Understanding Active Loudspeaker Crossovers
A correctly set up active sound system can have greater clarity, realism, and freedom from inter-modulation distortion

September 14, 2011, by John Lenard Burnett

crossovers
This article is provided by Lenard Audio.

 
Read John’s introduction to crossovers here and his discussion of passive crossovers here.

Active systems can be 2-way, 3-way, or 4-way. 

The line level signal from the preamplifier or mixer is electronically filtered into the correct frequency bands to be sent to independent amplifiers that drive the bass loudspeaker, mid loudspeaker and tweeter separately. 

The auditory difference between an active and passive loudspeaker system is noticed by everyone. A correctly set up active sound system can have greater clarity, realism, and freedom from inter-modulation distortion. 

For an active system to be compact and affordable the amplifiers can be included within the loudspeaker cabinets.

Traditionally, the audio retail market had been opposed to selling active systems, but this is changing. Audiophile retailers make greater profits marketing magic and mystical components, not technology. 

Magical loudspeakers connected to magical amplifiers with magical cable producing a magical sound which only they can hear. This may sound silly but these fraudulent business practices are real and highly profitable. 

The pro audio market is mostly driven by brand image and trends, rather than technology. Merchandisers gain greater control in how they profit from a sale by selling loudspeakers, cable and amplifiers separately.

The primary problem is greed, ego, ignorance, fear of technology and lack of education. Also, most sound engineering courses are superficial with little or no meaningful electro-acoustic content.

An active system incorrectly set up will perform worse than a passive system. Active systems require technical skill to set up correctly. People who make loudspeaker systems sometimes do not have electronic knowledge. Most electronic people have limited electro-acoustic knowledge. Most active applications are 2-way. An extra amplifier and loudspeaker for sub-bass. 

Active systems are often used for high-powered professional applications. Active is rarely taken to its fullest extent as a 4-way across the frequency spectrum, because of cost, time and limited technical understanding to manage the complexities.

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Electronic crossovers can be purchased as a rack mount unit 2-way, 3-way for stereo but rarely as a stereo 4-way. Crossover points can easily be changed. 

Slope (2nd-order) 12 dB/octave (Q 0.707) -3 dB at crossover point (Butterworth) will give the best performance with least complications. Higher order slopes 4th-order 24 dB/octave with -6 dB at the crossover points can be utilized to provide greater control over the loudspeakers, but are more critical to calibrate.

The compression driver and bullet tweeter must have a Capacitor in series with the amplifier. The turn on/off pulses and DC offset from amplifiers could easily destroy driver and tweeter. 47μF for the compression driver and 4.7μF for the bullet tweeter will provide an extra -6 dB/octave roll off. This will insure no bass energy will get to the drivers.

Linkwitz-Riley. Professional electronic crossovers referred to as Linkwitz-Riley, have fixed slopes, 4th-order 24 dB/octave (Q 0.5) and -6 dB at crossover point (Bessel). The crossover points can be changed.

This has been adopted as the standard for professional systems. The steep 24 dB/octave slopes give maximum protection, control and power capability for the drivers. This Linkwitz-Riley benchmark is a mathematical calibration that is dependent on the listener being unable to make changes once the system is calibrated. (www.linkwitzlab.com)

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Theory behind this method is comprehensive and can be questioned, not as being right or wrong, but suited to the outcome. One part of the theory states that, at the crossover point, sound comes from two sources, acoustic directivity is increased, therefore power to each speaker should be reduced -6 dB (1/4). 

This increased directivity is on-axis only, off-axis energy decreases. The total sound energy remains the same. For near-field monitor listening, -6 dB at the crossover point can be argued as correct, but for far-field -3 dB can be argued as correct.

This Linkwitz-Riley bench mark is dependent on the listener being unable to make changes after the system is calibrated. But with active systems, level to each loudspeaker can simply be changed to whatever balance the listener wishes to enjoy. 

Choice to change levels between loudspeakers for personal musical enjoyment makes bench marking technically meaningless. The decision between right and wrong, now becomes one of choosing or not choosing the right to choose.

Listen to a mid loudspeaker crossed over in isolation. 2nd-order 12 dB/octave crossovers attenuate the music without hearing a tone or color change. Higher order crossovers attenuate the music causing a subtle but noticeable tone or color change (hardening) in the music. When recombined with the other loudspeakers, this tonal change is not directly noticeable.

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Active crossovers are created by electronic L. C., resonant circuits, or software digital assimilations of them. The slopes of crossovers represent a part (one side) of a resonant note. This note is not heard directly, but the steep slopes of third and fourth order crossovers can reflect harmonic interference (sideband distortion) within the music. 

Early research referred to this ghost resonance as transient distortion. Conclusive research in this area has not been completed.

Chebychev, Butterworth, Bessel. These names are often used by people who wish to impress but do not know what they mean. They honor early physicists and represent the math behind filter shapes, used by electronic design engineers. The Q of the slope (Quality of resonance), refers to the circuit, and its D (Damping), including the geometry of shifting phase in the slope.

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Some electronic crossovers are designed for research and use computing power to change filter shapes, slopes, crossover points and time alignment. These crossovers are complex to set up.

Some live sound engineers love to randomly twiddle with these functions during live performances. This irresponsible behavior often guarantees distorted coloration in the sound system and destruction of compression drivers.

John Lenard Burnett is the founder of and product designer at Lenard Audio, a design and consulting company specializing in electro-acoustic engineering. He also has created the Lenard Audio Institute, an on-line educational resource.

 



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Understanding Active Loudspeaker Crossovers
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