Class D Amplifier FAQ - Part 2: Zobel Circuits, Testing, PCB Layout & Grounding

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:

R_Z = R_L

C_Z = 1 / (2 * π * f_C * R_L)

Where R_Z is the Zobel resistance and C_Z is the Zobel capacitance, R_L is the impedance of the loudspeaker, f_C is the desired cutoff frequency. For our 27.4-kHz full-bridge example, this works out to a C_Z 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 value. The schematic for this Zobel is shown in Figure 3.

Figure 3. A Zobel circuit normalizes the rising impedance of the loudspeaker.

Page 2: Can I use a ferrite bead instead of an LC filter?
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