Audio Engineering

728 Chapter 24

24.2.3 What Passive Crossovers Look Like to Amplifi ers

For conventional full-range loudspeakers with passive crossovers, the crossover
components stand between the amplifi er and the speaker. Unless the speaker’s drive
unit(s) is/are blown or have been disconnected or removed, then the crossover won’t
usually be “ seen electrically ” (by the amplifi er) on its own. Figure 24.11 shows the
impedancethat would be seen by the power amplifi er if the loudspeakers were 8-ohm
resistors and how it drops from about 10.5 ohms to just over 5 ohms at the crossover
point. As both drivers are being driven at this point, it’s what you might expect. Figure
24.12 shows how much the picture changes when the resistors are replaced by real
speakers: now there are two dips.

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2WPXORZ2.CIR Temperature  27

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Ohms (log)

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v(1) Frequency (Hz)

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v(4) v(5) Frequency (Hz)

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LF rescue XOF HF dip

Impedance seen through a 2 way passive crossover

Bass Mid/High Bass

Individual drive unit impedances

Figure 24.12 : Here, the resistors are replaced by drive-units having the impedance

characteristics shown in the lower graph. The upper graph shows how the impedance seen

by the amplifi er has changed—notably two dips where there was one.