Power Transformer For Audio

Power Transformers For Audio

Power transformers for audio – understand the different types of power transformers and select the suitable on for your audio applications. 

Power transformers are all the same. They provide power (voltage and current) to the audio equipment, period. True? NO! Power transformers are really like “Transformers” – more than meets the eye!  What you use on your gears will drastically impact the end result – sound! 

This page is dedicated to the matters and issues with power transformers that can be considered as low frequency transformers since they work with 50-60Hz from the mains. I will have another page talking about audio frequency transformers, like the output transformers, input transformers, inter-stage transformers and etc.

In short, power transformers convert mains (wall) AC (110Vac, 240Vac, depending on the country you live in) to equipment desired voltages. It’s the magnetic field from the primary winding that induces voltage/current on the secondary windings. 

These windings are mounted to an iron alloy core that allows the power transformer to work more efficiently. There are many types of core materials and is determined by the applications to be used in as the permeability of the cores are different and therefore the end results will also be different. 

The magnetic field not only couples between the windings but also couples to the surrounding components, and therefore causes noise as current/voltage is induced on the components, for example a wire or a PCB trace. This leaked field is called the stray flux. This noise is hard to debug or catch and is best to avoid at all cost.

The stray flux is proportional to distance and therefore the further the lesser the strength of magnetic field. In order to reduce the coupled noise, one can move the power transformer (aggressor) away from the victim. The best thing to do is to have this in mind when you start your chassis or mechanical design. It is harder to change when you’ve got your chassis made.

Important tip on reducing transformer induced noises:

• Shield/encase your transformers.
• Provide separate compartment for your transformers, away from the PCB/circuit/tubes.
• If you have multiple transformers on your design, align them to be 90 degrees with each other to reduce magnetic coupling.
• Space the transformers.
• Increase the distance between the transformers and the sensitive circuit.
• Core types

There are many shapes and format of power transformers. Usually, they’re wound on cores made from iron alloy that comes in sheets stacked together to form a complete core. They’re in sheets and stacked together to avoid currents from forming in the core itself – waste of energy. Power transformers are real life products and therefore they’re not 100% efficient. Energy is wasted as heat mostly at the core or the windings, lost stray flux and etc. Therefore, one of our goals is to minimize this loss as much as possible.

EI Core Power Transformer

It is named as EI core as the core shape looks like letter “E” and letter “I” combined. It is one of the most common types of power transformer available. The bobbin (the structure where the wires are wound, sometimes is made of plastic or paper) is mounted to the “E” core, where the “E” and “I” are stacked in alternating directions to form a power transformer. The “E” and “I” laminations need to be mounted as closer and as uniform as possible for best efficiency. 

Due to the way the cores are stacked together, leaving an air gap between the “E” and “I” lamination, it has the highest stray flux leaving the core. It is impossible to totally close the gap with how the core is formed. It can be improved with a conductive tape or metal wrapped around the exterior of power transformer to form a close loop, same direction as the windings. This will partially cancel out the stray flux leaving the power transformer. Yet another solution is to enclose/encase the transformer with ferrous material like mild steel that will contain the stray flux within the enclosure. 

Copper band around the windings

Important tip: the strongest stray flux radiates outwards on the winding plane. If you want to reduce noise, point it away from your sensitive circuit.

Toroid Power Transformer

Toroid or toroidal power transformer has the core that shaped like a doughnut or ring. The wires are wound around the ring directly instead of needing a bobbin. It comes in plastic wrapped coils or potted inside a case. The core is made of a continuously wound strip of metal, like a roll of duct tape. This enables a very efficient and low stray flux power transformer, nearly 1/10^th of what EI core power transformer is having. Toroid power transformer is quieter and more efficient. 

Since the toroid core is a continuous roll of metal, it is harder to wind as the wires need to pass through the center hole to be wound on the core itself. Toroid power transformer is more commonly seen on high end audio gears due to the efficiency and lower noise behavior compared to other types of power transformers.

C-core Power Transformer

C-core, or cut-core are named as how it looks like, 2 letters “C” cores joined together to form an “O” loop, secured by a metal tape. The windings can be wound on a bobbin and mounted to the “C” core before being joined back together to form the “O” loop and therefore is easier to make compared to toroid power transformer. Two C-cores can also be used to form a double C-core transformer. Wires can be wound on one side, or both sides of the core. 

C-core is better than EI core power transformer as it leaks lesser stray flux. The joints are hidden within the bobbin, and the symmetrical construction of the wires cancels the stray flux.  

There are other types of cores too, like the UI-core or the R-core but it won’t be touched here since they are less common in the market.

Final words

Plan up front on your design for what power transformers to be used. There will be size, cost and performance trade-offs between the different types of power transformers. 

J&K Audio Design

20/12/2013

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