Inductors

An inductor (inductance) is a frequency-dependent component: its AC impedance increases with frequency. This means that low frequencies pass almost unhindered, while high frequencies are blocked ever more strongly. The cause is the coil’s magnetic field: current flowing through a coil generates a magnetic field. When the direction of the electric current alternates, the magnetic field builds up and collapses. The changing magnetic field in turn induces a voltage in the coil’s own circuit that opposes the change (self-induction). It is as if an “inertia of the magnetic field” counteracted the change in current – the faster the alternation and the greater the coil’s inductance and thus the magnetic field, the greater the opposing effect. Crossover filters exploit this deliberately: used in series, the inductor primarily attenuates high frequencies (low-pass effect), while low frequencies pass preferentially.

How does an inductor act in a crossover?

Series use (in the signal path)

When an inductor is placed in series with the loudspeaker branch, it typically acts as a low-pass component: low frequencies remain comparatively unaffected, while high frequencies are attenuated more strongly with increasing frequency and are finally blocked almost completely.

Parallel use

Depending on the circuit and its combination with other components, an inductor can also serve to calm resonances, smooth impedance curves or block certain ranges – that is, to route them past the loudspeaker driver in parallel, short-circuiting them.

Basic principle: the greater the inductance (L), the lower – depending on the circuit – the effective crossover or intervention frequency can be.

What Mundorf offers

Our inductors are designed to deliver exactly what developers and loudspeaker builders need in practice: reliable filter action, control and sonic calm. To achieve this, we consider not only the inductance value, typically specified in mH (millihenry), but the entire effective package of DC resistance R_DC, other losses, core material and mechanical stability.

Key parameters – and why they matter for audio

In practice, an inductor has not just an inductance L but a whole bundle of electrical and mechanical properties. For demanding audio reproduction, the following are particularly relevant:

Inductance (L)

Determines the filter action and the operating frequency, e.g. in a crossover circuit.

DC resistance (R_DC)

Causes level loss and influences damping, control and efficiency – particularly critical in woofer branches, where high currents flow. High currents may heat the conductor material and, via its temperature coefficient, possibly change the DC resistance in a signal-dependent manner.

Parasitic capacitances and self-resonance

Every real coil has distributed or parasitic capacitances. Together with the inductance, this forms a resonant circuit that has a self-resonance. Above the self-resonant frequency the inductor loses its ideal behavior – depending on the application, this can become sonically relevant.

Practical notes for installation in crossovers

• Magnetic coupling: coils can influence each other if they are placed too close together or unfavorably oriented. This can effectively shift filter values.
• Proven measure: maintain spacing and orient the coil axes at 90 degrees to each other wherever possible.
• Metallic parts nearby: steel sheets and other metallic components can influence magnetic fields and generate additional losses.

Basic designs

Inductors differ not only in inductance and resistance, but above all in their design: air-core coil or cored coil, round wire or strip material, winding geometry, core material and mechanical fixation. Each design is a different bundle of advantages and undesirable properties – electrically, magnetically and mechanically. The “right” inductor therefore always follows from the task and the boundary conditions: current, frequency range, space requirements, desired linearity and sonic priority.

Frequency-dependent losses and distortions influence how “cleanly” an inductor works – especially in dynamic, high-current ranges. In practice, this is often more important than the mere nominal value of the inductance L.

Besides its inductance, every real coil also brings DC resistance, frequency-dependent losses, stray fields, parasitic capacitances and a mechanical structure. Air-core coils work without a core and therefore remain very linear, but usually require more space or have a higher DC resistance for the same inductance. Cored coils can be more compact and enable a lower DC resistance, but introduce core-related losses and, at high currents, the risk of nonlinearity or saturation. Which design makes sense is therefore always decided by the specific task: current load, frequency range, space requirements and availability, desired control and sonic priority.

Macro-microphony: when the coil winding vibrates along

Alternating currents flow in a coil and correspondingly generate an alternating magnetic field. This magnetic field exerts forces on the conductor itself and on the entire winding. If the winding is not sufficiently stabilized mechanically, it can start to vibrate as a whole. These vibrations act back on the music signal in two ways: as a loss of valuable energy needed to control the micro-movements of the loudspeaker diaphragm, and as electromagnetic feedback of the movement into the music signal.

