The EM Poll

Tech Basics Without Tears

Mar 14, 2008 5:29 PM, By Jon Chappell With Steve Oppenheimer

           

A PAIN-FREE EXPLANATION OF FIVE CONCEPTS ALL MUSICIANS SHOULD UNDERSTAND

Crossover Hits

The best way to deliver highs and lows is through two or more separate speakers, each optimized for that particular range. A woofer, or midrange speaker, simply can't deliver high frequencies the way a smaller tweeter or horn dedicated to that purpose can, and a tweeter or horn is similarly ill equipped to handle the low frequencies that a woofer is accustomed to belching out.

The question is, how do you then direct only the appropriate frequencies to each speaker: lows to the woofer and highs to the tweeter? The answer lies in the crossover circuitry, of which there are two types: active and passive.

Active Crossovers

An active (powered) crossover is more complex and expensive than a passive system, but it's much more efficient and versatile. In an active crossover system, the different frequency ranges are isolated before the amplifier stage, and then each band is sent to a separate amplifier that only needs to reproduce frequencies in that range.

Because active crossovers use powered electronic circuitry, they lose less signal power and can incorporate more precise and sophisticated filters than passive crossovers can. Since each range is amplified separately, you control the power to each element and can put separate protective limiters for the highs, mids, and lows (in a triamped system). To top it off, you can easily control the crossover frequency via a knob, a feature that would require swapping out the electronic components in many passive systems.

Passive Crossovers

In a passive (unpowered) crossover, the signal is split after the amplifier stage, as it's on its way to the speaker. Simple in-line components, an inductor and a capacitor, act as a low- and highpass filter, respectively.

Figure 2: In a typical two-way crossover, a capacitor (C) acts as a highpass filter and an inductor (L) as a lowpass filter.

In a two-way passive speaker system (woofer and tweeter or horn), current on its way to the woofer would flow through an inductor, which acts as a lowpass filter, allowing only the lows to reach the speaker. For the signal going to the tweeter, the current flows through a capacitor acting as a highpass filter, blocking lows and allowing only highs to pass (see Fig. 2).

In a three-way system — with separate drivers for high, mid, and low frequencies — the current on its way to the midrange speaker would flow through both an inductor and a capacitor, which would block lows and highs, allowing only the middle range to pass (that is, acting as a bandpass filter). A typical division in a three-way system would send the 30 to 800 Hz frequencies to the bass speaker; 800 Hz to 6 kHz to the midrange speaker, and 6 kHz to 20 kHz to the high-end driver.

Passive systems have the advantage of being simple and inexpensive, but they can present problematic loads for power amps at some frequencies, and they add some resistance to the circuit, so the system's sound quality can suffer, especially at higher power levels and at lower frequencies. That said, they can be an appropriate choice for smaller P.A. systems.

If you have to use separate speaker systems for three separate frequency bands, or you have a large P.A., use subwoofers, or for other reasons are using beefier power amps, go with an active crossover. If you just need a simple, two-way system and aren't using large amounts of power, a passive crossover will probably do the job.