Your beautiful Class-AB audio amplifier measures perfectly on a resistive dummy load, then sings into 20 kHz oscillation the moment you connect a real speaker. Welcome to the reactive-load stability problem — and its 100-year-old solution: the Zobel network, also called a Boucherot cell.

A loudspeaker is not an 8 Ω resistor. It's a voice coil in series with a mechanical resonance, presenting inductance that rises with frequency. At 20 kHz, an "8 Ω" driver might look like 8 Ω in series with 30–100 µH — an impedance of 20–50 Ω or more, with a phase angle approaching +90°. Your amplifier's feedback loop was designed assuming a resistive load. Feed that loop a load whose phase rotates 90°, and your carefully tuned phase margin evaporates. The result: parasitic oscillation, tweeter burnout, and a smoke show.

The Zobel network is a series R-C snubber placed directly across the amplifier's output terminals, right at the output stage. Typical values for an 8 Ω speaker amp: a 10 Ω resistor in series with a 100 nF capacitor. At DC and audio frequencies, the capacitor is high-impedance and the network is invisible. Above the audio band, the capacitor's impedance drops until the network looks like a pure 10 Ω resistor across the output — masking the speaker's inductive rise and giving the feedback loop a well-behaved, resistive load to see.

Design rule of thumb: Choose R roughly equal to the nominal load impedance (or slightly higher). Choose C so the corner frequency f = 1 / (2πRC) sits at the top of your audio band — typically 100–200 kHz. For 10 Ω and 100 nF, f = 1 / (2π × 10 × 100e-9) ≈ 159 kHz. Below that, the network is invisible; above it, it dominates.

Real-world example: Nearly every commercial audio power amplifier has a Zobel across its output binding posts, often paired with a small series output inductor (5–10 turns around a resistor) that isolates the amp from capacitive cables. This "Thiele network" combination — Zobel plus series L — is the standard defense against both inductive speaker loads and capacitive electrostatic-speaker or long-cable loads.

Watch the resistor power rating. A high-frequency oscillation or a program-material transient with lots of ultrasonic content will dump real watts into that 10 Ω. Use a 2–5 W non-inductive wirewound or metal-film resistor. And place the Zobel physically at the output devices, not at the connector — inches of trace inductance between amp and network will re-introduce the very phase shift you're trying to cancel.