Loudspeakers and negative impedances

A direct radiator moving coil loudspeaker driven by an amplifier whose output impedance approaches the negative of the blocked voice-coil impedance can be made to exhibit extended low-frequency response with reduced distortion. The results are not to be confused with the effects of a negative resistance source. In a typical case, neutralization of 70 per cent of the blocked voice-coil impedance completely damps the cone resonances as well as substantially reducing the nonlinear distortion below resonance. When the amplifier is compensated for the failing radiation resistance at low frequencies uniform output can be obtained to any arbitrary low frequency limited only by the ultimate power-handling capability of the amplifier and speaker. In this system, no additional amplifier power is required at frequencies down to the speaker resonance; additional power is required below that point.