Crossovers WHY go active?

Many DEQX owners first use their PDC or HDP preamp-processor to correct their passive speakers, capitalizing on its unique time and frequency domain processing capabilities.

This produces great results, but additional benefits can come from engaging the active crossover features; running each driver (bass, mid, tweeter) independently via dedicated power amplifiers

The short answer is that going fully-active typically results in dramatic improvements, particularly with regard to lower distortion and improved dynamics.

Passive speakers have been the practical, low cost hifi solution for consumer speakers for more than 80 years and, unlike pro-audio speakers, they are usually passive.

As a result, they remain the weakest link in affordable hi-fi systems by a widening margin as higher resolution digital audio becomes more common.

When a passive crossover is placed between a power amplifier and speaker drivers it must handle high voltage and high current (i.e. all of the power) so its design is kept as simple as possible.

In contrast, active crossovers, whether analogue or digital, work at line level and, therefore,
a small fraction of a watt. This allows more stringent and effective designs.

A legacy analogue active speaker typically exhibits reduced distortion, as compared to its passive cousin, primarily by using higher crossover filter slopes. The reduced dynamic driver distortions include: driver Inter-Modulation (IM), crossover and harmonic distortion from non-linear operation. Professional monitor speakers are almost exclusively active. A popular arrangement uses 4th order (24 dB/octave) filters feeding an internal 50 Watt power amplifier for the tweeter and 100 to 200 Watt amplifier for midrange and bass duties.

The reason for the relative improvement over passive crossovers is that the high power passing through passive crossovers implies shallow filter slopes of only 1st or 2nd order (6 dB/octave or 12 dB/octave, respectively). Some speakers use steeper 3rd order filters, which become expensive as tolerances are harder to maintain and the stray resistance in components tends to increase, potentially eroding the benefits of those higher-order filters. In contrast, active crossovers allow tight tolerances and steeper filters at lower cost, although of course additional amplifiers are needed to power each driver. However, now each amplifier can be right-sized for its mating driver. For example, a tweeter amplifier might be an elegant 25W chip amp compared to a 100-watt Class-A/B bipolar amp for mid or bass drivers. Audiophiles tend to favour some amplifiers for highs, and others for mids and bass. Low power S.E.T’s or Class-A might be used for highs, Class-A/B amps for the midrange and, increasingly, Class-D for bass or subwoofers. An active configuration naturally allows one to mix-n-match amplifiers for the best overall results.

There have been three generations of active crossover design:

Traditional; the classic analogue active crossovers typically use 12dB/octave to 24dB/octave filters, sometimes with one or more bands of EQ for correction of the most significant frequency-response errors.

Basic Digital; classic DSP crossovers mimic traditional analogue filters such as Butterworth and Likwitz Riley, but in the digital domain. These crossovers typically offer slopes from 6 to 24 dB/octave or steeper. Sometimes, multiple bands of EQ and/or time-delay adjustment for each output are included. The trade-off is that some of the advantages over a classic analogue active filter can be compromised by lost analogue resolution due to ‘budget quality’ A-to-D and D-to-A converters and support electronics.

Advanced DSP; capable of producing Linear-Phase crossovers implemented via long FIR (Finite Impulse Response) filters. These are capable of adjusting the signal phase to different degrees at multiple frequencies to maintain actual linear-phase response. Unfortunately, regardless of the DSP engine’s computing power, FIR processing normally adds hundreds of milliseconds of latency (delay) to the audio signal. This can be devastating when trying to synchronise audio with video, or when processing live signals, which has limited availability of such otherwise powerful filters. DEQX, a pioneer of Linear-Phase crossovers, has developed a patented technique that allows FIR/IIR filters to be implemented that exhibit very low latency of only about ten milliseconds. With this technology, DEQX takes Linear-Phase crossover design to its logical conclusion by combining the low-latency, phase-correct crossover filters to provide detailed correction of each speaker driver in both amplitude and phase and then aligning the inter-driver timing – all at each of hundreds of different frequency groups.

To evaluate the actual degree of time and amplitude errors produced by the speaker drivers at all frequencies, DEQX’s correction and crossovers must be computed using special software. The software enables the processor to ‘listen’ to each of the drivers during the setup procedure, taking actual anechoic (mids and highs) and room (bass) measurements. That data is analysed and the correction and crossover filters are computed according to the user’s requirements, and then loading into the processor for day-to-day use. Additionally, the user can adjust up to 10 bands of parametric filters – in real-time – for ‘voicing’ to personal preferences. The user can also save multiple profiles, each customized for their specific applications.

