IndexSignal & AmplificationOutput Impedance & Damping
Output Impedance & Damping
How source impedance and cable bend the response of a real loudspeaker load.
A single driver: resonance near 55 Hz and a rise from voice-coil inductance. Real multi-way speakers add a hump at each crossover.
Input
Solid state: about 0.01 to 0.1 Ω. Valve amplifiers: often 1 to 4 Ω.
How it works
Damping factor
Damping factor compares the speaker's nominal impedance with everything in series with it: the amplifier's output impedance and the resistance of the cable.
Why it changes the tone
Source resistance and speaker form a voltage divider. If the speaker were a plain resistor the only effect would be a small, even loss. A loudspeaker's impedance swings with frequency, so the division ratio swings with it:
Where the impedance is high (bass resonance, upper treble) more signal reaches the driver; where it dips, less. The response starts to trace the impedance curve. With 0.05 Ω the ripple is a few hundredths of a decibel. With 3 Ω it can exceed 2 dB, an audible tonal shift that is often credited to the amplifier's character.
How much is enough
Beyond a damping factor of about 20 the speaker's own voice-coil resistance, which sits in the same loop, dominates and further gains are inaudible. Chasing figures in the hundreds is pointless if the cable adds 0.2 Ω.