A Wave Digital Filter Model of the Fairchild 670 Limiter
This paper presents a circuit-based, digital model of the prized 1950’s vintage Fairchild R 670 vacuum tube limiter. The model uses a mixture of black boxes and wave digital filters, as a step toward a fully wave digital filter design. Wave digital filters provide an efficient, modular way to digitally simulate analog circuits. A novel model for the 6386 triode is introduced to simulate the active component in a wave digital filter model of the Fairchild 670’s signal amplifier. The signal amplifier is integrated with a hybrid wave digital filter/black-box sidechain amplifier model to form a complete model of the Fairchild 670. Model test results for music and pure tones are discussed, highlighting the device’s static gain characteristics and gain reduction dependent distortion. Finally, this paper discusses the model’s salient features and their implications for designing dynamics processors.