In an ideal world, a loudspeaker should present a resistive
load to the power amplifier. However, in the real word,
it is far from this.
majority of moving coil loudspeakers are reactive loads.
In electrical terms, what this means is the resistance
is not constant but changes with frequency. Reviews frequently
emphasize how low the impedance of a speaker as a measure
of how demanding the loudspeaker is to the power amplifier.
by itself, is not the only parameter affecting amplifier.
What is often taken for granted is the electrical phase
displacement. This can place even more stress on the power
amplifier than low ohms.
go back to some basic
electrical fundamentals. The best power transfer is
when the voltage and current are in phase. This would
be a purely resistive load, like a filament bulb. What
happens when the load is purely capacitive? There will
be a phase shift where the voltage lags the current by
90 degrees. When the load is purely inductive, the opposite
happens, ie the voltage leads the current by 90 degrees.
Whether it leads or lags, the end result of phase displacement
is more power needs to be delivered to the load.
Take Swift-III as an example (Fig 29). This kind of impedance
and phase responses are typical in speakers (Fig 30).
In the crossover region, the power amplifier is subjected
to two different demands.
is Impedance. The impedance at 2 ohms means the speaker
wants more power from the amplifier.
is the phase shift. Quite a bit of power is not used by
the speaker due to the +/- 45 degrees phase shift. To
make up for it, the power amplifier must now deliver more
power than normal. Looking at it another way, Phase Shift
= Power Inefficiency.