Abstract
An audio transformer is used in a guitar amplifier to match the impedances of the power amplifier and a loudspeaker. It is important to understand the influence of the audio transformer on the overall sound quality for realistic tube amplifier emulation. This paper proposes to simulate the audio transformer using a wave digital filter model, which is based on the gyrator-capacitor analogy. The proposed model is two-directional in the sense that it outputs the loudspeaker current, but it also connects backward to the power amplifier thus affecting its behavior in a nonlinear manner. A practical parameter estimation procedure is introduced, which requires only the measurement of basic electrical quantities but no knowledge of material properties. Measurements of a Fender NSC041318 and a Hammond T1750V transformer are presented as case studies, as well as parameter fitting and simulation for the Fender transformer. The results show that these practical transformer designs introduce distortion at low frequencies only, below about 100 Hz for the Fender and 30 Hz for the Hammond transformer, and that the proposed model faithfully reproduces this effect. The proposed audio transformer model is implemented in real time using the BlockCompiler software. Parametric control allows varying and also exaggerating the model nonlinearities.
Highlights
There are new and cheaper technologies for guitar effects and amplification, vacuum tube amplifiers remain extremely popular
This paper proposes to simulate the audio transformer using a wave digital filter model, which is based on the gyrator-capacitor analogy
The proposed audio transformer model is implemented in real time using the BlockCompiler software
Summary
There are new and cheaper technologies for guitar effects and amplification, vacuum tube amplifiers remain extremely popular. The magnetic material in audio transformers may present a frequency-dependent saturation, which differs significantly from the saturation caused by vacuum tube amplifier stages. A nonlinear model for offline vacuum tube guitar simulation is proposed in [15] using the Jiles and Atherton model [16] for magnetic losses. This paper proposes a WDF implementation of a nonlinear audio transformer model for real-time guitar amplifier emulation. The complete model for audio transformer includes, in addition to the nonlinear transformer behavior, parasitic phenomena such as leakage inductance, winding resistance, and input/output capacitance, which are important for matching the transformer’s frequency response.
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