Active vs Passive Loudspeaker Design

Passive Loudspeaker Systems and Crossovers

A passive loudspeaker system requires a power amplifier to produce very large current swings due to parallel connections of drivers in the passive crossover circuit. A very stable amplifier design is required also because of the highly reactive character of the passive filter needed for this. The passive filter must handle the full audio bandwidth with minimum distortion. All of this might mean a heavy and expensive design.

When the passive loudspeaker is driven the electrical interaction of the drivers and the non-linearity in the crossover circuitry can cause undesired electrical changes in the loudspeaker performance.

A passive crossover is made entirely of passive components, arranged most commonly in a Cauer topology to achieve a Butterworth-type frequency response in each driver output channel of the crossover filter.

Passive crossover filters use non-reactive resistors and primarily reactive components (capacitors and inductors). Very high performance passive crossovers are likely to be more expensive than active crossovers.

The components capable of good performance at the high current and voltage levels the loudspeaker systems are driven at, are hard to make and therefore expensive. Polypropylene, metallized polyester foil, and paper-electrolytic capacitors are common. Inductors may have air cores, powdered metal cores, ferrite cores, or laminated silicon steel cores, and are mostly wound with enameled copper wire. Some passive networks include additional devices such as fuses, temperature-dependent resistors (PTC devices), bulbs or circuit breakers, to protect the loudspeaker drivers from accidental overloading.

Modern passive crossovers increasingly include impedance equalization networks (also called Zobel networks) that compensate changes in the load impedance presented by the loudspeaker across the audio frequency range, a property inherent in virtually all loudspeakers. The issue is complex as part of the change in the loudspeaker electrical impedance is due to acoustic loading changes across a driver's frequency range.

Problems with passive crossover networks include,

• Passive networks may be expensive if executed properly. They can be bulky and cause power loss.
• Passive crossover networks are not only frequency-specific, but also load impedance specific. This prevents interchangeability of crossover filters or filter components with speaker systems of different impedances and designs.
• An ideally working crossover filter including load impedance equalization networks, can be very difficult to design as passive electronic components behave and interact in complex ways.

Active Loudspeaker Systems and Crossovers

An active crossover filter built in an active loudspeaker system can be very precise. The power amplifiers are connected directly to the drivers of an active loudspeaker, resulting in the power amplifier’s load becoming much simpler and well known. Each driver-specific power amplifier has only a limited frequency range to amplify (the power amplifier is placed after the crossover) and this adds to the ease of design.

An active crossover contains electronic amplifier components. Active crossovers are operated at signal levels suitable for power amplifier inputs in contrast to passive crossovers that operate at the high signal levels of the power amplifier's outputs, having to handle high currents and in some cases high voltages.

Active crossovers are followed by power amplifiers for each driver output. Thus a 2-way active crossover needs two power amplifiers — one for the woofer and one for the tweeter. This means that an active loudspeaker using the active crossover approach will often cost more than a system using passive crossover filters although none of the power amplifiers needs to provide an output as high as that of an equivalent sound level full-frequency-band power amplifier in a passive system, which on the other hand can reduce costs.

Any cost and design complications in systems using active crossovers are offset by the following gains:

• The frequency response becomes independent of any dynamic changes in the driver's electrical characteristics or the drive level.
• There is an increased flexibility and precision to adjust and fine tune each output frequency response for the specific drivers used.
• Each driver has its own signal processing and power amplifier. This isolates each driver from the drive signals handled by the other drivers, reducing inter-modulation distortion and overdriving problems.
• The ability to compensate for sensitivity variations between drivers.
• The possibility to compensate for the frequency and phase response anomalies associated with a driver’s characteristics within the intended pass-band.
• The power amplifiers are directly connected to the speaker drivers, maximizing the control exerted by power amplifier damping on the driver’s voice coil, reducing the consequences of dynamic changes in the driver electrical characteristics. This may improve the transient response of the system.
• There is a reduction in the power amplifier output requirement. With no energy lost in the passive crossover filter components, the amplifier power output requirements are reduced considerably (by up to 1/2 in some cases) without any reduction in the acoustic power output of the loudspeaker system. This can reduce costs and increase audio quality and system reliability.

The flat frequency response of a high-quality active loudspeaker is a result of the combined effect of the crossover filter response, power amplifier responses and driver responses in a loudspeaker enclosure.

Using the active approach enables frequency response adjustments and optimization of the full loudspeaker system, placed in various room environments, without expensive external equalizers. The end result is a simpler, more reliable, efficient, consistent and precise active loudspeaker system.