The improvement of low-frequency driver performance using an amplifier with negative output impedance

T. D. Kite and E. L. Hixson
J. Acoust. Soc. Am. 96, 3318(A), 1994.

Abstract

A low-frequency dynamic driver (woofer) was modeled on PSPICE, an electric circuit simulator. The far-field pressure response of the driver was computed as the dc resistance of the voice coil, Re, was varied. In the limit of zero Re, the on-axis response for a constant voltage input was shown to rise at 6 dB/octave. The input to the driver was therefore equalized with an integrator to produce a flat pressure response with frequency. A beneficial side effect of reducing Re was the improvement of the transient response of the driver, since it became possible to critically damp the system. The system was found to be stable for values of Re greater than zero. The simulated results were tested in an anechoic chamber using a power amplifier with variable negative output resistance. The far-field pressure response of the driver was found to be substantially flat over a broad range of frequencies aroudn the natural resonance of the driver, and transient response was improved, as predicted by the circuit simulation.