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Question 925

What is a Zobel network in an audio amplifier and how does it work? Is it an effective device or not? — CV, Cape Town, South Africa
My understanding is that a Zobel network consists of a resistor in series with a capacitor and that the capacitor is normally connected to ground. When you attach the free end of this network to a wire carrying an audio signal, the network acts like a frequency-dependent load. At very low frequencies, the capacitor has plenty of time to charge through the resistor and the network has little effect on the audio signal—it acts as though it weren't there. At very high frequencies, the capacitor has no time to charge through the resistor and behaves like a wire. As a result, the network acts as though it were just the resistor connecting the audio signal wire to ground. So the impedance of the Zobel network varies from infinite at low frequencies to become equal to the resistance of the resistor at high frequencies. The crossover between these two behaviors is related to the RC time constant. I think that Zobel networks are used in audio amplifiers to dampen out high frequency oscillations that might occur in the absence of loads at high frequencies.

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