Macro-microphony manifests itself not only as a clearly measurable error, but also as a loss of calm, precision and control. Dynamic signal components are particularly affected: bass impulses lose contour, fine details appear less stable, and the sound image seems more restless and diffuse. The higher the currents and the larger the winding, the stronger these effects can become.

Our countermeasures – graded by effectiveness

To effectively suppress macro-microphony, we deliberately employ mechanical stabilization techniques. These are not cosmetic, but a central component of the sonic optimization.

1. Rigid winding geometry

The winding itself is already designed for mechanical stability. Defined winding tension, clean layer guidance and controlled geometry reduce unwanted mechanical degrees of freedom of the winding from the very beginning.

2. Bake-varnish wire – self-stiffening winding

With baked varnish wire, the conductors carry an additional activatable varnish layer. After winding, the turns are bonded together and form a rigid composite. This significantly reduces the mechanical “working” of the winding. Bake varnish is a very effective measure against macro-microphony in round-wire and stranded-wire windings.

3. Resin soaking – alternative fixation

As an alternative to baked varnish, we use resin soaking for certain designs. The resin penetrates the winding, fills cavities and fixes the winding as a unit. Mechanical stability increases significantly, and vibrations are strongly damped. Bake varnish and resin soaking are alternative principles, not additive measures.

4. Foil coils – maximum calm by design

Foil coils are inherently the mechanically calmest. The flat, dimensionally stable winding structure has hardly any mechanical degrees of freedom and shows practically no tendency to vibrate along. This is why foil coils exhibit very low macro-microphony even without additional measures.

5. Resin-soaked foil coils – maximum stability

Additional resin soaking further stiffens the already very stable foil winding. This represents the highest level of macro-microphony suppression and is used where maximum control and calm are required – particularly in high-current bass and lower-midrange branches.

Result for the listening experience

The consistent reduction of macro-microphony leads to a calmer, more controlled and more precise sound image. Transients remain cleanly defined, bass impulses gain contour, and even at high levels the reproduction remains stable and unadulterated. This is exactly where the difference shows between a coil that "merely" works – and a sonically optimized inductor.

Core materials

Air-core coils

The optimal core material for inductors is air. For physical reasons, air-core coils are superior to all metal-core inductors in impulse fidelity and freedom from distortion, because no core-related losses, nonlinearities or saturation effects can occur. This allows them to be used in all sections of a crossover: as an inductor in the midrange branch, as a bass inductor with a large conductor cross-section for low resistance, or as an equalizing inductor in a correction network with a thinner wire cross-section. Especially in high-quality loudspeakers, air-core coils convince with fine detail, precision and dynamics. They thus form the basis for realistic, harmonically beautiful music reproduction. The prerequisite is appropriate dimensioning of inductance and DC resistance as well as clean installation, so that the inductor can fully exploit its potential.

Strengths

• no core saturation
• very linear behavior (inductance remains stable across current)
• no core-related distortion or core losses

Typical side effects

• larger and heavier at high inductance
• R_DC can be higher (depending on wire cross-section)
• at high frequencies, depending on geometry, a stronger influence of current displacement (skin effect)

Sonic relevance

• often very “free”, “open”, clean in fine resolution
• in the bass, the RDC is often decisive: lower RDC = more control and level

Cored coils

Ferromagnetic core materials (i.e. materials that magnetize very strongly in a magnetic field because their internal magnetic domains align) are inserted into a coil to increase its inductance. This turns the air-core coil into a cored coil with significantly higher inductance at the same DC resistance (R_DC) – or conversely: at the same inductance, a lower R_DC can be realized in a more compact design. This is precisely what makes relatively compact and affordable inductors with high inductance and low R_DC possible.

