It is well known that amplifiers that use negative feedback in most cases need a zobel network on the output to avoid the risk for oscillating or ringing at high frequencies. The speaker together with the speaker cable becomes a reactive load at high frequencies. And the phase/gain margin for amplifiers with negative feedback normally decrease with increasing frequency. The reactive load forms an oscillator together with the amplifier unless a zobel network is used. A few amplifiers on the market lack an embedded zobel network at the output or have poorly dimensioned zobel networks. Such amplifiers can start ringing depending on the speaker cable and depending on the speaker design. Some ringing can cause the sound to be harsh, cause listening fatigue or in more severe cases, destroy the output transistors of the amplifier.
For conditions where ringing may be an issue, we have developed impedance equalizers to be attached to the speaker terminals. These simple devices are zobel networks build for minimal losses. We use FEP-insulated wires, a very low loss polypropylene capacitor and a wirewound resistor.