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Loudspeaker Inductors with Air-Cored and Ferrite Bobbins

Typical Ferrite Loudspeaker Inductors

Passive loudspeaker crossover using tufnol board

   Most homes have a number of sound reproducing systems / items be it TV, Radio, Computers or Hi-Fi. A common link between these systems are Loudspeaker Inductors and two other component types which process electrical signals into an acceptable acoustic experience.
      One of the key components in most of these enclosures, (where there is more than one speaker), is an electrical circuit called a crossover. This distributes electrical signals in a carefully prescribed manner to minimise speaker / driver distortion and provide a listenable result. These crossovers contain three main types of components, i.e. Resistors, Capacitors and Loudspeaker Inductors. Collectively they are generically known as passive crossovers as they are operable without additional power input.
      Crossovers used in Cinemas, Theaters, Sound reinforcement and, to a much lesser extent, high end hi-if systems, use electronic crossovers which are self-powered and contain no inductors, so enough said there then.

      In simple terms, loudspeaker inductors can be viewed as frequency dependent resistors, i.e. they work with AC signals, the higher the frequency, the higher their resistance. In AC circuitry the term 'resistance' is replaced by a concept called 'impedance'. Often in conjunction with other components, the property of increasing, or decreasing component impedance with frequency can be used to control the signal level being applied to individual speaker units.

      Whilst resistors have a property called resistance which is measured in Ohms and is essentially independent of frequency, inductors have a property called inductance, (no surprise there then), which is measured in Henries and "is" frequency dependent. In addition, two other parameters are sometimes specified (1) its DC resistance and (2) its resistance, strictly impedance, at a particular frequency, well not so often in Audio work.

2. Assorted Air-cored Bobbins

Air-cored nylon bobbins

      Depending upon your viewpoint, Inductors are rather clever devices or just rolls of wire. Most inductors are wound on Formers or Bobbins being either circular or rectangular in shape and which are usually manufactured from materials which for ferrite materials have very specific electrical properties. Air-cored bobbins are not so fussy as they use air as their operating medium.

3. Ferrite Cored Bobbins

Ferrite bobbin core

      The two most widely used inductor styles use either Air-cored or Ferrite cored bobbins. There are many combinations of these arrangements which include Nylon bobbins with a central air core making it an air-cored inductor. The air cored centre may also be substituted for Ferrite core in which case it becomes a ferrite inductor. Furthermore, the whole bobbin may be constructed from ferrite, making it also a Ferrite Inductor.

      For loudspeaker crossovers / audio use, most loudspeaker inductors fall within an inductance range of 10mH (milli-henries) to around 0.1mH. Note 0.1mH is equivalent to 100uH (micro-henries). The larger values are generally used to control the lower frequencies associated with bass and mid-bass drivers, whilst the smaller values are used to control the tweeter frequency response.
      Inductors are not lossless devices and some energy is always lost as heat. The bass crossover section uses inductors wired in series with the loudspeaker driver. Any significant inductor losses result in a reduction in system efficiency or loss of sound level. Increasing the input power, current, turning up the volume, often done), may result in increased heating of the coil, melting of the insulated coating on the wire, shorted coil turns and rapid demise of the device as an inductor.

4. Air-cored Inductors

Wire wound air-cored inductor

Typical audio Inductors for Mid to Low frequency uses.

      So, why the two types?? Well, a property called 'coil saturation' comes into effect. This is a measure of an Air-Cored Inductors ability to maintain its properties / inductance value with increasing current / power. Remember, we want our crossover circuit properties to be consistent over a wide range of power levels so the stability of our inductors is an important point....

5. Ferrite Cored Inductors

Small audio ferrite inductor.

      Now, air-cored inductors do not saturate easily and so their performance is more robust / linear in performance than their equivalent designs and so they are often thought to provide a better sound quality. However, there is always a downside. For any specific inductance requirement the length of wire required for an air-cored coil is significantly longer than that of an equivalent ferrite. The longer the wire, the higher its DC resistance and the greater its loss in power.
      Loudspeaker inductors optimally require low value DC resistance for best performance if they are not to be too lossy and waste valuable amplifier power. In many standard production loudspeakers the inductors are, therefore, ferrite based, saving wire, cost and size for a reasonably good performance.
      To lower the DC resistance we could use thicker wire, i.e. a larger wire diameter, but this reduces the number of turns per coil layer, requires a much larger coil and, hence, significantly increases the copper cost.
       The DC, (Inductor DC Resistance), also forms part of the crossover's internal electrical design characteristics and so swapping a ferrite coil for an air-cored coil, (a dubious upgrade without careful matching), can result in a bad experience. Increasing the DCR of Bass coils, for instance, can result in increased coil heating and subsequent coil failure.

6. Notes on Coil Winding Wire

      Most / nearly all loudspeaker inductors are wound using copper wire. Depending upon the crossover application and power requirements, wire diameters may vary from around 0.5mm to 1.5mm or (24 to 15)AWG. If cost is no object then you can move to using Flat copper tape or pure Silver wire. In all cases the wire or tape has to be electrically insulated. The acoustic improvement is debatable.