However, this comes at the price of three fundamental, core-related effects which – depending on material and design – can become sonically relevant:

Hysteresis (remagnetization) losses

These losses occur independently of power whenever the alternating current of the audio signal passes through zero and its polarity reverses – the magnetic field then also changes direction. However, the ferromagnetic material does not completely lose its magnetization at the zero crossing of the current. A residual magnetization initially remains, which must first be dissipated by the subsequently opposing magnetic field of the coil before, as the cycle continues, the magnetization of the core material also reverses. This effect is called hysteresis. It is precisely in this region around the zero crossing that subtle distortions arise, which are noticeable even at low levels.

Saturation distortion

Saturation distortion behaves differently: it occurs when the core is almost completely magnetized and the electrical signal – and with it the coil’s magnetic field – continues to rise. The effective magnetic field amplification provided by the core material then drops abruptly – the coil’s inductance collapses. As a consequence, the filter action of an inductor circuit shifts under load, for example: higher frequencies that should actually be attenuated more strongly are instead passed on with distortion.

Eddy-current losses (frequency-dependent additional losses)

In electrically conductive core materials, e.g. iron, the alternating magnetic field can induce eddy currents. These currents flow within the core material itself, generate heat and thereby extract energy from the magnetic field. Eddy-current losses increase with frequency or with the rate of field change and thus act as additional, frequency-dependent losses. They are deliberately reduced by suitable, poorly conducting materials (e.g. ferrites, iron powder) or by constructive measures such as a core built from thin, mutually insulated laminations (e.g. of grain-oriented silicon iron).

Aronit core inductors

Aronit cores consist of special iron powder pressed with plastic resin. This produces a core material that, for comparable geometry, can handle higher loads than ferrite cores. At the same time, Aronit exhibits lower remagnetization losses than Feron. All in all, Aronit is thus a core material that tolerates high currents well and can at the same time enable very efficient magnetic behavior.

• High level handling and “power” in a compact design (more load-tolerant than ferrite of similar geometry)
• Very clean behavior under dynamic load thanks to low remagnetization losses
• In bass and lower-midrange branches, a balanced mix of control, reserve and calm – without the space requirements of large air-core coils

Feron core inductors

Feron core inductors are based on grain-oriented silicon iron with a lamination thickness of 0.35 mm – a material class that can be called the top league among core laminations. These high-performance transformer laminations are not to be confused with ordinary transformer core sheets. Through targeted material and process quality, Feron core inductors achieve a combination of compact design, low DC resistance and exceptionally low core-related distortion.

• Very high composure under level and impulse loads (low core-related compression)
• High control and efficiency thanks to a low series resistance (RDC)
• Clean, “calm” reproduction even under load (minimal baseline distortion and remagnetization losses)

Ferrite core inductors

Ferrite cores are sintered from electrically non-conductive, ferrimagnetic materials, usually iron oxide, possibly in combination with further metal oxides and metal carbonates. The HP3616 ferrite material we use differs significantly from that of the widely used, inexpensive Far East cores: it is designed for low baseline distortion and a fast remagnetization capability (rapid reversal of the field direction). On this basis, ferrite core inductors are created that are not only compact but also work cleanly and precisely in sonically sensitive applications.

• Very precise, “fast” behavior thanks to high remagnetization capability (hardly any delay of the music signal)
• Low baseline distortion → clean equalization and correction without “veiling”
• Particularly strong in correction networks and in the midrange; at low
• Remagnetization distortion = baseline distortion around the zero crossing (polarity reversal), effective independently of level
• Saturation distortion = occurs at high levels when the core is fully magnetized (“saturated”): the inductance collapses and the filter action changes in an undesirable way

Our core materials at a glance

Air (no ferromagnetic core)

Strength: Maximum linearity, no hysteresis baseline distortion, no saturation, no other core-related distortion

Trade-off: Larger design; R_DC depends strongly on the conductor cross-section; stronger stray field (layout-sensitive)

Classification: The reference solution for maximum impulse fidelity and minimal distortion – particularly convincing in the bass when the conductor cross-section is large enough

Ferrite (HP3616, ceramic, sintered)

