Signal Processing Fundamentals: Passive & Active Crossovers

A loudspeaker array is the optimum stacking shape for each set of cabinets to give the best combined coverage and overall sound.

You’ve no doubt seen many different array shapes. There are tall towers, high walls, and all sorts of polyhedrons and arcs. The only efficient way to do this is with active crossovers.

Some smaller systems combine active and passive boxes. Even within a single cabinet it is common to find an active crossover used to separate the low- and mid-frequency drivers, while a built-in passive network is used for the high-frequency driver. This is particularly common for super tweeters operating over the last audio octave.

At the other end, an active crossover often is used to add a subwoofer to a passive 2-way system. All combinations are used, but each time a passive crossover shows up, it comes with problems.

One of these is power loss. Passive networks waste valuable power. The extra power needed to make the drivers louder, instead boils off the components and comes out of the box as heat—not sound. Therefore, passive units make you buy a bigger amp.

A couple of additional passive network problems has to do with their impedance.

Impedance restricts power transfer; it’s like resistance, only frequency sensitive.

In order for the passive network to work exactly right, the source impedance (the amplifier’s output plus the wiring impedance) must be as close to zero as possible and not frequency-dependent, and the load impedance (the loudspeaker’s characteristics) must be fixed and not frequency-dependent (sorry, not in this universe; only on Star Trek).

Since these things are not possible, the passive network must be (at best), a simplified and compromised solution to a very complex problem. Consequently, the crossover’s behavior changes with frequency—not something you want for a good sounding system.

One last thing to make matters worse. There is something called back-emf (back-electromotive force: literally, back-voltage) which further contributes to poor sounding speaker systems.

This is the phenomena where, after the signal stops, the speaker cone continues moving, causing the voice coil to move through the magnetic field (now acting like a microphone), creating a new voltage that tries to drive the cable back to the amplifier’s output! If the speaker is allowed to do this, the cone flops around like a dying fish. It does not sound good!

The only way to stop back-emf is to make the loudspeaker “see” a dead short, i.e., zero ohms looking backward, or as close to it as possible—something that’s not gonna happen with a passive network slung between it and the power amp.

All this, and not to mention that inductors saturate at high signal levels causing distortion—another reason you can’t get enough loudness. Or the additional weight and bulk caused by the large inductors required for good low frequency response. Or that it is almost impossible to get high-quality steep slopes passively, so the response suffers.

Or that inductors are way too good at picking up local radio, TV, emergency, and cellular broadcasts, and joyfully mixing them into your audio.

Such is life with passive loudspeaker systems.