Digital Crossovers

Digital crossovers are one of the key core strengths of the miniDSP range of products. In this app note, we'll provide an overview of digital crossovers and how they differ from passive crossovers and other types of active crossover. With a high degree of flexibility, user-friendly interfaces, and unbeatable value, miniDSP digital crossovers offer the perfect solution for both the DIYer and the loudspeaker/system professional.

Passive crossovers [Top]

A passive crossover uses only components such as resistors, coils (inductors), and capacitors to divide the signal from the power amplifier into different frequency bands for the different drivers — woofer, midrange, tweeter, for example. (The term "passive" refers to a device or circuit that is not able to control electron flow supplied from a power source, like a transistor, tube, or opamp.)

The following diagram shows a typical system configuration, where volume control (volume control symbol) is done in the preamp and the passive crossover is located in the loudspeaker 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. Also, more complex speakers, such as a three-way or four-way, will have a lot more components than shown here.

Passive crossover diagram
Block diagram of system with passive crossover

Active crossover [Top]

An active crossover, in contrast, divides the frequency band using the line-level signal and is typically positioned between a preamplifier and the power amplifiers. Each loudspeaker driver has its own dedicated channel of amplification, as shown in the diagram below.

Connecting a driver directly to an amplifier channel improves damping factor and gives the amplifier greater "control" over the driver. (A capacitor is often used in series with the tweeter to protect it from possible low-frequency or DC transients, especially at turn-on or turn-off.) The large — often expensive — passive components between the amplifier channels and the speaker drivers are no longer needed. This advantage is even greater for three-way and 4-way speakers, as they need larger component values for lower crossover frequencies.

Active crossover diagram
Block diagram of system with active crossover (with analog inputs)

Digital crossover [Top]

Until recently, most active crossovers were implemented with analog circuitry, typically using op-amps to realize specific types of circuit topologies. Switches or plugin-modules select different crossover frequencies. This type of active crossover is limited by the fact that each filter has to be realized with a physical circuit. For example, making the crossover slope steeper would require additional analog circuitry - not 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 the 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. The following diagram shows a typical system configuration, where volume control can be done digitally either 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 system with active crossover (with digital inputs)

The miniDSP advantage [Top]

Digital crossovers from miniDSP incorporate many additional functions, enabled by flexible onboard DSP and our friendly user interfaces. We have also put together an extensive library of application notes to help you make the most of these features!

Flexibility. miniDSP crossovers span the range from simple two-way up to complex four-way (or even five-way) configurations. The following app notes explain how it's done:

Parametric equalization. All our crossovers include extensive parametric equalization capabilities, for correction of loudspeaker driver response, addressing problematic room modes, and for tailoring overall system response. Please see the following app notes:

Advanced biquad programming. The miniDSP crossovers incorporate a feature that allows almost infinite flexibility and customizability of driver and system response.

Time delay/alignment. Time alignment on all output channels is essential to ensure smooth response through the crossover region. See the application note How to time-align speaker drivers.

What else is needed? [Top]

In addition to the active crossover itself, you will need loudspeaker boxes with drivers. You can either build a set from scratch, or convert an existing loudspeaker into an active speaker by removing the passive crossover.

You will need sufficient channels of amplification: one per loudspeaker driver. You may even have enough stereo amplifiers available already. Otherwise, multi-channel amps intended for home theater applications will work well and are available at very reasonable prices, and if you have a DIY inclination then Class D power amplifier circuit boards with two, four, and even six channels are also available. Or, our PWR-ICE series of plate amplifiers might be a perfect match for your project.

You will also need to be able to perform acoustic measurements. We recommend the free program Room EQ Wizard (REW), together with our UMIK-1 USB measurement microphone. We've put together an app note to show you what to do: Loudspeaker measurement with UMIK-1 and REW! (If you have an existing measurement microphone and/or software, that is fine too, as long as you are able to obtain reasonably accurate measurements.)

Finally, you will need to be willing to learn! Mastering active crossovers is an adventure, and we're delighted to be part of it. Please register and join in on our forum to join other like-minded folks building active speaker systems.