Strength: Very low electrical conductivity → low tendency to eddy currents; compact designs possible; HP3616 enables very precise function with very low baseline distortion (zero crossing)

Trade-off: The saturation reserve to be taken into account and the load capacity are more limiting at the same size than with Aronit or Feron; the design remains decisive

Classification: Very good for correction networks/equalization and midrange when particularly clean small-signal behavior and controlled remagnetization behavior are required

Aronit (iron-powder core, powder composite, pressed)

Strength: Special iron powder pressed with plastic resin → robust and current-tolerant; particles insulated from each other by the plastic reduce eddy-current losses; relatively low baseline distortion with good design

Trade-off: Loss and distortion behavior are strongly type- and design-dependent; not automatically the finest material for every midrange task

Classification: Particularly attractive for high-current branches (bass/lower midrange) and robust applications; with the right design also very clean

Feron (grain-oriented silicon iron, 0.35 mm, annealed)

Strength: Extremely low losses and distortion combined with extremely high load capacity; very low baseline distortion (at the zero crossing thanks to a small → coercivity)) and a very high saturation reserve – depending on core shape and design

Trade-off: Stray field and optimal integration depend on the core shape (I-core PCB-mountable; E-core/zero-ohm usually enclosure mounting)

Classification: The quality difference compared to common industrial core laminations lies in the material class and process control. Feron enables extremely low R_DC values at high inductance – an essential prerequisite for precise, tight and controlled bass reproduction

Practical classification by task in the crossover circuit

Bass / mid-bass (high currents)

Primarily decisive: R_DC and saturation reserve

Typically strong: Feron (maximum cleanliness under load, very low R_DC), Aronit (robust/compact, current-tolerant), air (if the conductor cross-section is large enough and space is available)

Midrange (high sensitivity to baseline distortion):

Primarily decisive: hysteresis baseline distortion around the zero crossing, low losses

Typically strong: Air-core coils; HP3616 ferrite for equalization and specific midrange tasks; Feron where additionally high reserves and a very low R_DC are required

Correction networks / equalizer branches:

Decisive: very clean small-signal operation (minimal baseline distortion), defined losses

Typically strong: HP3616 ferrite, depending on the task also air or a suitable cored solution (Aronit/Feron)

Conductor construction

With inductors, it is not only inductance and core material that matter, but also the construction of the conductor. A given inductance and core type determine the conductor length in the winding, and this, together with the wire cross-section, determines the DC resistance (R_DC). The geometry, together with various processes (e.g. baked varnish, potting), determines the mechanical calm of the winding.

Factors determined by the conductor

DC resistance (R_DC)

More R_DC usually means less efficiency and less electrical damping – often, especially in the bass range, audible as reduced control.

AC losses (current displacement, skin effect)

With alternating current, the current is not distributed evenly in the conductor. With increasing frequency it shifts more and more toward the surface of the conductor (skin effect). How much this matters in practice depends on the application.

Parasitic capacitance

Every winding has a parasitic capacitance. The geometry of the winding structure is decisive. In our foil coils, this capacitance is typically lower than in comparable round-wire coils.

Mechanics and macro-microphony

Currents generate magnetic fields, and these in turn generate forces in current-carrying conductors. Macro-microphony describes unwanted mechanical-electrical feedback and energy losses that modulate and distort the signal.

Round-wire coils

Principle: the winding is built from solid round wire.

Strengths

• Proven, easily calculable construction
• Good packing density, many inductance values practicable

Typical limits

• Without fixation, the winding can work mechanically under load
• Depending on the design, frequency-dependent additional losses (skin effect) can weigh more heavily

Classification

Round wire is the solid standard. In high-quality crossovers it is significantly improved as soon as the winding is consistently calmed mechanically – here, baked varnish or resin soaking is a clear quality lever.

Bake-varnish wire – the bonded winding as a quality lever

Principle: Bake-varnish wire is varnish-insulated wire with an additional activatable layer that becomes adhesive when heated. The turns are thereby fixed into a composite.

