By Bryan Geyer
Loudspeaker systems
are designed with the expectation that each internal driver will be operated
within its optimum frequency range. The need to comply with this restriction is
traditionally accomplished by sectioning the input signal into several
(commonly 2 or 3, sometimes 4) frequency-defined passbands that feed the
individual drivers. These bands present sufficient overlap (“cross over”) to
create a flat and seamless integrated output. The analog crossover networks that
formulate these passbands and link them to the allocated drivers are generally
derived by either of two basic means…
(1) As a low
impedance passive filter network, composed entirely of discrete passive
parts, that’s inserted into the high current signal path between the power
amplifier output and the loudspeaker drivers. (The parts are normally secured
inside the sealed loudspeaker enclosure.) A significant advantage implicit with
this passive network is that it facilitates the use of a single power amplifier
to feed all of the derived passbands and related drivers serving that channel.
(2) As a separate active
filter network (with high Zin), composed of active solid-state circuitry in
a dedicated external control box (with its own power supply), that is inserted
into the line-level signal path after the preamp stage (or after the
master volume attenuator when there’s no preamp) and before the ensuing
(hence multiple) power amplifiers. Each of the derived frequency-defined
passbands will then connect (via low Zout) directly to its own
independent power amplifier, and each such amplifier will output directly
to a designated driver within the speaker system. An obvious disadvantage with
this active approach is the escalating cost. There’s the initial expense
incurred in creating the active filter circuits + enclosure, as well as the
cost of providing a captive power amplifier for each of the output passbands.
Passive low impedance
crossovers can be effective when they’re properly implemented with care, but doing
so becomes a challenge. Because the passive elements will be inside the path
between the power amplifier and the loudspeaker, they must be able to tolerate
high current signals and still stay stable. That dictates beefy parts, i.e. the
use of wire-wound resistors, low gauge (fat) wires for the inductor chokes, and
large value non-polarized capacitors. These comprise the sort of parts that
aren’t normally stocked in tight tolerances, although precision is essential to
assure that the filter network will be accurate. This mix of requirements isn’t
mainstream; obtaining and/or creating such components often becomes difficult.
Passive crossover
design can be deceptive. Erroneous assumptions and unexpected complexities are
frequently experienced. Below are four such examples, as previously cited by
prolific DIY contributor Tom Perazella, in a letter that appeared in the
September, 2017 issue of the Boston Audio Society’s “Speaker” journal…
(a) Using the driver
manufacturer’s designated impedance as a guideline for the selection of the
required passive component values might lead to significant error. The real
impedance of the woofer at the intended point of crossover is likely to
be appreciably different in value than the manufacturer’s stated impedance. Using
the labeled value could cause significant initial error.
(b) Matching the
different drivers’ relative passband sensitivity is inherently difficult when
using a passive low impedance network. In order to keep the crossover points
stable you must use L-pads to achieve the corrected levels. Again, the
manufacturer’s stated sensitivity might not be entirely accurate, so the
components required for the L-pad should be selected only after measuring
individual driver sensitivity at the intended crossover frequency.
(c) After all of the
requisite driver measurements are made, and all of the passive components fully
specified, it is very likely that the desired values won’t be available.
Complex series/parallel groupings will probably be required to approximate the
target values. This component selection challenge becomes particularly vexing
in the case of air core inductor chokes, where you’ll need to specify
higher-than-needed values, and then remove some of the wire turns while making
constant in-process measurements.
(d) Last, when the
construction phase is done and you conduct final measurement and listening
tests you will generally want to make some adjustments. This will often require
exchanging expensive passive components. The consequent rewiring will likely
force layout changes, and the related delay makes new comparison measurements
more difficult, less reliable. For these reasons, completing those critical
final adjustments gets quite arduous.
Clearly, it’s
apparent that some of the pitfalls described above might go unrealized—or get
ignored—or simply remain unsolved—when a low impedance passive crossover
network is created. In such case, the related speaker system’s output will
forever reflect the inaccuracy of that oversight. Personally speaking, I believe
that numerous commercially marketed “high end” speaker systems do exhibit some
of these same errors.
