A Study of Tube Amplifier Modeling Using Nonlinear Wave Digital Filters

Project Type: Student Project
Student: Matthias Hotz

Abstract

In recent years the simulation of vacuum-tube amplifiers, in particular guitar tube amplifiers, gained the attention of companies and the scientific community. One of the published approaches is based on nonlinear wave digital filters, which have the unique advantage of providing a structural relationship between the physical amplifier circuit and the digital model. In this work the theory behind nonlinear wave digital filters as well as the tube stage circuit is discussed thoroughly and on this basis a real-time capable wave digital model is derived. All necessary simplifications are highlighted and an improvement to the existing model is proposed and evaluated. Concluding, practical considerations regarding the accuracy and real-time capability are presented.

Full Text & Additional Material

The project report can be downloaded here. The simulation models are distributed under the terms of GNU GPLv3.

The audio samples were generated using a slightly modified circuit, in particular Ci was replaced by a 10 k resistor to suppress blocking distortion. Furthermore, a rudimentary speaker simulation (2nd-order high-pass at 50 Hz and two 1st-order low-pass filters at 5.5 kHz and 7 kHz, respectively) was applied to enable evaluation in a more practical context.

Theme 1: Picking
Input signal theme1_dry.wav
Output signal if the input has a maximum amplitude of 1 V. Note that the input signal is slightly compressed. theme1_sim_1.wav
Output signal if the input has a maximum amplitude of 30 V. The input signal is severely distorted. theme1_sim_30.wav
Theme 2: Power Chords
Input signal theme2_dry.wav
Output signal if the input has a maximum amplitude of 30 V. Again, the input signal is severely distorted. theme2_sim_30.wav