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# What is Slew Rate and Damping Factor?

by August 30, 2004

Q: What are slew and dampening factor? I saw it asked in one of your forums and its not in your glossary.

A: Slew Rate: This is a term used to describe how quickly the output of an amplifier can track its input. Slew Rate is usually measured in V / usec. The higher the value (up to a point), the better the amp is at potentially reproducing the subtle nuances and dynamics associated with music reproduction.

Damping Factor: This is a quantity which defines how quickly the amplifier can stop a reproduced frequency such as a bass note. The higher the damping factor, the better the amp will control the woofer and help reduce overhang distortion (again to a point). The damping factor of an amplifier is mostly dependent on the output impedance of the power amplifier and the ability of the power supply which feeds the power amp.

Damping factor is inversely related to amplifier output impedance so if the output impedance is low, than the damping factor is high.

Amplifier Output Impedance - This again is an important metric as it determines the amount of interaction or frequency response variation an amplifier may produce when driven by a reactive loudspeaker load. Usually lower is better, but some do prefer high output impedance tube amps for example since they can excessively roll off high frequency response and result in a warm sound.  We like to see amplifier output impedance at 100mohms or lower for the entire audio bandwidth.  Class D amps tend to have higher output impedance because of the output filter.  The Axiom A1400-8 as shown below is an exception to what we've seen with most Class D amps.

Solve for Zo = > Zo = RL*Vopen/VLoad - RL

Damping Factor = Zload / Zo

Where RL = Load Resistance; Zo = Amplifier Output impedance