Building a crossover network with air core inductors
A passive crossover network determines how an amplifier’s signal is divided between the woofer, compression driver, and any additional high-frequency sections. In a horn-loaded loudspeaker, that division is especially important because the drivers can have very different sensitivities, impedance curves, and acoustic origins. The inductor value may be calculated on paper, but the finished network must also work with the real driver, cabinet, horn, and listening position.
Air core inductors are often chosen for high-quality passive loudspeakers because their coils contain no ferromagnetic core. This avoids core saturation and the associated non-linear behavior at high signal levels. Their drawbacks are equally practical: they can have higher resistance, larger physical dimensions, and greater sensitivity to placement than compact iron-core alternatives.
For a system such as those developed by Sunship Audio, the crossover is part of a wider acoustic design. A bi-radial wooden horn, TAD-Pioneer compression driver, woofer, enclosure volume, and time alignment all influence the final response. The network should therefore be treated as a measured system rather than an isolated collection of component values.
Why air core inductors matter
An inductor opposes changes in current more strongly as frequency rises. In a low-pass filter, this allows lower frequencies to reach the woofer while progressively reducing high-frequency energy. In a high-pass or impedance-equalizing circuit, an inductor may perform a different supporting role, but its resistance and linearity remain significant.
An air core coil uses a non-magnetic former or a self-supporting winding. Since there is no magnetic material to approach saturation, it can handle demanding musical transients with predictable behavior. This is valuable in a high-sensitivity horn system, where small changes in series resistance or distortion can be audible.
The main penalty is winding resistance, often called DCR. A series inductor with excessive DCR reduces woofer output and changes the effective electrical Q of the filter. It can also alter the crossover slope because the driver no longer sees the intended source impedance. A larger wire gauge usually lowers DCR, although it increases cost, weight, and coil size.
Start with the electrical targets
Begin by defining the intended acoustic crossover frequency, target slopes, driver polarity, and impedance range. A textbook second-order filter based on nominal impedance is only a starting point. Real loudspeaker impedance varies with frequency, and a compression driver attached to a horn may show pronounced resonances below its usable passband.
For a simple first-order low-pass, the approximate inductance is:
L = R / (2πf)
Here, L is inductance in henries, R is the assumed load resistance, and f is the crossover frequency. This equation is useful for estimating a starting value, but it does not account for the driver’s electrical rise, cabinet alignment, acoustic roll-off, or the resistance of the coil itself.
A practical design uses impedance and frequency-response measurements to refine the network. Zobel circuits, notch filters, padding resistors, and L-pads may be needed before the acoustic slopes match the target. In a passive horn system, attenuation of the compression driver is commonly required because its sensitivity can be substantially higher than that of the woofer.
Choose inductors by circuit role
The safest place to spend on a low-DCR air core inductor is usually the series position in the woofer circuit. This component carries the full woofer current, so its wire size and thermal capacity affect compression, damping, and tonal balance. A parallel shunt inductor generally experiences less direct signal demand, though its current can still be substantial depending on the filter topology and impedance.
Inductor geometry also affects performance. A larger diameter can reduce proximity effects and distribute heat more effectively, while a multilayer winding can make a compact component with different parasitic capacitance. The manufacturer’s stated inductance tolerance, DCR, current rating, and measurement conditions are more useful than the component’s appearance.
Keep inductors physically separated and rotate adjacent coils by approximately 90 degrees. This reduces magnetic coupling between windings. Avoid placing a coil directly beside a transformer, steel screw, crossover terminal, or other ferrous hardware. Secure each component firmly with non-magnetic fasteners or suitable adhesive so vibration cannot produce mechanical noise.
Compare the practical trade-offs
The best component is determined by its location in the circuit, required value, available space, and acceptable resistance. There is no universal rule that every branch must use the largest possible coil. An oversized part may bring lower DCR but create layout problems, unwanted parasitic effects, or unnecessary expense.
| Inductor type | Main advantages | Main limitations | Typical passive loudspeaker use |
|---|---|---|---|
| Small-gauge air core | Affordable, compact, easy to source | Higher DCR and lower current capacity | Low-current shunt branches and modest filter values |
| Large-gauge air core | Low DCR, strong transient handling, no core saturation | Larger, heavier, and more expensive | Woofer series sections and high-output systems |
| Foil air core | Very low resistance and high current capability | Bulky, costly, and mechanically demanding | Premium series paths where space permits |
| Iron or ferrite core | High inductance in a compact package | Possible saturation and greater non-linearity | Space-limited designs with carefully controlled levels |
A crossover for a high-efficiency horn speaker may operate at lower current in the compression-driver section than in the woofer section, even when both components handle the same amplifier output. That difference allows the designer to prioritize low DCR and current capacity where they have the greatest audible and electrical effect.
Build around the real horn and driver
The acoustic center of a compression driver is not automatically aligned with the mouth of the horn. The driver throat, adapter, phase plug, and horn profile all affect the wavefront. Before finalizing a passive network, review the method for aligning driver and horn so the physical interface supports the intended time relationship.
Time alignment changes the way the two drivers sum through the crossover region. If the drivers are offset in depth, a filter that appears correct in an electrical simulation may produce a dip, peak, or altered directivity in the listening window. This is one reason a passive network should be voiced after the cabinet, horn, and mounting geometry are fixed.
A heavily braced birch plywood enclosure also contributes to the result. Panel vibration, internal damping, woofer loading, and port behavior can shift the measured response that the crossover must correct. Mount the network away from strong woofer magnet fields, use short high-current paths, and label every connection before installation.
Measure, refine, and document
Measure each driver separately at the intended acoustic position, then measure the combined response with the final polarity and physical spacing. Near-field woofer data, gated far-field data, impedance sweeps, and listening tests each reveal different information. A measurement microphone and a reliable impedance jig are more valuable than repeated component swaps based only on taste.
Verify the actual inductance and DCR of every production component. Two coils with the same nominal value can produce different results if their tolerances or resistance differ. Record values, wire lengths, resistor ratings, terminal locations, and polarity in a schematic that can be reproduced later.
Keep the first prototype easy to modify. Use a rigid board, generous terminal spacing, and separate mounting points for alternative capacitors or resistors. Once the measured response and listening balance are stable, replace temporary wiring with secure connections and add strain relief where the cabinet may experience vibration.
Practical recommendations for a durable network
A reliable air core inductor crossover benefits from disciplined choices rather than expensive parts everywhere.
- Use the lowest practical DCR for series woofer inductors.
- Confirm inductance, resistance, tolerance, and current rating before installation.
- Space adjacent coils apart and rotate their axes to reduce magnetic coupling.
- Measure the drivers in their final horn and cabinet positions.
- Keep a complete schematic and parts record for future servicing.
A crossover board should be designed for service as well as performance. Leave enough room around hot resistors, prevent coils from touching one another, and avoid routing signal wires in long parallel runs. When a network is installed in a custom loudspeaker, accessibility can save significant time during final voicing or later component replacement.
The finished system should be judged by its acoustic integration: stable imaging, consistent power response, natural vocal presence, clean treble, and controlled woofer output. Air core inductors provide a strong foundation, but their value is realized only when resistance, layout, driver alignment, and measured filter behavior are considered together.
For a custom horn-loaded loudspeaker, have the crossover evaluated as part of the complete enclosure and driver package. Visit Sunship Audio’s Berlin listening room or discuss a measured, time-aligned network for a system built around the intended TAD-Pioneer components and acoustic goals.