Appendix E - Inductors
Saturday, 11-Oct-2008, 13:37:38 GMT
Last modified: 25-Mar-2007, 19:49:48 GMT



 Inductor considerations
Inductor recommendations
Alpha-Core/Goertz
Erse
Intertechnik
Mundorf
Solen


Inductor considerations:

Compared with capacitors, inductors are dirt simple - you wind a conductor into a coil and hook it up. Nothing to it, right? Well, there's a little more to in than that. Inductors can be classified by the type of conductor and the type of magnetic core, if any, they use.

Inductor conductors are usually one of several distinct types:

  1. METAL FOIL   Extremely good electrical characteristics, difficult to terminate, slightly worse distributed capacitance, extremely linear inductance vs. frequency curve.

  2. SOLID WIRE   Good for general-purpose crossover inductors.

  3. STRANDED WIRE   Easier to work with and terminate, otherwise the same as solid wire.

  4. LITZ WIRE   Popular before metal foil became popular. As much or more trouble to terminate than foil, its performance falls between regular wire (solid or stranded) and foil.

The inductance of any coil will depend on the dimensions of the coil, the number of windings, and the magnetic permeability of its environment. Using a ferromagnetic core of some variety will greatly increase the inductance of any coil. Nothing is free, though. While magnetic fields behave perfectly in air, once they go through a metal core, things happen. The most significant thing is called hysteresis. This means that the magnetic field is no longer a linear function of the signal. Worse yet, ferromagnetic metals all will saturate with a high enough magnetic field. Once they begin saturating, lots of distortion is introduced.

You will typically find inductors with one of several types of cores:

  1. AIR CORE   By avoiding the use of any sort of metallic core, the air core inductor offers the greatest linearity and lowest distortion. It's also by far the largest and requires the most turns, each of which may introduce problems.

  2. FERRITE CORE   Ferrite core inductors have extremely poor large signal characteristics for high current inductors. Avoid them at all costs!

  3. STEEL CORE   Steel (typically, an iron/nickel alloy) core inductors are sometimes necessary when a suitable air core inductor would be too large or offer too much resistance. When selecting a steel core inductor, you will need to consider three things:

    1. POWER RATING   This is extremely important! It tells you there's enough iron in the core to avoid problems with magnetic saturation.

    2. LAMINATION   Bulk steel alloy is subject to induced parasitic current loops called eddy currents. These are avoided by building the core of a stack of thin steel sheets, electrically insulated from one another.

    3. AIR GAP   The core should have an air gap somewhere in the magnetic circuit. In order to avoid non-linear effects, the magnetic circuit should be interrupted, if only briefly. The easiest way to do this is to use a coil wound on a straight armature, like a bar magnet. If using a core which resembles a transformer, it should be designed so that there's a small internal air gap.

Since inductors are so straight forward, there has been much less voodoo surrounding them than capacitors. This is probably one reason that there are so fewer recommended manufacturers.

Having said that, there is one bit of information that really is much more critical to a wire (as opposed to foil) inductor's performance than many suspect, and that is how the windings are arranged. If you look at a coil in cross section, it will look like a group of identical coins laid out on a table. Maximum packing density is achieved when each wire (or coin, in the model) touches all adjacent wires. When done this way, a hexagonal pattern will result. This is known to topologists as hexagonal close packing. Inductor manufacturers simply refer to it as "perfect lay" winding. Aside from looking neater, a perfect lay wound inductor will closely match any other "identical" perfect lay wound inductor. Without using perfect lay winding, the two inductors may look superficially the same and may even measure the same inductance, but the resistance may be far from identical.


Inductor recommendations:

The following inductors have been recommended by experienced DIY builders. Unlike capacitors, this is a short and reasonably exhaustive list. With only a few acknowledged industry leaders, I had to do much less selection of who to list.


Vendor recommendations - Alpha-Core/Goertz:

Alpha-Core and Goertz (all inductors are sold under the Alpha-Core name, but many people still know them as Goertz thanks to the Madisound catalog) manufacture the following lines of metal foil inductors. All are recommended as probably the best air core inductors on the market.
  1. 16 AWG copper foil inductors in values from 0.05 - 8.5 mH.
  2. 14 AWG copper foil inductors in values from 0.05 - 8.5 mH.
  3. 12 AWG copper foil inductors in values from 0.05 - 8.5 mH.
  4. 14 AWG silver foil inductors in values from 0.05 - 8.5 mH.
Goertz/Alpha-Core distributors & dealers:
North America Madisound, GR Research, Michael Percy Audio, and Speaker City,
Taiwan THL Audio
Poland Akkus


Vendor recommendations - Erse:

Erse is an apparently small company which makes a number of quite attractive inductors. Be forewarned that they're difficult to find, at least under the Erse name. It's been suggested that some other vendors' inductors are supplied by Erse, but I haven't had any confirmation of that. Erse makes a several lines of interest to serious DIY-er's.