Active crossovers are relatively more complex than passive
networks, but easier to enable. They’re generally more precise, also inherently
cleaner. (Separately amplified passbands foster less IM than with a passive
network. The latter uses only one amplifier.) But the most obvious and vital
advantage that an external active crossover bestows is that it’s both accessible
and controllable. It’s not buried inside a sealed speaker enclosure, and
the output levels can be varied, they’re not fixed. Given this combination of
convenience and flexibility, it’s easy to alter the tonal subtlety of the sound
to favor a particular program or genre, and then rapidly reset to “standard
profile” when desired. It’s this versatile control advantage that makes an
external active crossover network so compelling in use.
For more insight on
the many advantages implicit with active analog crossover networks, check some
of these informative sites…
Application…
Both types of analog
crossovers, active and passive, are often applied together. One very effective
case is to use an active crossover to split the incoming line-level
signal at 70 to 100 Hz, and send the low-pass output to a pair of self-powered
subwoofers with the high-pass output to the main stereo power amplifier and
main speakers. The latter’s internal passive crossover will then
generate new frequency-defined passbands and link to a designated driver. (In
such case, any crossover that was supplied as an integral feature of the
self-powered subwoofer is simply bypassed* and unused. Bypass mode option is
normally enabled via the subwoofer’s control panel.)
Of course, it’s also
possible to apply active analog crossovers at the main speakers too. In such
case, the high passband from the initial subwoofer/main speaker split would
feed a cascaded active network, with those outputs linked to multiple captive
power amplifiers. Each such power amplifier would then connect directly to a
driver inside the main enclosure. (The internal passive crossovers would have
to be deactivated or removed.) However, let’s review some basic fundamentals
first…
In terms of the
potential benefit, it’s particularly rewarding to apply active crossovers at
the bass end of the spectrum. This is because…
…the low bass is where passive networks become
more difficult; there are vexing component problems.
…it’s important to confine high energy low
bass to the subs, so effective high-pass filtering is essential.
…the bass range is where an active
Linkwitz-Riley full 4th order filter slope is most helpful.
Further, at
mid-to-treble range frequencies the use of a traditional passive crossover becomes
more practical, or more fitting, because…
…the passive parts get smaller in size than
those needed for the low bass, so they’re easier to implement.
…the passband “handoff” frequencies becomes
less critical, more forgiving with respect to accuracy.
…a passive crossover avoids the need to
provide a captive power amplifier for every passband.
That last aspect is
decisive. Dedicating a captive stereo power amplifier (the “self-power” amp) to
the subwoofers while also providing another stereo power amplifier to drive the
ensuing stages seems prudent and practical. But adding still more power amps in
order to serve the main speaker’s upper bass, mid, and treble drivers borders
on obsessive. This seems especially so when considering that the primary
audible benefit derived with captive bandpass power amplifiers fades at
frequencies beyond the middle bass (400 to 600 Hz) region. In truth, the upper
bass, mid, and treble drivers can be effectively served by properly designed
passive low impedance crossovers. Such networks have proved acceptable in
that role for decades. Their continued use seems logical and warranted.
Main speakers only…
All of the
applications noted above assume the use of paired subwoofers in concert with
the main speakers. That combination is especially helpful when “mini-monitors”
comprise the primary speakers, but active networks are just as helpful when
full range speakers are used without any supplementary subs. The
existing passive crossover networks within the full range speaker enclosures
would then be (wholly or partially) deactivated, and the related drivers linked
directly to the active network’s independent power amplifiers. A single active
crossover controller with just two output passbands could serve to drive the
woofer and all of the higher frequency drivers (requires two stereo power
amplifiers). Or a dual active crossover controller—such as this product:
https://sublimeacoustic.com/products/k231-stereo-3-way-active-crossover—could
serve to provide three distinctly separate output passbands to drive the
woofer, the mid-range, and the tweeter sections independently (requires three
stereo power amplifiers). In either case, convenient access to fully
independent ± gain controls would be provided for each separate passband. This
latter feature represents a critical advantage that can’t be achieved by means
of a passive network.
BG (December 2019)
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