Modeling the Pultec EQP-1A with wave digital filters

Abstract

This paper details the development of a virtual analog model of the passive equalizer stage of the classic Pultec EQP-1A studio equalizer using Wave Digital Filters. The modeling process involves analyzing the unit’s schematics, LTspice simulations, and implementing a Wave Digital Filter structure using the pywdf library in Python. The proposed structure initially used R-Type adaptors, that are removed in a second model, better optimized for realtime. The Python prototype is compared to LTspice simulations, showing that, at sufficiently high sampling rates, the discrepancies between the two are marginal. The Wave Digital Filter model was then ported to C++ using the chowdsp_wdf library. Both implementations of the model in C++ (the one that uses R-Type adaptors and the real-time optimized one) are tested. Our optimized implementation proved to be much faster than the other opensource, Faust-based emulation of the EQP1-A that we evaluated, and a more accurate emulation of the original unit’s equalizer stage.