Of course, even speaker wire can affect IM distortion when large bass currents must share the same conductor with smaller high frequency currents in a single-way passive speaker configuration. In the high-end audio world, dominated by passive speakers, even the subtlest advantages provided by exotic speaker cable is extolled by vendors and magazines. On the other hand, active systems make the choice of speaker cables and interconnects far less critical; simply because those undesirable IM distortions have been addressed at their source!

Active - how many ways?

DEQX-HD™ supports a 3-way active regime because most affordable speaker drivers can perform well over only three to four octaves out of ten (20Hz–20kHz). Co-incidentally, this can
be described as the three decades of the audio spectrum:

BASS:	20 Hz to 200 Hz (3.3 octaves)
MIDRANGE:	200 Hz to 2 kHz (3.3 octaves)
HIGHS:	2 kHz to 20 kHz (3.3 octaves)

Speaker designs often vary these crossover points by an octave or so either way depending on the design and drivers employed, while 2-way designs normally only use one driver from bass and midrange. For serious audiophiles and professional audio engineers alike, a system using sensibly chosen amplifiers in an active arrangement often represents little or no additional cost compared to the high-end, high-cost passive alternative. This is especially the case when one considers the total cost of a heroic audiophile amplifier that must drive ten octaves of signal frequency, perfectly, into equally heroic passive crossovers and custom drivers through ‘transparent-at-all-frequencies’ cables. In an active configuration, each amplifier is only required to deliver three to four octaves (one decade) of bass, mid- or high-frequency energy, instead of the usual ten, and as a result tends to perform more effortlessly and transparently regardless of complex dynamics.

The pros and cons of shallow and steep filter slopes

Sporting shallow filter slopes, a passive crossover struggles to keep one driver’s output from interfering with its neighbouring driver. This is especially true over the two to three octave range on either side of where the driver’s frequency response, distortion and dispersion are most desirable. Steeper slopes also help by reducing driver cone motion in response to ‘out-of-band’ signal energy that exacerbates dynamic cone break-up. This better quarantines each drivers low and high frequency output to their three or four octave ‘comfort zone’, further minimising crossover and IM distortion and smoothing dispersion patterns.

Some audiophiles say they don’t like active systems because they use steep slopes, and it is true that many active systems use 4th order filters and beyond. However, as important as steeper slopes are to help minimise various types of distortion, there’s no requirement for active systems to use unduly steep slopes if good driver integration can be achieved with lower-order filters – and this is difficult to achieve with affordable drivers. To determine what filters slopes and crossover frequencies work best for any given design and driver combination, all DEQX processors provide the tools necessary to implement active crossovers with slopes from 6 dB/octave (as shallow as it gets) to 300 dB/octave (as steep as it gets). In fact four entirely different crossover set-ups can be stored for instant recall to allow A/B/C/D comparison until the right combinations are found. In fact several might ultimately be used for different types of music and chosen accordingly. A caveat here is that if DEQX’s patented Linear-Phase anechoic driver correction is to be used, the minimum slope is 48 dB/octave. If shallower slopes are desirable, correction of the shallower traditional crossovers can be accomplished with DEQX’s multiband minimum-phase parametric equalizer.

Examples

In a typical consumer 2-way speaker, the relatively shallow passive low-pass filter driving the
6” bass/midrange driver will allow significant high frequency energy to ‘leak’ into that driver – usually only 6dB to 12dB ‘down’ at twice the crossover frequency, resulting in significant cone break-up distortion and ‘beaming’. The similarly shallow high-pass filter supplying the tiny
1” tweeter struggles to protect it from low-frequency energy well below the crossover frequency and this causes tweeter distortion by pushing it towards its maximum excursion limits. When these distortions combine with each driver’s frequency-response and frequency related phase errors, the resulting output sounds congested, or… well … passive!

Leaving aside planar and electrostatic speaker drivers for the moment, though the same fundamentals apply, the diameter of typical round speaker drivers might be say; 12” for bass,
5” for midrange and 1” for highs. The bass driver will cross over to the midrange driver via a low-pass filter that starts attenuating its output above say 200 Hz at typically 12 dB per octave, so two octaves above that (~800 Hz), a metal cone driver will be ringing, or a paper cone is breaking up–either way is not good, yet this unwanted output is only 24 dB down only an octave or so away from the tweeter crossover. Meanwhile, the midrange driver’s high-pass filter only attenuates say 100 Hz bass energy by 12 dB, so in loud bass moments, its excursion tends towards maxing out where distortion and congestion set in. Several octaves higher, the same phenomenon is responsible for tweeter distortion, again causing congestion and loss of resolution.

