Power Transformer

Power Transformer

There are many types of power transformers. We are we still seeing a lot of EI core power transformers, especially for tube amplifiers? Why aren’t we using toroid transformers that have a lot of excellent electrical properties of being more efficient and lesser stray flux, not forgetting easier to manufacture and save on material? 

Toroid power transformers:

• Pros: lower material cost (smaller core size and lesser wires), higher efficiency, easier to automate for mass production.

• Cons: high turn-on surge, mechanical longevity (higher chances to develop mechanical rattling over long usage), higher noise coupling from primary/mains, complicated or less feasible insulation implementation, size customization constraint.

EI power transformers:

• Pros: primary coupled noise is lower (good for isolation), easy size customization, comparatively low turn-on surge, easier on insulation implementation, matured technology, easier for semi-automatic winding, easier to repair and reuse

• Cons: more manual, less efficient, comparative size is bigger

Weighing on the pros and cons between the two, we would still choose EI core over toroid core and would recommend users to do so.

There are a lot of considerations when come to power transformer design. This is very important to meet the “for audio use” or audiophile criteria.

• Operating temperature – 65 degC or lesser is better, due to core loss and winding IR dissipation.

• Mechanical vibration – zero hum tolerance, this could come from the windings or the core. If core is undersized, possibility of audible vibration will be higher.

• IR loss – as low as possible, or meeting the regulation requirement. If magnet wires used, IR loss will be significant, and that will also increase the winding temperature. Therefore some will increase the current rating or VA rating of the taps to reduce such effects to maybe 2x or more. (It is not necessary if order from us as we would take voltage regulation into consideration when designing your transformers.) There are risks of oversizing, one being inaccurate output voltage and therefore turning into risks of burning out the tube rectifiers and various other problems.

• Efficiency – ~90%

There are 2 methods of power transformer design. First method is to match the load current with the required output voltage. When it is loaded, the voltage will drop to the correct specification. This should be the right way. The second method is to match the voltage with no load – not for audio use but for some other applications. 

For HIFI use, we would highly recommend users to get the best you can for your transformers. The power transformer is the supplier of power to your device. With enough juice from the power transformer, only your device will sound right. Ensure your balance is right. Don’t spend thousands on a cable and skimp on the transformer. The order is simply incorrect. If you use all audiophile components, make sure you spend adequately on the transformers or the overall impact to sound will be heavily discounted. 

J&K Audio Design

20/12/2013

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