The advantage of active crossover
What else is needed?
A passive crossover only uses components such as resistors, coils (inductors) and capacitors to divide the signal from the power amplifier into different frequency bands for the different drivers, such as woofers, midranges and tweeters. (The term “passive” refers to a device or circuit that cannot control the flow of electrons provided by a power source, such as a transistor or valve.)
The following diagram shows a typical system setup, where the volume control (volume control symbol) is performed in the preamplifier, and the passive crossover is located in the speaker cabinet. The diagram shows a second-order two-way crossover, which has relatively gradual cutoff slopes on the woofer and tweeter: more components will be needed for steeper cutoff slopes. Additionally, more complex speakers, such as three- or four-way speakers, will have many more components than those shown here.
Passive crossover diagram
Block diagram of the system with passive crossover
An active crossover, in contrast, divides the frequency band using the line-level signal and is typically placed between a preamplifier and power amplifiers. Each speaker driver has its own dedicated amplifier channel, as shown in the diagram below.
Connecting a driver directly to an amplifier channel improves the damping factor and gives the amplifier more "control" over the driver. (CAUTION: A capacitor is often used in series with the tweeter to protect it from possible low-frequency or DC transients, especially when turning the power on or off.) The large, often expensive passive components between the amplifier channels and the amplifier drivers speakers are no longer needed. This advantage is even greater for three-way and 4-way speakers, as they require higher component values for lower crossover frequencies.
Active crossover diagram
Block diagram of the system with active crossover (with analog inputs)
Until recently, most active crossovers were implemented with analog circuits, typically using operational amplifiers to realize specific types of circuit topologies. Switches or plug-in modules select different crossover frequencies. This type of active crossover is limited by the fact that each filter must be made with a physical circuit. For example, steepening the crossover slope would require additional analog circuitry, which isn't easy once a unit is in the field.
With modern DSP (digital signal processing) technology, active crossovers can be implemented entirely with digital computation. This means that audio processing can be changed much more easily, without any hardware changes. The amount of audio processing is limited only by the DSP power available. Digital crossovers also support direct digital input from a digital source, such as a computer or streamer. The following diagram shows a typical system setup, where volume control can be done digitally in the source or in the crossover itself. (Note that digital crossovers still support analog input as in the diagram above.)
Active crossover with digital input
Block diagram of the system with active crossover (with digital inputs)
The advantage of DSP
DSP digital crossovers incorporate many additional functions, enabled by the integrated DSP and very intuitive user interfaces.
Flexibility. Digital crossovers range from simple two-way configurations up to complex four-way (or even five-way) configurations. Here are some links for crossover design:
- Building a 2-way crossover - basic tutorial on how to build an active two-way speaker.
- 3/4 way stereo crossover - basic tutorial on how to build a 3/4 way active speaker.
Parametric equalization. All crossovers include extensive parametric equalization capabilities, for correcting speaker driver response, addressing room concerns, and tailoring overall system response. Below are some links for parametric equalization:
- PEQ vs Graphic EQ - basic explanation of parametric EQ and how it compares to a graphic EQ.
- Auto-EQ tuning with REW - how to use Room EQ Wizard (REW) to generate correction curves for your system and room.
Advanced biquad programming. DSP crossovers incorporate a feature that allows for nearly infinite flexibility and customization of driver and system response.
- Linkwitz Transform: How to implement a Linkwitz Transform to improve low-end response.
- Advanced Biquad Programming - learn more about advanced biquad programming.
Time delay/alignment. Time alignment across all output channels is essential to ensure smooth response across the crossover region. See the How to Time Align Speaker Drivers application note.
What else is needed?
In addition to the active crossover itself, you will need speakers with drivers. You can build a set from scratch or convert an existing speaker into an active speaker by removing the passive crossover.
You will need sufficient amplifier channels (amps for each driver or multi-channel amplifiers): one for each speaker driver. You may also already have enough stereo amplifiers available.
You will also need to be able to take acoustic measurements. We recommend the free Room EQ Wizard (REW) program, along with our UMIK-1 USB measurement microphone. We've put together an app note to show you what to do: Speaker Measurement with UMIK-1 and REW! (If you already have a microphone and/or measurement software, that's fine too, as long as you can get reasonably accurate measurements.)