Sunship Audio

Designing a time-aligned crossover using only passive components

Horn-loaded loudspeakers built around TAD-style compression drivers and dedicated woofers present a peculiar challenge: the acoustic centres of those drivers rarely sit on the same vertical plane. Even a few centimetres of mechanical offset can blur transients around the crossover region. A properly aligned passive network compensates for this geometry through topology choice, slope design, and physical driver placement, all without resorting to digital delay lines.

The appeal of an all-passive approach runs deeper than nostalgia. Capacitors, inductors, and resistors can be repaired in any Australian capital city, from a small workshop in Hobart to a component supplier in Brisbane, decades after an active processor has become obsolete. A well-built passive crossover also introduces a particular grain and rhythm to the music that many listeners value in high-efficiency horn designs.

Australia's hi-fi community has been receptive to horn-loaded systems for decades, with strong enthusiast groups around Melbourne, Sydney, and Perth. Annual gatherings at the Melbourne International Hi-Fi Show regularly feature large horn systems, and the country's spacious suburban listening rooms suit the imaging that aligned passive networks produce. That culture provides a useful testing ground for refinements that prioritise long-term coherence over short-term convenience.

Designing such a network requires equal attention to electrical behaviour and mechanical layout. The crossover's transfer function on the bench tells only part of the story; cabinet geometry, horn flare rate, and listener position all shape the final outcome. With passive parts alone, the designer relies on disciplined measurement and iteration rather than algorithmic correction.

Fundamentals of time alignment in passive networks

Time alignment refers to the arrival of acoustic energy from each driver at the listening position. When two drivers produce the same frequency at slightly different times and those outputs are summed in the air, comb-filter dips and a loss of focus result. Networks of capacitors and inductors cannot directly delay a signal the way a digital buffer can, but they can shape relative phase across frequency to mimic alignment when combined with careful driver spacing.

In practice, alignment matters most within a narrow band around the crossover frequency between woofer and compression driver. Outside that band, level matching and directivity dominate perception. Designers exploit this by selecting a topology whose phase behaviour at the crossover point correctly offsets the physical offset of the drivers.

Bi-radial wooden horns, the kind featured on the about page of Sunship Audio, have a fixed mouth depth that sets the practical limit on driver placement. Once the horn is bolted to the baffle, the remaining levers are crossover topology and small shimming behind the woofer. Treating both as one design problem yields the most coherent results.

Choosing components for stability and tone

The quality of passive parts matters more in a time-aligned network than in a casual design, because any drift in value shifts both magnitude and phase at the crossover point. Film capacitors with stable dielectrics, air-core or iron-dust-core inductors, and non-inductive wirewound resistors are all worth specifying. Australian climate swings, from dry heat in Alice Springs to coastal humidity in Cairns, can stress lesser components, so sealed or encapsulated parts earn their modest premium.

Sourcing specialist parts internationally introduces its own complications. Customs paperwork, parcel tracking, and regulatory shifts affect consumer goods across every category, and even unrelated industries have seen supply chains rewritten overnight, as the EU logistics changes show for cross-border parcels. The lesson for the crossover builder is to maintain a small surplus of matched critical components so a finished design remains serviceable regardless of overseas shipping disruptions.

Topology and slope trade-offs

The slope of a passive crossover determines how aggressively neighbouring drivers roll off, and this directly affects how much phase shift intrudes into the operating band. First-order networks (6 dB/octave) preserve phase coherence most faithfully but offer poor driver protection and limited off-axis control. Third-order networks (18 dB/octave) keep each driver quiet well outside its passband but introduce substantial phase shift that requires careful mechanical compensation.

Second-order networks occupy the practical middle ground that most horn systems favour, particularly those using a 1.4-inch compression driver crossed over in the 800–1,200 Hz region. The slopes are steep enough to protect the diaphragm from low-frequency excursion yet gentle enough that phase rotation around the crossover frequency remains manageable.

The comparison below summarises the trade-offs for a typical bi-radial horn system with a 15-inch woofer:

Network order Insertion loss Phase shift at XO Driver protection Component count
First-order (6 dB) Lowest Minimal Poor Lowest
Second-order (12 dB) Moderate Moderate Good Moderate
Third-order (18 dB) Highest Substantial Excellent Highest
Fourth-order (24 dB) Very high Pronounced Excellent Highest

Most Australian-built horn systems land on second-order for the woofer-to-mid band and third-order for the mid-to-tweeter section, accepting slightly higher insertion loss in exchange for driver protection. The phase penalty is then corrected by physically recessing or advancing the relevant driver on the baffle.

Cabinet bracing and driver offset compensation

A heavily braced birch plywood cabinet behaves almost like a solid block at audio frequencies. That rigidity matters because any cabinet flex would smear the crossover's careful work. Once the enclosure is stiff, driver offset can be calculated from horn mouth depth, magnet depth, and basket thickness, and applied uniformly across both channels of a stereo pair.

Mechanical offset works best in 5–10 mm increments, since finer steps require either a custom-depth baffle or a precision spacer ring. Where the geometry demands a larger offset, designers sometimes tilt the horn slightly toward the listener, a trick borrowed from professional sound reinforcement. The slight angular offset preserves arrival time at a sweet spot while relaxing the requirement for perfect depth alignment.

Photographs of finished systems, showing precisely how drivers are mounted and offset on the baffle, can be reviewed in the gallery. Studying several examples side by side helps a new builder internalise the relationship between horn depth, woofer depth, and the chosen crossover topology.

Measuring, listening, and refining

A completed crossover should be verified with both instruments and ears. Impulse-response measurement, ideally captured at the typical seated distance in a finished room, makes phase behaviour visible across frequency. A symmetrical impulse across the crossover point is the practical sign of correct alignment. Step-response plots reveal storage of energy that may indicate misaligned drivers or a slope choice that pulls too much low-frequency energy into the compression horn.

Room treatment matters as much as crossover design. Thick curtains, heavy rugs, and absorption at first-reflection points tame the long decay times common in Australian homes with high ceilings. Without that treatment, reflections reach the listener milliseconds after the direct sound, smearing even a textbook-aligned network. Pairing proper alignment with thoughtful room work produces a soundstage that holds together at any volume.

Practical guidance for the first build

A few habits tend to separate a serviceable time-aligned crossover from a truly coherent one. Keeping the design simple reduces the number of parts that can drift or fail. Buying matched drivers in pairs, where budget allows, lets the builder account for natural production variation between samples. Local listening groups around Brisbane and Adelaide are an excellent way to expose a new design to critical ears before committing to a final layout.

Recommendations for designers approaching their first all-passive time-aligned network: