The nominal impedance of a magnetic headset isn't necessarily a good description of its actual impedance as a function of frequency. A headset's fairly complex equivalent circuit may even lead to parasitic oscillations in a driving amplifier. But designers can suppress these oscillations by adding a shunt capacitance across the headset.
A test setup was used to determine a circuit model for the particular headset under consideration (Fig. 1). This circuit-modeling technique was described previously (see "Mimicry Finds Defective Buried Components," John Dunn, electronic design, July 9, 1992, p. 76).
The resulting RLC headset model was analyzed for its impedance-versus-frequency property and found to have a parallel resonant peak in the megahertz range. Unfortunately, the amplifier used to drive this headset tends to exhibit a parasitic oscillation at a 1-MHz frequency. Shunting the headset with increasing values of capacitance tended to reduce that propensity to oscillate, suppressing it altogether at 1µF.
This is because increasing the capacitance across the headset's equivalent circuit lowers the frequency of the resonant peak and decreases the magnitude of the peak impedance (Fig. 2). At a capacitance of 1µF, the resonant peak has been eliminated entirely and the amplifier no longer tends to oscillate.