I thought I'd follow the same basic progression for introducing the Duelund-Altec Project to you as I did for the Duelund-Westminster Project, where I discussed each Duelund CAST component in detail, then detailed each step as I breadboarded the crossovers, modified the loudspeakers, and then constructed the final version of the crossovers, followed by updates of any DIY refinements or tweaks I discovered along the way.

In this post I want to tell you more detail about the Duelund CAST tinned-copper (Sn-Cu) Air Core Inductors that I'll be using in the Duelund-Altec Project for the grand old Stokowski Altec's crossovers as shown in the photograph above, and as depicted in blue in positions L1, L2, and L3 in the crossover digram below.

An inductor (also called a choke) is a passive device that stores energy in its magnetic field (a capacitor stores energy in its electric field, for example).

The Duelund CAST Sn-Cu Air Core Inductor – as the name implies – utilizes an air core that annealed soft tinned-copper foil in natural paper and oil is wrapped around, instead of being wrapped around a magnetic core as with many other inductors.

The 6.6mH Duelund CAST Sn-Cu Air Core Inductor is one paper-in-oil (PIO) foil coiled up, and is a passive device that stores energy in its magnetic field.

To construct a Duelund CAST Sn-Cu Air Core Inductor a foil of tinned-copper is initially wound with paper around an air core to a certain value of inductance that is greater than the needed inductance (which is 6.6mH in this case).

Then the coiled Sn-Cu foil is put under vacuum pressure in an oil tank at high temperature for about a week to permeate everything with oil. Then the Sn-Cu foil is coiled by hand in a humidity and temperature-controlled room to an exact inductance specification, and then it is placed back into the oil. Immediately after this the winding is sealed with a special lacquer that takes a week to cure.

During this process a ring of vacuum impregnated paper has been readied with a base of CAST material (above).

As you can see in the photo above the completed CAST base has a leather-like surface texture.

Then the coiled foil is heated and placed in the ring & base assembly and CAST material is poured on top of it in five stages.

A precisely controlled flame is then quickly applied over the surface to drive any air bubbles out of the CAST material (the black glossy material you see in the photo below).

Following this step the Duelund CAST Sn-Cu Air Core Inductor is measured and checked against controls to ensure quality. The total production time is typically six to eight weeks to make each inductor.

Each 6.6mH Duelund CAST Sn-Cu Air Core Inductor is 7 inches across (17.78cm), 1.5 inches tall (3.81cm), and weighs 3257 grams (7.18 pounds) - they're big and heavy!

The leads are Duelund DCA12GA tinned-copper wire.

Frederick told me, “As far as I know, the Duelund CAST air core inductors are the only paper-in-oil inductors commercially available to this day.”

Frederik pointed out to me that many people mistakenly think inductors aren’t as important as other circuit components because they typically deal with lower frequencies.

He said, “In fact it’s the total opposite, the lower registers are far more resonant in nature than high frequencies, which means resonance control becomes even more important. People are always slow to try the inductors, but when they eventually do, they are always amazed at the difference.”

Final pricing for the new 6.6mH Duelund CAST Sn-Cu Air Core Inductors is not yet available, but I will provide it for you as soon as it becomes available.

Frederik told me regarding pricing, "The inductors, I do not know – but should be slightly more than the regular CAST Cu inductors due to the more expensive foil.”

To give you an idea of pricing, for example, a 5mH pure copper foil Duelund CAST Air Core Inductor MSRP is USD $1,779, and can be ordered from Parts Connexion for  USD $1,513, so an equivalent Duelund CAST Sn-Cu Air Core Inductor would be a little more.

A 6.6mH Duelund CAST Sn-Cu Air Core Inductor would be more expensive than that by several hundred or so USD, I suppose.

Many thanks to Frederik for making the Duelund-Altec Project possible by providing all these beautiful custom and handmade Duelund CAST components for the project - you're awesome, Frederik!

The Duelund-Altec Project is a world’s first for crossovers built exclusively with the new Duelund CAST Sn-Cu components, and are the only ones in existence at this time.

As always, thanks for stopping by, and may the tone be with you!