  1. Erse makes a full line of economically-priced perfect lay 18 AWG air core wound wire inductors, in values ranging from 0.10 - 3.3 mH.
  2. Erse makes a full line of economically-priced perfect lay 14 AWG air core wound wire inductors, in values ranging from 0.10 - 6.2 mH.
  3. Erse makes probably the best, most attractive, and easiest to work with steel-cored bar inductors available. Available mostly in 16 AWG, in values ranging from 1.5 - 20.0 mH.
Eerse distributors & dealers:
North America Zalytron and Parts Express


Vendor recommendations - Intertechnik (I.T.):

Intertechnik, in Germany, is a large German company marketing parts and drivers for audio and autosound applications. Among these are a popular line of capacitors. For crossover work, many are of interest:
  1. CFI copper foil inductors are available in values from 0.15 - 4.7 mH.
  2. Tritec inductors are perfect lay stranded wire air core designs, available in metric wire sizes corresponding to approximately 16 AWG, 14 AWG, and 12 AWG in values from 0.15 - 12.0 mH.
  3. Air Therm inductors are I.T.'s premium line of smaller perfect lay air core designs wound on plastic bobbins, available in a metric wire size corresponding to approximately 18 AWG in values from 0.14 - 1.8 mH.
  4. Luftspule inductors are smaller perfect lay air core designs wound on plastic bobbins, available in a metric wire size corresponding to approximately 21 AWG, 18 AWG, 15 AWG, 12 AWG, and 9 AWG, in values from 0.10 - 3.9 mH.
  5. I-Punkt inductors are laminated iron core bar inductors, available in metric wire sizes corresponding to approximately 18 AWG, 17 AWG, and 16 AWG in values from 1.2 - 12.0 mH.
Intertechnik distributors & dealers:
Germany Intertechnik and Open Air Loudspeaker
The Netherlands Speakerland/Speaker & Co
Australia GSO Audio


Vendor recommendations - Mundorf:

Mundorf makes several lines of inductors of interest to the DIY-er:
  1. The CFC16 series of 16 AWG copper foil inductors in values from 0.10 - 6.8 mH.
  2. The CFC14 series of 14 AWG copper foil inductors in values from 0.10 - 12.0 mH.
  3. The CFC12 series of 12 AWG copper foil inductors in values from 0.10 - 12.0 mH.
  4. The CFC10 series of 10 AWG copper foil inductors in values from 0.22 - 6.8 mH.
  5. The N106 series of "Zero-ohm coils" use a steel transformer core ("E"&"I" laminations) with an internal air gap. Available in values ranging from 1.5 - 18.0 mH. If using these, seriously consider spending some money on their vacuum impregnation or baked varnish options.
  6. The N130 series of "Zero-ohm coils" use a steel transformer core ("E"&"I" laminations) with an internal air gap. Available in values ranging from 3.3 - 30.0 mH. If using these, seriously consider spending some money on their vacuum impregnation or baked varnish options.
  7. The N106 series of "Zero-ohm coils" use a steel transformer core ("E"&"I" laminations) with an internal air gap. Available in values ranging from 10.0 - 33.0 mH. If using these, seriously consider spending somee money on their vacuum impregnation or baked varnish options.
Mundorf distributors & dealers:
North America E-Speakers
Spain Audiohum
Malaysia & Singapore Leda Resources


Vendor recommendations - Solen:

Solen is generally acknowledged as the premier supplier of wound wire, air core inductors. They supply two different grades using solid wire and Litz wire, their "Standard" and "Hepta-Litz" lines, respectively. All are wound using perfect lay high-density hexagonal windings.
  1. The S20 series of 20 AWG solid wire inductors features values from 0.16 - 6.2 mH.
  2. The S18 series of 18 AWG solid wire inductors features values from 0.11 - 15.0 mH.
  3. The S16 series of 16 AWG solid wire inductors features values from 0.11 - 30.0 mH.
  4. The S14 series of 14 AWG solid wire inductors features values from 0.16 - 30.0 mH.
  5. The S12 series of 12 AWG solid wire inductors features values from 0.33 - 30.0 mH.
  6. The S10 series of 10 AWG solid wire inductors features values from 0.68 - 30.0 mH.
  7. The L16 series of 16 AWG Litz wire inductors features values from 0.10 - 15.0 mH.
  8. The L14 series of 14 AWG Litz wire inductors features values from 0.16 - 30.0 mH.
  9. The L12 series of 12 AWG Litz wire inductors features values from 0.33 - 30.0 mH.
  10. The L10 series of 10 AWG Litz wire inductors features values from 0.68 - 30.0 mH.
Solen distributors & dealers:
North America Solen, Madisound, Parts Connexion, Meniscus Audio, Parts Express, Michael Percy Audio, and Welborne Labs
Australia Speakerbits
Taiwan THL Audio




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