Active vs. Passive Loudspeakers – What’s the difference?

Despite the fact that the majority of loudspeakers designed for studio monitoring, including all ADAM Audio studio monitors, are active, the world of consumer hi-fi is still dominated by passive loudspeakers. So, in this blog post we’re going to unpack the differences between active and passive loudspeakers, and unpick some of their different technical characteristics.

The world’s first active studio monitor was the Klein & Hummel “OY” which was launched in 1967. This three-way design had an electronics backplate with XLR input, acoustical controls, three-way active crossover, system protection, three amplifiers, power supply, and even some directivity control in the form of a horn loaded tweeter. The next significant milestone was the Genelec S30 launched in 1978, which had a vertical rather than horizontal layout and a ribbon tweeter, but it was also a three-way design and remarkably similar to the OY in its general concepts. Many other designs by these two companies were launched in the 70s and 80s.  Then came the two-way Meyer HD-1 in the early 90s. Despite these long running products dating back many decades the market was predominant a passive one until the early 2000s when active designs were truly embraced by almost all studio monitor brands. Passive loudspeakers driven by remotely located amplifiers and connected by lengths of loudspeaker cable – much like most consumer hi-fi loudspeakers. So, now we have a timeline for the divergence of active monitors and passive hi-fi loudspeakers, it’s probably an appropriate point to describe their fundamental differences.

Passive

In a passive loudspeaker, the drivers are connected to a single power amplifier channel via a crossover circuit comprising an arrangement of inductors, capacitors and resistors. The crossover circuit divides the incoming full-range audio signal into frequency bands appropriate for the different loudspeaker drivers and matches their overall sensitivity. In a two-way loudspeaker, for example, the crossover will comprise a low-pass filter feeding the low frequency driver (woofer) and a high-pass filter (usually with an attenuating resistor) feeding the high frequency driver (tweeter). The diagram below illustrates the schematic of a simple two-way passive crossover.

Active

An active loudspeaker is effectively defined by the position of the crossover in the signal chain; it comes before, rather than after, the power amplification, as illustrated in the diagram below. However, this crossover position implies that, with the signal divided into frequency bands before the amplification, multiple power amplifiers are needed: one for each frequency band (and driver). And along with resulting in multiple power amplifiers, the position of the crossover prior to the power amplifiers means that it operates at line level rather than at amplifier power level, and this has a number of significant technical consequences.

1. In operating at amplifier power level, passive crossover components are necessarily physically large. A passive crossover inductor for example comprises a coil of copper wire that may number hundreds of turns. It will as a result be expensive and heavy, but more importantly, it will display significant electrical resistance that results in signal attenuation. Inductors can also saturate electrically and cause distortion. Passive crossover components generally are far from perfect inductors and capacitors, so they invariably result in some degree of signal loss. In contrast to the lossy and non-ideal nature of passive crossover components however, active crossover components are typically extremely compact and demonstrate behaviour much closer to their idealised characteristics. There is also an advantage from the direct connection of the drivers to the power amplifier outputs, rather than via non-ideal passive crossover components. The advantage arises because direct connection optimises the dynamic control the power amplifier has over the drivers.
2. Secondly, in being located between the power amplifier and the drivers, passive crossovers are affected by the variable impedance of the drivers to the extent that the behaviour of the loudspeaker can vary depending on both frequency and volume level. Conversely, with the separation provided by the power amplifiers between an active crossover and the drivers, the downstream impedance seen by the crossover is fixed, so no response errors occur.
3. Active crossover filters can be engineered to be much more intricate and accurately targeted than passive crossovers. For example, the passive crossover filter slopes are typically 1st or 2nd order resulting in 6-12 dB/octave stop-band attenuation. With active crossovers however, one typically sees 4th order (24 dB/octave) in analog designs and higher in DSP designs. Active crossovers can also incorporate targeted EQ beyond simple filtering, or even inter-driver time delays, that are both well beyond the capabilities of passive crossovers.
4. Finally, power amplifier level crossovers heat up during operation and electronic components that change temperature change their properties, e.g. resistance increases with increasing temperature which changes the shape of the filtering. An active low-level crossover positioned before the amplifiers do not heat up much and therefore the system maintains its performance no matter how loud the loudspeaker is playing.

So the technical characteristics of active crossovers already suggest that active loudspeakers ought to demonstrate some significant performance advantages over their passive siblings, and in general, if appropriately designed and engineered, they do. However, as we’ve already mentioned, the majority of consumer hi-fi loudspeakers are passive, so why is that?

Firstly, installation factors play more of a role in consumer hi-fi than they do in professional audio, and in not requiring connection to mains power, passive loudspeakers are often easier to install in home environments. A stereo pair of loudspeakers positioned, for example, three metres apart, might not both be within easy reach of a mains power socket, and that can be a mundane yet significant issue in a domestic living room. Secondly, hi-fi amplifiers are usually integrated units that incorporate pre and power amplifier functions in one box, so the opportunity to route line-level signals to active loudspeakers isn’t typically available. And thirdly, the consumer hi-fi speaker market is extremely price competitive, so the manufacturing cost of extra power amplification, especially at entry-level and mid-market price points, is often not affordable.

The long discontinued ANF10 2-way nearfield monitor – The only passive studio monitor in ADAM Audio’s history of 25 years.

Finally, the crossover architecture and engineering of a loudspeaker is but one element of a complex whole. There are many other engineering and design factors that can equally fundamentally influence performance so, it’s invariably a mistake to assume a single technical element of any loudspeaker is its defining feature. And, although we’ve described a bunch of technical advantages of the active format, they don’t negate the fact that passive loudspeakers can be designed and engineered to reach extremely high levels of performance. There are passive hi-fi loudspeakers out there that are capable of giving any active studio monitor some serious competition.