Effect in practice

• The winding becomes mechanically stable and works significantly less
• Micro-movements between the turns are reduced
• This primarily reduces macro-microphonic effects (the winding works less) and increases the “calm” in operation

Classification

With an otherwise identical design, a baked varnish coil is always better than the corresponding round-wire coil without baked varnish, because it is bonded and therefore remains mechanically calmer.

Resin soaking as an alternative to baked varnish wire

If no baked varnish wire is used, a resin soaking carried out under vacuum can take over the mechanical fixation. The resin penetrates the winding, reduces cavities and degrees of freedom of movement, and transforms the winding into a stable composite. The result: significantly less macro-microphony and an overall calmer, more precise operation of the inductor.

Stranded wire as hepta strand (7 individual wires, baked varnish)

Basic idea: splitting the conductor into several individual wires can favor the current distribution in the conductor and thus reduce additional losses in certain applications.

Strengths

• Can support a clean, controlled current distribution in suitable tasks
• Bake varnish enables very good mechanical fixation of the winding
• Interesting for correction networks (equalizing inductors) and selected midrange tasks

Classification within the range

If hepta strand is understood not as an all-rounder but as a tool for specific tasks in the midrange, it can be used to good effect.

Foil coils with polypropylene as insulator

Principle: the conductor is a foil made of (Angelique) copper, silver or (Angelique) silver-gold.

Electrical strengths

• Very low R_DC achievable at high inductance
• High current-handling capacity → particularly attractive in the bass and lower midrange
• Parasitic capacitance of foil coils lower than that of round-wire coils

The decisive advantage: the calmest winding

Foil coils are constructively much more stable than round-wire windings – and in practice the mechanically calmest. Precisely because the foil is flat, dimensionally stable and tightly guided, the winding shows particularly little tendency to work mechanically under load. In this respect, foil coils are noticeably calmer – even compared to very good baked varnish round-wire coils.

Resin-soaked foil coil with paper as insulator

Principle: resin soaking transforms the already very calm foil winding into an even more stable composite.

Effect

• Even fewer remaining degrees of freedom of movement in the winding (macro-microphony further reduced)
• Maximum mechanical calm under load
• Audible as additional composure, contour and background calm – especially in the bass/lower midrange

Foil is already the calmest design. Resin soaking is the next step when mechanical calm and stability are to be pushed to the maximum.

Classification

Round wire

Round-wire coils are the proven standard design: a solid conductor is formed into a winding and delivers an easily calculable inductance. Sonically, the DC resistance (R_DC) is decisive here, because it directly influences efficiency and control – especially in the bass. Depending on wire cross-section and area of use, the skin effect can also play a role. The mechanics are also crucial: a loosely built round-wire winding can work under load and impair the calm. Our round-wire coils are therefore optimized for a tight winding and fixed with damping so that they remain mechanically calm. Correctly dimensioned, round wire is thus a very strong all-rounder.

Bake-varnish wire / resin-soaked round wire

Bake-varnish wire and resin soaking are two paths to the same goal: a mechanically calm, firmly fixed winding. With baked varnish wire, the turns “bond” into a composite – this significantly reduces macro-microphony and makes the coil audibly calmer and more precise. Alternatively, a resin soaking can transform a winding of ordinary enameled wire into a rigid composite. Both lead to more contour, less mechanical “working” in the winding and thus more cleanliness in the sound. The macro-microphony reduced in this way means a considerable gain in detail and fine dynamics across the entire sound image. In this respect, a fixed round-wire winding is clearly superior to ordinary round wire.

Hepta-litz (7 individual wires, baked varnish)

Hepta-litz consists of seven individual wires, each of which is already baked varnish wire and can therefore be stabilized mechanically very well. Splitting the conductor into several individual strands favors the current distribution in the conductor bundle as frequency increases and can thus reduce additional losses in certain applications. Typical fields of use are correction networks and equalizer branches as well as selected midrange tasks.

Foil (copper / silver / silver-gold)

Foil coils use a flat, dimensionally stable foil conductor made of copper, silver or silver-gold and thus enable a very low RDC at high inductance. The decisive point, besides the electrical performance, is the mechanics: foil coils are constructively the calmest winding design. They work particularly stably under load. The further reduced macro-microphony means a considerable gain in detail and fine dynamics across the entire sound image. This makes the reproduction appear exceptionally composed, precise and controlled.

