be