Is Damping Factor Effective?

Often we are told we need good amplifier damping factor to control the excursions of loudspeaker drive units, but will damping factor exert all that control?

If your speakers have two or three drivers they will invariably use a passive circuit called a crossover. This circuit uses inductors and capacitors, and often resistors, to split the frequencies to the different drive units.

Inductors will have no impedance below their operating frequency of course... think again!

Inductors are windings of a long piece of copper wire. They have a DC resistance, and although not big, it's big enough to give some series resistance right down to DC.

To keep this resistance low, large cross-section wire sizes are used, but even then there will be resistance.

The woofer has to have the largest inductor, and that means more turns, so will be quite a length of wire - and it will be quite resistive. Values between 0.1 Ohms to 0.4 Ohms being quite common.

The inductor is in series with the amplifier output, and the amplifier damping factor might be quite fantastic, but even the most rugged of inductors is going to be around 0.1 Ohms.

If we add 0.1 Ohms to the virtually zero output of a high damping factor amplifier, the inductor will dominate.

The damping factor due to the inductor is simply calculated as speaker impedance (let's call it 8 Ohms) divided by it's impedance including its DC resistance.

For the 0.1 Ohm inductor example, the damping factor cannot be any better than 8/0.1, which is 80, and so does an amplifier damping factor of say 400 really exert all that control? No.

And not all speakers will have heavy duty inductors. One I came across is 0.4 Ohms, and the damping factor there will be 20.

And the above doesn't include for the rising impedance of the inductor with frequency, so in real terms, damping factor will be even less.

So should we worry about amplifier damping factor too much?