Resin-soaked foil

A resin-soaked foil coil takes the already very high mechanical calm of foil to the extreme. The impregnation further reduces remaining degrees of freedom in the winding and makes the composite even more stable. The result is maximum composure under current and a particularly “black” background calm. Especially in the bass and lower midrange, this can deliver audibly more contour, punch and composure, without harshness or restlessness. Resin-soaked foil is thus the logical choice when, in addition to a low R_DC, maximum mechanical stability and thus minimum macro-microphony have priority.

Conductor material

Metals: how are they structured, why do they conduct current, and what happens to the signal in the process?

From the outside, metals appear to be a uniform material, but internally they are finely structured – and it is precisely there that the differences arise. This “fine structure” consists of crystal grains (with a certain grain size) and their transitions; added to this are subgrains, twin boundaries, lattice defects such as dislocations, vacancies and interstitial atoms, as well as precipitates, inclusions and very thin oxide or boundary layers at internal and external surfaces. There is also a texture (a preferred orientation of many grains) and internal stresses, which can be modified by forming and heat treatment. Purity, heat treatment and deliberately small alloying additions can stabilize this internal order – for example a small gold content in silver, or silver/gold additions in copper, which often make the reproduction appear calmer and more faithful in timbre.

Metals conduct current because they possess free conduction electrons that can move through the crystal lattice. How well this works depends above all on how strongly these electrons are scattered along the way – by temperature (lattice vibrations), by foreign atoms/alloying additions, by the aforementioned lattice defects, by transitions between grains and by boundary layers. This is why silver, copper, gold and aluminum are all good conductors yet differ in their conductivity: silver has the highest, followed by copper, then gold and aluminum.

For the music signal this means: the current does not flow through an ideal, completely “silent” medium, but through a real material with exactly the internal structures described above. These can produce tiny, signal-dependent changes in behavior – not as a gross error, but as fine differences, for example with regard to calm, focus and the stability of timbres.

Another example is small gold or silver additions in conductor materials. A small gold content in silver makes the material more stable in the long term and reduces fine temporal restlessness, without losing the typical transparency of silver. And silver/gold additions in copper can additionally calm and stabilize the internal structure – the result often comes across sonically as a calmer, clearer reproduction with more stable timbres. In short: element and conductivity are the basis – the deliberately engineered internal structure determines how “relaxed” and musical a metal plays in detail.

It is precisely this level of “inner signal calm” that is explored in depth in the next chapter. Learn more → chapter “Micro-microphony”

Copper

Copper is highly conductive and mechanically robust. Sonically, this results in a balanced, full-bodied precision with good substance and a natural foundation. How finely and calmly copper plays in detail depends on the respective copper quality and its internal structure. That is why we use high-purity, thermally post-treated copper grades and select them so that the signal remains stable, faithful in timbre and free of fine restlessness.

Angelique copper

Angelique copper is copper with deliberate silver/gold additions (Ag/Au). These additions stabilize the boundary regions in the metal and reduce temporal micro-restlessness, smoothing the noise spectrum. Sonically, this leads to more calm, more stable timbres and better focus, without losing the full-bodied character of copper.

Silver

Silver stands for neutral, very precise reproduction with high clarity and clean structure. It adds little of its own and therefore often appears more sober, yet delivers maximum resolution.

Silver-gold

Silver-gold remains as transparent as silver but adds noticeably more tonal color and natural fullness. It sounds clear and at the same time more relaxed, with more stable timbres and a more musical overall character.

Angelique silver/gold with copper content

Angelique silver/gold with copper content combines the transparency and tonal fidelity of silver/gold with more warmth and body. The gold content stabilizes the material and reduces fine temporal restlessness, while a copper content gives the sound more substance and natural fullness. The reproduction thus remains clear and transparent, yet at the same time appears calmer, more relaxed and more stable in timbre.