EI Core versus C-Core Transformer

EI core versus C-core Output Transformer

There’s always this perception that C-core or double C-core output transformers are far more superior to the standard EI core output transformers.

C-core transformer versus EI core transformer

 Well, it is true to some extent that C-core has lesser stray flux compared to EI core, and therefore lost is smaller, and impact to the surrounding is also lesser in comparison. The core itself will radiate stray flux, especially with the 90 degC sharp corners that the EI has, more is radiated compared to the rounded edge of the C-core transformer. 

For EI core, the section where the cores are connected inevitably will have minor gap and therefore there will be stray flux radiated from there. For C-core, the section is smaller, and it is usually covered by the bobbin + windings, and therefore stray flux is lesser comparatively. 

One reason limiting the popularity of C-core is the flexibility of size. Usually the C-cores are predetermined in size and therefore to customize it according to applications will be difficult, resulting in wastage due oversize core. Users cannot stack more laminations to the core to increase the capacity. This is something equipment manufacturers do not like. There are also several other reasons too that involves materials and manufacturing difficulties and costs but it will not be covered here.

If the EI core winder has enough experience, know-how and expertise to wind a good EI core transformer, the cost / performance ratio can match such that of a C-core transformer. The winder just needs to address the shortcomings of the EI core transformer and apply remedies to resolve it. With the flexibility to adjust the EI core size (capacity), it brings a lot of benefit for fine tuning the size requirement of the transformer.

So, don’t be too superstitious. EI core or C-core are all usable and can sound pretty good. What matters is where you get it from, whether it is wound by an amateur or a true professional that demands for quality and reliability, with the right partnering components like the magnet wire,  insulation material, winding technique, insulation technique, output taps drawing methodology and etc. 

Just like what HIFI guru always says, design is far more important than material. You can have a so-so material, but with the right design and application, it will still be a good match. With better material, it would be better! If you have the best material, but with the wrong design, it’s worse that what’s mentioned above. 

J&K Audio Design

19/12/2013

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