      The copper wire is insulated and the wire is graded as Grade 1 or Grade 2. The wire is drawn through a chemical dipping bath containing the insulation. One pass is a grade 1 wire and for a thicker coat it passes through twice making it grade 2. The opportunity exists to apply two different coatings for higher temperature coatings, (electric motors) or low friction coatings as used in high speed winding machines. In addition thermo bonding of the coils is possible, using a special coating, (prevents coil buzzes and noises), whereby the insulating coat has a carefully calibrated high value electric current passed through it to partially melt the outer coating and bond all the turns / layers together. This also permits bobbin less air-core coils, which saves the cost of a bobbin. The bobbin / former less process is usually performed by machines with collapsible winding mandrels.

      The copper wire can also be graded regarding the impurities it contains. For general use we have Ordinary copper winding wire, (nominally 9996 to 9999, known as 4 nines, i.e. 99.99% pure copper) or OFC (Oxygen Free Copper, notionally 6 nines 99.9999% pure copper). If you're wishing to wind your own coils you may find it difficult to source OFC copper wire unless you want a pallet load.
      For loudspeaker inductors grade 2 is the best option as it provides a more robust insulated wire and can be better worked without the coating stripping off. The wire diameter increases marginally due to the coating and may therefore decrease the number of turns per layer, confounding coil calculations slightly.
       For the minimum coil size / inductor efficiency and maximum copper content, it is possible to use rectangular cross section wire but this is difficult to handle, wind, obtain and naturally more expensive.

7. Winding your own Loudspeaker Coils

A Sophisticated Desktop Winder

Computer controlled desktop coil winder

      Whilst this sounds a good idea, it is rarely worth the cost or effort to wind your own loudspeaker inductors. You need an Inductance Meter, Bobbins or Ferrite Cores, some form of winding device and, of course, wire.
      In addition, you need to calculate the turns etc, the accuracy isn't great unless your winding process is concise. Haphazard non-layered windings do not provide for small tolerance inductors. It is important to tension the wire correctly in order to remove wire kinks and achieve uniform layering.

      The above image shows a typical mid 90's computer controlled desktop coil winder capable of accurately winding a wide range of coil / bobbin configurations. Image curtesy of Marsilli& Of course, this type of equipment is far outside the general DIY users pocket, for, as the saying goes, "If you had to ask the price you probably couldn't afford it."

8. Air-cored Audio and RF ferrite coils

Air-cored audio and RF ferrite inductors.

      The wire tension can vary from a few grams for hair-like fine wire to some (8 - 9)Kgms for thicker 1.5mm material. This type of tension is difficult to accurately apply without specialised tooling.
      Any kinks reduce the number of turns per layer. As the inductance is non-linear with turns a lost turn or two cannot be just added back on at the end of the process. If you want tight tolerance coils it is better to purchase coils or overwind to a slightly higher Inductance value and then carefully unwind a few turns, measuring the inductance as you go, until you arrive at the required value.
      If you have very special needs then, maybe, winding your own coils is for you but purchasing stock items is probably a better way to go.
      Here at QTA Systems, we wind our own coils and are able to wind both Air-cored and Ferrite types using the following wire diameters (0.8, 1.0, 1.12, 1.25 and 1.32)mm to specific customer Inductance values. We can also supply loose bobbins and / or copper wire if you care to try your hand at a bit of DIY. For loose materials or specific loudspeaker inductors, view the Inductors link at Page Bottom

9. Mounting the Coils

      Because of their size and power handling most loudspeaker inductors generate an external electric field and if spaced too closely together they may interact, producing unwanted non-linear side effects. The mounting of air core bobbins is usually straightforward as one bobbin end has a central hole and this can be bolted to the crossover board using a non-ferrous bolt, (Nylon or Brass).
      For mounting more than one coil, these should be spaced as far apart as the board design will permit. If space is tight then each coil is recommended to be fixed on a different orthogonal, (right-angled axis). In this case, because of coil geometry, only one coil can be bolted. To the rescue comes Silicon adhesive, set the remaining coils, each on a different axis, into a good bed of adhesive.

10. A Simple Tie Wrapped Ferrite Coil

Mounting a ferrite inductor with tie wraps

      Ferrite coils require a different mounting technique. The comments regarding spacing and orientation still apply but no bolt hole may be available, in which case they can be set in silicon adhesive. Long Nylon bolts easily stretch when over tightened and may permit rattles so .... Again Silicon adhesive is useful to bed the coils into. Once set however, this can be difficult to remove and a scalpel knife used to cut through the lower silicon bond may be the only way to free up the device. So, if you want to experiment, move your layout around a little, then double sided tape, (non-permanent fixing, it often softens and releases with heat), or Nylon tie wraps may be the answer. You only need make a few holes in the board to thread the wraps through them.

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