- A Review of Digital Techniques for Modeling Vacuum-Tube Guitar Amplifiers
Although semiconductor technologies have displaced vacuum-tube devices in nearly all fields of electronics, vacuum tubes are still widely used in professional guitar amplifiers. A major reason for this is that electric-guitar amplifiers are typically overdriven, that is, operated in such a way that the output saturates. Vacuum tubes distort the signal in a different manner compared to solid-state electronics, and human listeners tend to prefer this. This might be because the distinctive tone of tube amplifiers was popularized in the 1950s and 1960s by early rock and roll bands, so musicians and listeners have become accustomed to tube distortion. Some studies on the perceptual aspects of vacuum-tube and solid-state distortion have been published (e.g., Hamm 1973; Bussey and Haigler 1981; Santo 1994).
Despite their acclaimed tone, vacuum-tube amplifiers have certain shortcomings: large size and weight, poor durability, high power consumption, high price, and often poor availability of spare parts. Thus, it is not surprising that many attempts have been made to emulate guitar tube amplifiers using smaller and cheaper solid-state analog circuits (e.g., Todokoro 1976; Sondermeyer 1984). The next step in the evolution of tube-amplifier emulation has been to simulate the amplifiers using computers and digital signal processors (DSP).
A primary advantage of digital emulation is that the same hardware can be used for modeling many different tube amplifiers and effects. When a new model is to be added, new parameter values or program code are simply uploaded to the device. Furthermore, amplifier models can be implemented as software plug-ins so that the musician can connect the guitar directly to the computer’s sound card, record the input tracks, add effects and/or virtual instruments, and then compile the song as a CD or upload it to the Internet. This is especially useful for home studios and small ad hoc recording sessions, because it eliminates several tedious tasks of acoustic recording, such as setting up the amplifier and recording equipment, selecting a microphone position, finding a recording room, etc.
This article attempts to summarize real-time digital techniques for modeling guitar tube amplifiers. Although a brief overview was presented in Pakarinen (2008), to the authors’ knowledge, there are no previous works that attempt a comprehensive survey of the topic. Because this topic is relatively new and commercially active, most of the reference material can be found in patents rather than academic publications. Judging from the large number of amateur musicians and home-studio owners, as well as the huge number of discussion threads on Internet forums, this topic is potentially interesting for a wide spectrum of readers. Thus, a conscious choice has been made to try to survey the modeling techniques at an abstracted level, without delving into the underlying mathematics or electric circuit analysis.
This review is organized into four sections. We first describe the sources of the nonlinearities in guitar amplifier circuits. Then, we review published methods for modeling the linear stages of guitar amplifiers. The heart of this survey is the review of methods for nonlinear modeling. Finally we [End Page 85] mention various other guitar-amplifier related technologies and present conclusions.
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The purpose of this section is to present an overview of the operation of vacuum-tube amplifiers and to illustrate the complex nature of their important nonlinearities. An overview of vacuum tubes used in audio applications can be found in Barbour (1998), and a detailed tutorial on classic vacuum-tube circuits is provided in Langford-Smith (1954). The physical principles governing the operation of vacuum tubes are reviewed in Spangenberger (1948). Excellent Internet articles discussing the design of guitar tube amplifiers can be found online (e.g., at www.aikenamps.com and www.ax84.com ).
A typical guitar tube amplifier consists of a preamplifier, a tone-control circuit (i.e., tone stack), a power amplifier, and a transformer that couples to the loudspeaker load...