What is a Zobel?
A Zobel is an impedance matching circuit often used with loudspeakers. A Zobel circuit is also known as a Boucherot cell or somewhat incorrectly as an RC snubber. The nominal impedance of a loudspeaker is not constant, and rises significantly at the upper end of the audio band. In order for a Class D amplifier filter to work as designed, this rising impedance needs to be included in the design. The easiest way to compensate for this is a Zobel, a simple series resistor and capacitor across the loudspeaker terminals. Although the selection of Zobel components depends on a number of factors, the following equations are a good starting point: RZ = RL CZ = 1 / (2 * π * fC * RL) Where RZ is the Zobel resistance and CZ is the Zobel capacitance, RL is the impedance of the loudspeaker, fC is the desired cutoff frequency. For our 27.4-kHz full-bridge example, this works out to a CZ of 0.73 µF. In most applications, either a 0.47 µF or 1 µF capacitor can be used as this is not a very sensitive v
