Design Article
Class D Amplifier FAQ - Part 2: Zobel Circuits, Testing, PCB Layout & Grounding
John Guy, National Semiconductor Corp.
12/3/2008 2:27 PM EST
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 value. The schematic for this Zobel is shown in Figure 3.
Figure 3. A Zobel circuit normalizes the rising impedance of the loudspeaker.
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AcousticCliff
12/3/2008 4:26 PM EST
Great Article.. One more thing to be aware of in the lab when using Class D amplifiers in breadboard situations are long cables. Doing acoustic experiments on small enclosures can be misleading if you add cables to a Class D Amplifier. The RF emissions can affect other low level signals, like microphones, thereby corrupting acoustic crosstalk measurements. Those "speaker" wires are not carrying audio signals.
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