Professional audio amplifier output matching circuit

Output matching circuit of audio amplifier.

The role of the resistor in the picture is to limit the current at a higher frequency to reduce the rating requirements for the capacitor. The resistance value of the resistor ranges from 4.7Ω to 10Ω. In most cases, the value of the capacitor is selected to be 0.1μF to reduce the load effect of the output speaker at high frequencies. As the output level increases, the current captured into the 'Zobel' network will also increase. Therefore, the rated power of the components in the output network must be sufficient under any conditions, and it is recommended to be 3W to 5W. Affected by the output impedance, cable impedance, and inductance of the amplifier connected in series, the frequency response of a real speaker will exhibit unstable characteristics, which is more complicated than a simple parallel resistor and capacitor. This phenomenon may cause high-frequency instability.

A small coil inductance in series with the output of the amplifier will increase stability. This approach can isolate the amplifier from the associated capacitors without causing significant losses at the audio frequency. The inductance value is generally 1 to 7μH. Choosing the appropriate inductance value can avoid high-frequency roll-off under a certain load impedance. It is recommended to use air-core inductors to avoid magnetic saturation. In addition, it is also recommended to add a damping resistor across the output coil to reduce the Q factor, overshoot and ringing of the output LC network. Traditionally, winding damping resistors with several ohms have been used to avoid self-inductance. The lower the resistor, the less effective the overshoot and ringing. Because the size of the output current depends on the output power and load impedance. Therefore, 1W to 5W rated damping resistor should be selected.