Taking a driver’s cone excursion towards its limit moves it into a non-liner operating region where distortion skyrockets, especially for its higher break-up frequencies that the crossover attempts to quarantine. In a typical 2-way design, a 6” paper cone mid/bass driver might start breaking-up somewhere above ~700 Hz, which is an octave or two before the tweeter is supposed to take over. However, if a high-pass filter is added at ~200 Hz to minimise the 6” driver’s excursion, its break up will be less noticeable, perhaps to ~2 kHz, where the tweeter then takes over.

In a 3-way active design, filters of about 24dB/octave to 60dB/octave can often be used, dramatically reducing the ‘crossover region’ where unwanted driver outputs interact with each other. Meanwhile the steeper high-pass filters driving the midrange and tweeter better quarantine unwanted low frequencies reducing displacement and potential distortion accordingly. Likewise, steeper low-pass filters better quarantine low-definition midrange output from the bass driver and lower definition highs from the midrange driver. This usually also provides a more natural frequency dispersion because high frequency beaming is reduced from both the bass and midrange drivers.

Going Active + Calibrated with DEQX

A DEQX processor-crossover-preamp is truly a multi-purpose sonic repair kit. A unique strength is its patented ability to correct frequency-related timing (group delay) and frequency-response (amplitude) errors in the speakers, and for that matter, the amplifiers and cables driving them – because they’re all measured and accounted for. As a result, many DEQX users are happy to leave their passive crossovers in place and simply apply the automatic time and frequency response correction to the speaker’s native behaviour, optionally adding a touch of room correction afterwards.

Apart from their correction processing and EQ features, the DEQX PDC-2.6 and HDP-3 processors can provide 2- and 3-way active crossovers from their stereo analogue and digital inputs [4- and 5-way active configurations are possible using two units]. The setup software runs in a Windows™ PC environment (or hosted on a Mac), and provides the necessary tools
to measure, analyse and compare a wide variety of filter and correction options. With all crossovers and full correction running, the DSP engine’s 32-bit floating-point processing has essentially unlimited internal resolution and provides THD+Noise in the digital domain that’s below -140 dB. Extensive listening tests confirm no discernable quality degradation or fatigue caused by the digital processing – ignoring the tremendous sonic benefits of full-active calibration.

If an active speaker design is not being employed, a passive speaker an be made active, or partly active, by having its internal crossovers bypassed if bi- or tri-amping is provided for. DEQX processors allow four different setups to be stored and instantly recalled from using the remote control or PC. This allows direct comparison between shallow or steep, traditional or DEQX-HD™ linear-phase active crossovers for example, or all might be steeper crossovers but experimenting with different crossover frequencies or degrees of correction.

The DEQX Wizard set-up software makes the setup and calibration process relatively simple, with default values offered for most parameters. If you already have some knowledge of speaker design and crossovers, you’ll find a wealth of tools that make calibration, tuning and tweaking fast and efficient. The four presets can have subtle or dramatically different configurations stored for easy A/B/C/D comparisons. A simple room measurement can be made as a reality-check, and up to ten-bands of parametric EQ can be adjusted in real time while listening to music. The I/O Control Panel allows the level of each of the six outputs (stereo 3-way) to be adjusted in real-time in 0.5dB steps, while each output can be muted, soloed or phase reversed to check each driver and amplifier individually. Delay can be added in real-time to the ‘group’ of midrange and tweeter outputs for example, to time-align external subwoofers that will typically be placed against a room’s rear walls or corners for most coherent bass delivery.

In summary, taking advantage of the active crossover option in DEQX preamp-processors; you can reduce dynamic, crossover and IM distortion from your speakers, amplifiers and cables.
By only delivering that part of the frequency spectrum that each driver handles best; distortion
is reduced at its source rather than corrected, so that break-up, crossover distortion caused
by comb-filtering, beaming, and resonances are minimised; driver efficiency is increased,
and musical vigour is preserved. And by correcting for amplitude, phase and timing anomalies intrinsic to all speakers; the rhythm, pace, tonality and dynamics of music are reproduced accurately and true to the recorded signal; spatial perception is improved, and subtle detail
is revealed.

So if you have a DEQX unit and haven’t yet tried going active, we hope you’ll discover the benefits of full-active calibration soon!